Ministry of Environment

Soils

1.3. Geology, Landforms And Surficial Materials

June M. Ryder

Introduction

The physical landscape of British Columbia is composed of a great variety of landforms and materials. It ranges from spectacular, soaring peaks in the Rocky Mountains, to alluvial plains in the Lower Fraser Valley and lava plateaus in the central interior. This section describes, and to some extent attempts to explain, the distribution of bedrock types, land forms and surficial materials - unconsolidated sediments that have been deposited in geologically recent time.

The present landforms of British Columbia and the nature and distribution of surficial materials have resulted from the interaction of three factors - rock type, tectonic history and climate. The reaction of bedrock to weathering and erosion influences the character of associated landforms and surficial materials. Tectonic history is the sequence of earth movements that have brought about the gross arrangement and relative elevation of major topographic elements. Climate controls weathering, erosion and deposition (geomorphic processes) and the rates at which they operate. The history of climatic change is also recorded in the landscape.

To discuss the resulting complex physical landscape, physiographic regions are delimited. Each of these consists either of a relatively uniform landscape or a repeated pattern of associated landforms. The land- forms within such regions are commonly underlain by related bedrock types and have undergone the same geomorphic history. British Columbia has been divided into five physiographic regions as shown in Figure 1.1.1.

Geology

The landforms that develop upon a particular kind of bedrock are related to its structural features such as bedding planes, joints, folds and faults, and to its mineralogical composition. Rock structure determines both major landforms and details of the landscape. For example, the straight, steep-sided section of Fraser Valley between Boston Bar and Lillooet is fault-controlled, or the size of rock fragments on a talus slope is related to the spacing of joints and bedding planes on the cliff above. The rate of physical weathering or mechanical disintegration of a rock, is influenced by the presence of lines of weakness and pore spaces that allow water, air and temperature changes to penetrate beneath the surface. Thus, densely jointed rocks such as basalt, or very porous material such as sandstone, are more readily fractured than massive, sparsely jointed granites.

The mineralogical composition of a rock determines its susceptibility to chemical weathering, since minerals differ in their resistance to chemical attack.

A geological map of the province (see: Douglas, R.J.W. in Further Reading) shows many different rock types arranged in an intricate pattern. Even on a simplified map (Figure 1.3.1) the outcrop distribution is complex and does not correspond very closely with physiographic regions. Accordingly, in order to avoid duplication, the following descriptions are arranged by rock type rather than by physiographic region. The geology of each physiographic region is shown in Figure 1.3.1 and Table 1.3.1.

Intrusive Igneous Rocks (Plutonic)

These are typically coarse crystalline rocks that in British Columbia range from syenite, through granitic types, to diorite. They occur in a variety of forms ranging from simple stocks, where outcrop area may be tens or hundreds of square metres, to the huge, complex batholith of the Coast Mountains.

Despite variations in mineralogical composition, plutonic rocks tend to be associated with a particular type of terrain. They are relatively resistant to weathering since they are composed largely of durable minerals (quartz, hornblende, feldspar) arranged as a cohesive fabric of interlocking crystals. As a result, slopes on these rocks are generally steep and topography is rugged. Spectacular, glacially formed cliffs survive with little modification, especially in massive or sparsely jointed rocks. Joints, and less commonly, faults, constitute lines of weakness and are followed by cliffs, gullies and major depressions.

However, despite their relative resistance to weathering, some disintegration and decomposition of plutonic rocks inevitably occurs. Mechanical breakdown due to processes such as frost shattering tends to produce relatively large fragments due to the wide joint spacing. Extremely coarse rubble on colluvial slopes and large boulders in till are characteristic of this terrain type. A combination of physical and chemical weathering typically causes granular disintegration. Olivine, pyroxene and calcic feldspar are most susceptible to attack by chemical weathering; crystals of quartz and potash and sodic feldspar are released to form a sandy or gritty residue. Although in most parts of the province the span of post-glacial time is too short for much chemical weathering to have occurred, weathered rock residue of long interglacial periods forms a major portion of the tills and other drift of the last glaciation. Sandy and gritty tills and much sandy outwash are thus associated with areas of coarse crystalline rocks.

Table 1.3.1
Geology and structure of the major physiographic regions.

Major Physiographic Regions
Physiographic Subdivisions (Holland)
Geology and Structure
Coast Mountains and Islands Outer Mountain Area Saint Elias Mountains granitic and metasedimentary rocks
Queen Charlotte Mountains granitic and volcanic (basic and intermediate) rocks
Vancouver Island Mountains folded sedimentary and metamorphic rocks with granitic batholiths
Coastal Trough Hecate Depression flat or gently dipping basaltic lavas and sedimentary rocks, a few granitic intrusions
Georgia Depression sedimentary rocks with scattered volcanic rocks and Quaternary sediments

in Fraser and Nanaimo lowlands granitic rocks in northeast

Coast Mountain Area Coast Mountains granitic (chiefly granodiorite and quartz diorite) rocks with minor gneiss and schist.
Cascade Mountains folded metasedimentary and volcanic rocks with granitic batholiths

 

Interior Plateau Interior Plateau Nechako Plateau flat or gently dipping volcanic rocks with a few granitic stocks
Fraser Plateau and Basin flat or gently dipping basaltic flows
Thompson Plateau flat or gently dipping lavas; sedimentary and volcanic rocks, granitic stocks and batholiths
Quesnel Highland schistose metamorphic rocks with minor volcanic and sedimentary rocks
Shuswap Highland gneiss and schistose metamorphic rocks
Okanagan Highland metamorphic rocks (chiefly gneiss) with granitic intrusions

 

Columbia Mountain and Southern Rockies   Rocky Mountains (south) folded and faulted sedimentary and metasedimentary (chiefly limestone,

quartzite, schist and slate) rocks

Rocky Mountain Trench (south) chiefly Ouarternary sediments
Columbia Mountains Purcell Mountains folded sedimentary and metamorphic rocks (chiefly quartzite, argillite

and limestone) with granitic intrusions

Selkirk Mountains folded sedimentary and metamorphic rocks with granitic stocks and batholiths
Monashee Mountains folded sedimentary and metamorphic (chiefly gneiss) racks with intrusions
Cariboo Mountains folded sedimentary and metamorphic rocks

 

Northern and Central Plateaus and Mountains   Rocky Mountain Foothills folded sedimentary rock (limestone, siltstone, sandstone and shale)
Rocky Mountains (north) folded and faulted sedimentary and metasedimentary rocks (chiefly

limestone, quartzite, schist and slate)

Liard Plateau folded sedimentary racks
Rocky Mountain Trench (north) chiefly Quaternary sediments
Northern Plateau and Mountain Area Liard Plain sedimentary rock
Yukon Plateau folded sedimentary and volcanic racks with granitic intrusions
Central Plateau and Mountain Area Cassiar Mountains granitic core surrounded by folded metamorphic and sedimentary rocks
Omineca Mountains granitic surrounded by folded sedimentary, metamorphic and granitic rock
Stikine Plateau folded and non folded sedimentary and volcanic rocks with minor intrusions
Skeena Mountains folded sedimentary and metasedimentary rocks (argillite, shale and greywacke)
Nass Basin sedimentary and volcanic racks
Hazelton Mountains sedimentary and volcanic rocks intruded by small granitic stocks and batholiths

 

The Great Plains Interior Plains Fort Nelson Lowlands flat and gently dipping shale and sandstone
Alberta Plateau flat and gently dipping shale and sandstone

Sedimentary Rocks

The resistance of sedimentary rocks to weathering and erosion depends upon many factors, but of prime importance are mineralogical composition, degree of induration, i.e. the hardening due to cementation, pressure and heat, and the spacing of bedding planes and joints. Poorly indurated rocks, such as clay, mudstone and shale, disintegrate readily and are easily eroded. Well indurated, massive sandstones, particularly quartz sandstones, and conglomerates are relatively resistant to erosion. Limestones generally form upstanding topography and also appear to be relatively resistant.

Horizontally bedded, alternating resistant and less resistant strata give rise to plateau and escarpment, or mesa-like topography. Stepped slopes are also characteristic, with steep "risers" of resistant rock, and gently sloping or flat "treads" of non-resistant strata. Terrain of these types occurs in the Great Plains region of northeastern British Columbia. Where the strata are dipping, asymmetric ridges (cuestas) are formed where the harder rock outcrops. Ridges and intervening valleys run parallel to the strike of the rocks and thus topography has a pronounced "grain" or linearity. Terrain of this type is found in the foothills of the Rocky Mountains.

Physical disintegration of sedimentary rocks produces fragments of a size related to the spacing of lines of weakness in the bedrock. Thus shales, which are thinly bedded, produce small, platy particles that readily disintegrate further. At the other extreme, massive conglomerates may shed huge blocks which survive intact during glacial transport.

Weathering of sedimentary rocks also results from chemical removal by solution or other means, of the cementing agent. Simple disintegration of the rock then occurs, forming a residue that is texturally similar to the original sediment. In British Columbia, products of weathering are mixed with products of glacial abrasion in the surficial materials. Thus, for example, clay tills overlie shales, and sandy tills overlie sandstones and conglomerates. Silica-cemented rocks such as sedimentary quartzites resist this effect and are extremely hard and durable.

In terrain developed upon faulted, folded and steeply dipping sedimentary rocks, there is a strong relationship between rock type, structure, and the ensuing topography. In belts of intense folding and faulting such as the Rocky Mountains, structures are expressed as strongly linear topography. Individual ridges or mountains are commonly asymmetric with steeper scarp and gentler dip slopes; dips of over 45¼ produce extremely jagged ridges. Prominent cliffs commonly coincide with fault planes or steep bedding planes.

Volcanic Rocks

Topography upon these rock types tends to be similar to that developed upon sedimentary rocks of similar structure. Flat-lying lava flows form plateaus bounded by escarpments and stepped hillsides. Terrain of this type is widespread in south-central British Columbia.

Most of the relatively undeformed volcanic rocks in the province were erupted from fissures and shield volcanoes during Tertiary time. Basalts, andesites and pyroclastic rocks (air-fall deposits) are probably most common. The lavas are fine textured and closely jointed. Basalt typically displays regular columnar jointing.

Mechanical weathering of lavas and pyroclastic rocks proceeds in accordance with the arrangement of fracture planes. The presence of open vesicles or spaces left by gas bubbles in many lavas, gives additional opportunities for weathering attack. The physical weaknesses which augment mechanical disintegration also affect chemical weathering. Basalt, consisting of plagioclase feldspar, pyroxene and olivine, is most susceptible, and rhyolite (quartz and feldspar) least susceptible to decomposition. With sufficient time, basalt breaks down completely to clay minerals. The weathering products of lavas are generally fine textured and result from the combined effects of chemical and mechanical weathering.

Metamorphic Rocks

No particular type of terrain can be ascribed to metamorphic rocks in general. Their resistance to weathering and erosion depends upon their individual characteristics.

Many metamorphic rocks such as gneiss and schist are foliated, that is, the minerals are segregated into bands within the rock. As a result, differential weathering may produce linear topographic features as in the Omineca Mountains and Finlay Ranges. Metamorphosed sedimentary rocks such as marble and quartzite commonly retain their original bedding and fold structures which control their topographic expression. Metamorphic quartzites are extremely resistant to weathering and form some of the highest peaks in the Columbia Mountains.

Gneissic terrain occurs in the highlands along the southeastern margin of the Interior Plateau. Weathering products are similar to those of coarse textured igneous rocks of the same composition. Schists occur in the Quesnel Highlands, and elsewhere in association with plutonic rocks; they are broken down relatively rapidly by weathering, and produce strongly linear or stepped topography.

Tectonic History

A brief account of tectonic history is necessary here, since the major topographic features of the province are tectonically controlled and many mountainous areas owe their present high elevation to recent uplift, rather than resistance to erosion.

The mountain systems of the province have been built during several episodes of mountain building since late Precambrian time. Successive episodes have tended to emphasize already established structural trends. Two great crystalline belts, the Coast-Cascade Mountains and the Cassiar-Omineca-Columbia Mountains, have been the sites of repeated uplift, deformation and igneous intrusion.

The features of the present landscape emerged during the Cenozoic Era (Table 1.3.2). Folding and thrusting of geosynclinal sediments to form the Rocky Mountains occurred roughly 70 to 35 million years ago. The Coast and Omineca belts also experienced re-elevation at this time. During mid-Eocene time, fissure and shield volcano eruptions produced the first of the two sets of lavas that cover much of the Interior Plateau.

The mid-Tertiary was an erosional interval during which the general outline of present day drainage began to emerge. An erosion surface of 450 to 600 m relief was formed over most of the area lying west of the Rockies. During Miocene time the second phase of volcanism occurred. There were renewed fissure and shield volcano eruptions and lava spread over parts of the mid-Tertiary erosion surface. The Plateau Basalts, now a prominent landscape feature of the Thompson Plateau, date from this time.

General uplift followed, resulting in differential elevation of the erosion surface and its superposed lavas. It was upwarped along the old Coast and Omineca axes and along the Insular Mountains. The erosion surface constitutes the oldest landform that is preserved today. Over large portions of the Interior Plateau it was modified only slightly by glacial processes. However dissection by rivers has been severe around the plateau margins, whilst in bordering mountains, isolated remnants of flat or gently sloping terrain and accordant summit levels are all that remain. Parts of the up- land surface survive on interfluves in the Insular Mountains of central Vancouver Island.

Quaternary History

The geomorphic and climatic history of British Columbia during the Quaternary Period is of greatest significance with regard to present day landforms and surficial materials. Climatic fluctuations during the Pleistocene brought about alternating glacial and longer nonglacial intervals (Table 1.3.2). There is geological evidence for up to four glaciations in British Columbia. However, it is likely that there were additional glaciations for which no local evidence is available, since more complex Pleistocene sequences are preserved in other parts of northern North America.

In British Columbia, sediments that predate the final deglaciation do not outcrop over sufficiently extensive areas to warrant attention here. Accordingly, the following description is restricted to the effects of the last, the Fraser or late-Wisconsin, glaciation and postglacial geomorphic processes. During the Fraser Glaciation ice accumulated in the high mountains of the Cordillera. Glaciers that were several kilometres wide and over a thousand metres deep occupied the main valleys in the mountains. An unbroken ice-sheet covered interior plateaus and thick glacier tongues coalesced along the coast. The Keewatin part of the Laurentide ice-sheet extended into the Great Plains region of northeastern British Columbia. Figure 1.3.2 shows ice-flow directions and other features of the Fraser Glaciation.

The effects of total erosion by all Pleistocene glaciers are widespread, and most apparent on high plateaus and mountains. Several distinct modes of erosion and associated landforms can be identified. True alpine glacial topography is found in mountains that contained glaciers, but were not totally covered by ice; horns, aretes, and cirques are diagnostic features of this type of terrain. Where mountainous topography was overridden by ice, glacial erosion produced typically rounded summits and ridge crests. Landscapes are commonly found which consist of a combination of these two terrain types where cirques and glacial troughs exist below rounded ridges and summits. Grooved and fluted terrain occurs on plateaus and plains and usually results from both glacial erosion and deposition. In valley and trough areas, erosion was greatest where the valleys were aligned parallel to the direction of ice flow. This was the case in the Okanagan Valley for example, where the bedrock valley floor now lies as much as 200 m below present day sea level.

Fraser ice reached its maximum about 15,000 years ago, when it extended south to 47¼N. latitude in Washington State. The subsequent retreat of the ice margin was relatively rapid. The Strait of Georgia and parts of the Fraser Lowland were ice free by 13,000 years ago. Minor readvances of ice into the eastern part of the Fraser Lowland occurred until 11,000 years ago. Major valleys in southern interior British Columbia were ice-free by 11,000 years ago. A Carbon-14 date from peat near a modern glacier terminus in the Coast Mountains shows that ice cover had shrunk to approximately its present extent by 9500 years ago. Sea-level was relatively high at the end of the Fraser Glaciation. As ice retreated from the Vancouver area, for example, sea-level stood at +175 m, and in the Kitimat-Terrace area it was at +230 m during deglaciation. Sediments deposited during the Fraser Glaciation and during the retreat are described below.

Holocene Climate and Geomorphic Processes

A comprehensive climatic record for Holocene (postglacial) time in British Columbia has not yet been assembled. The information presently available has been obtained chiefly by analysis of pollen from Carbon-14 dated bog and lake sediments. Climatic changes are deduced from the vegetation succession recorded by pollen stratigraphy. Most information comes from the southwest coastal area and adjacent parts of Washington State. No studies have been reported from British Columbia north of a line through the Skeena and Pine Rivers.

Climatic warming occurred rapidly either just before or during deglaciation, and vegetation quickly colonized the newly exposed surfaces. A cold climate, i.e. colder than at present, of one or two thousand years duration is indicated only at sites where the pollen record extends back to the time when ice was still present. There is no record of tundra vegetation. Trees were everywhere a part of the immediate postglacial flora.

There was a relatively warm and dry interval during Holocene time, but its time of occurrence and duration have not yet been precisely determined. There is some evidence for a mid-Holocene "xerothermic" or "Hypsithermal" interval, approximately 3000 to 8000 years ago, which was preceded by cool, moist conditions. However, most recent investigations have indicated that the relatively warm and dry conditions of the Hypsithermal prevailed during the first half of Holocene time, roughly from 10,000 to 6500 years ago (Table 1.3.2).

Cooler and wetter conditions followed the Hypsithermal and lasted until the present time. During this period, minor climatic fluctuations have occurred, and short-lived cool and moist intervals have coincided with temporary expansions of mountain glaciers (referred to as "neoglaciation"). Although these periods of activity are somewhat irregular, in general three episodes of glaciation have been identified at 5800 to 4900 years and 3300 to 2300 years ago and during the last 1000 years (Table 1.3.2). In fact, many glaciers appear to have reached their greatest Holocene extent within the last few centuries.

Table 1.3.2
(click image to view larger image)

Geomorphic processes during the Holocene period have been controlled in part by the effects of Fraser Glaciation and in part by Holocene climatic conditions. Immediately following deglaciation there was a period of intense activity when much glacial drift was reworked and redistributed by streams and rivers, and mass wastage processes modified drift deposits on slopes. Aggradation along rivers, alluvial fan deposition, and lacustrine sedimentation occurred at this time. This activity gradually diminished as the supply of easily erodible drift became exhausted. In many valleys postglacial aggradation was succeeded by a phase of downcutting by streams and rivers. Terraces were cut into moraine, fluvial and lacustrine sediments.

With one or two exceptions, the effects of Holocene climatic change are hard to discern in the British Columbia landscape. Increased warmth and dryness during the Hypsithermal period in south-central British Columbia appears to have induced aeolian processes by causing reduction in vegetative cover. Sand dunes in Okanagan Valley and other aeolian deposits have been attributed to this origin. In alpine areas, periglacial and glacial activity seems to have increased during the neoglacial events described earlier.

Surficial Materials and Associated Landforms

Surficial materials that form the parent material of soils were formed during and since the Fraser Glaciation. Drift of this glaciation includes till and fluvioglacial, glaciolacustrine and glaciomarine sediments that were deposited during the ice retreat. Throughout Holocene time, colluvial materials have accumulated on or at the foot of slopes, and fluvial, lacustrine and organic sediments have been deposited on valley floors and in depressions. The physical properties of these materials, their distribution, and the landforms with which they are typically associated are described here.

Morainal materials were deposited directly from glacier ice. They include Pleistocene till and the rubbly deposits of Holocene glaciers. Till is probably the most extensive of all surficial materials in the province. It covers level to moderately sloping surfaces that lie above valley floor areas affected by recent fluvial activity and below rock and colluvial slopes of the alpine zone. Till plains occur where till has infilled irregularities in the underlying surface or covered level surfaces of structural or erosional origin. Drumlins and fluted till sheets are common on plateaus and plains particularly the Interior and Alberta Plateaus.

Till is a compact, non-sorted and non-stratified sediment which contains a heterogeneous mixture of particle sizes. However, it can vary abruptly from place to place with changes in the nature of source material. Where till was derived from bedrock, its mineralogical composition is related to the local rock (as described earlier). Where till was derived from unconsolidated sediments, it reflects their texture and mineralogy. For example, in valleys of the Thompson Plateau, till consists of rounded pebbles and cobbles in a matrix of sandy silt. The stones come from older fluvial gravels and the fines from older lacustrine silt.

Sandy tills are porous and so water passes rapidly through them. At the other extreme, clay tills tend to be impervious and poorly drained. In addition, impeded drainage may occur where a thin cover of till overlies impervious bedrock.

Fluvioglacial materials are deposited by meltwater either in contact with glacier ice or beyond the ice margin as outwash. Ice-contact deposits vary from well defined kames, kame terraces and eskers to irregular and discontinuous spreads of gravel overlying till. They consist of gravels and sand that may be sorted and stratified. Where bedding is present, it is typically distorted as a result of partial collapse when the supporting ice melted. Kames and eskers tend to be well drained and often constitute the driest, or most xeric, sites within a landscape.

Within British Columbia, ice-contact materials have a wide distribution and are found in most areas with the general exceptions of narrow glacial troughs and the higher terrain of alpine glaciation. Major eskers and esker complexes are found on the Nechako Plateau, Fraser Plateau and Liard Plain. Kames and eskers typically occur in association with meltwater channels on the surface of the Interior Plateau. In areas of higher relief, kames occur in cols or high-altitude through-valleys. Kame terraces form the highest terraces within most valleys.

Proglacial outwash deposits constitute gravel plains or terraces that may be kettled. The material ranges from sand to boulders and is sorted and poorly bedded. Outwash gravels originated as floodplains occupied by braided rivers. In Holocene time old outwash plains have been modified due to changing river regimes and have undergone terracing or have been occupied by meandering rivers. Outwash gravels are forming in present-day proglacial situations along valleys draining from glaciers in the Coast, Columbia and Rocky Mountains.

Outwash gravels are porous and permeable and generally constitute well drained areas unless the water-table lies close to the surface.

Glaciolacustrine materials collected in proglacial lakes during or shortly after deglaciation (Figure 1.3.2 shows the major lakes). They consist of sediment brought into the lakes by meltwater streams. Silt and fine sand are most common but coarser sand and gravel occur close to points of inflow. Clay-sized sediment can occur locally. Bedding in lacustrine silts and sands ranges from massive to laminated. Where materials were deposited in contact with glacier ice, slumping and settling ensued during melting giving rise to irregular undulating or kettled topography.

Figure 1.3.2

In general, the nature of terrain associated with glaciolacustrine sediments depends upon their thickness. In many basins sediments are sufficiently thick that all former irregularities have been masked and the old lake bed now forms a level or slightly undulating surface. Scarps in lacustrine silts tend to be steep with vertical upper sections. Headward erosion of gullies in lacustrine silt is rapid, and where this material occupies large areas, extensive dendritic gully systems may have developed, as for example in the South Thompson Valley near Kamloops.

Near-surface drainage and groundwater movement within glaciolacustrine materials depend upon their texture. Fine silt and clay beds form impervious layers which will reduce infiltration rates if near the surface or cause lateral migration of groundwater in perched water tables at depth. In the latter situation, high pore pressures may develop and cause loss of cohesion resulting in slump and flow failures along slopes. Steep scarp slopes are particularly susceptible to this process. In some places this has been man-induced by addition of extra water, for example from irrigation, on the surface above the scarp.

Glaciomarine materials ("marine drift") were formed during the time of high sea-level at the end of the Fraser Glaciation by the accumulation of particles released from floating, melting ice. It consists of stony, silty clay which is non-sorted and may show faint, irregular stratification. The material is superficially similar to till, but in places contains the shells of Pleistocene marine molluscs. As sea-level fell, the areas mantled by marine drift gradually emerged and were systematically worked over by wave action to form a thin covering of beach sands and gravels.

Water percolation through unmodified marine drift is slow due to its high clay content. Drainage is rapid through beach gravels and sands.

Fluvial materials are transported and deposited by streams and rivers. They consist of gravel, sand, or silt and are generally well-sorted by comparison with most other surficial materials.

In general, floodplains of rivers that have steep gradients and high sediment loads (bedload) tend to have shallow, braided channels and to be underlain by gravelly material. A thin cover of flood-deposited sand may occur away from active channels. Streams of this type occur within steep, mountainous terrain. Floodplains of low-gradient rivers that are transporting fine textured sediment have meandering channels, levees and backwater or backswamp areas. They are underlain by sand and silt resulting from overbank deposition. Rivers of this kind are found on deltaic floodplains and upstream from lakes, rock sills and other local base-level controls.

Fluvial terraces are underlain by materials similar to those underlying floodplains, but gravels usually predominate and overbank fines are less common.

Alluvial fans are formed where streams emerge from steep, narrow valleys onto flatter ground. Material is deposited here due to reduced gradient and widening of the channel. Material is coarsest, commonly boulder gravel, close to the fan apex, and becomes systematically finer outwards, grading to fine gravels, sand or silt at the periphery. Many alluvial fans are relict features in the present landscape and date from the early post-glacial phase of geomorphic activity.

A delta is simply an extension of a floodplain. Texture of the material within a delta is relatively uniform and depends to a great extent upon the gradient of the river upstream. The delta of a steep mountain torrent will consist of boulder gravels whereas the delta of a low-gradient river will be formed of silt or sand, as for example the delta of the Kootenay River near Creston.

Percolation of water through sand and gravel is potentially rapid due to their coarse texture. Drainage of fluvial surfaces thus depends chiefly upon the depth of the groundwater table. Where this is high, as in the case of deltas and floodplains, areas of impeded drainage will occur, and slightly elevated areas, such as levees, will be relatively dry. The water table is also close to the surface in places on alluvial fans, particularly near the stream and at the fan toe where seepage zones may be present. Seasonal fluctuations in the levels of lakes and rivers will induce variations in the level of the water table in related landforms. Terraces and dissected fans are well drained sites.

Colluvial materials are the products of mass wastage. They include any unconsolidated materials that have lost their original form due to downhill movement, as well as distinctive types of colluvium such as mudflows, landslide deposits and the debris of shattered bedrock.

Mudflows consist of clasts of various sizes surrounded by a "mud" matrix of silt and clay. This mass flows as a viscous liquid when saturated and is capable of transporting large boulders. Colluvial fans consist of stratified sequences of thin mudflows. They tend to be moderately well drained as a result of surface slopes of 5 to 15¼, although water percolation through mudflow material is usually slow due to its high silt and clay content.

Landslide deposits vary from fine textured masses of slumped lacustrine silt to huge blocks and rubble of rockslides. They generally form hummocky topography below a steep landslide scar. Fine textured slide deposits are poorly drained and may contain pockets of standing water. There may be seepage zones near the foot of the slide scar. Rockslides are usually very well drained, xeric sites.

Bedrock fragments loosened individually by frost shattering and other weathering processes accumulate as talus slopes or sheets on hillsides. As a result of its coarse texture and steep surface gradients, material of this type is generally very well drained, although seepage sites may occur at the foot of slopes.

Solifluction deposits are widespread in high alpine areas. They consist of water-saturated masses of weathered rock or drift that flow slowly downhill. The high moisture content is derived from melting snow and summer precipitation, and in addition, drainage may be impeded due to underlying permafrost or impervious bedrock.

Eolian materials are wind deposited sands and silts. They form dunes where abundant material is blown along the ground surface and heaped up by the wind, and they form thin layers over older landforms by the settling of airborne particles. Dunes consist of medium to fine sand, whilst eolian mantles consist of fine sand and silt. These materials occur throughout the province, even in the alpine zone, but they are thickest and most extensive on terraces, fans, floodplains and outwash surfaces.

Dunes are generally well drained. The drainage of eolian mantles depends to some extent on the permeability of the materials that they overlie and the associated landform.

Marine materials consist of off-shore deposits of stratified and locally fossiliferous clays and silts that settled from suspension, and shoreline materials of beach sands and gravels that formed by wave action and longshore drift. Terrain underlain by marine clays and silts is usually very gently sloping. Beach gravels and sands occur as low ridges. Drainage on the fine textured off-shore materials is restricted; beach materials are generally well drained.

Organic materials consist of peat that has accumulated in depressions and other areas of high water table. They are most abundant in the northern part of the province in areas such as the Liard Plain, Fort Nelson Lowland, and Queen Charlotte Lowlands. Elsewhere, they are restricted to relatively small areas in rock basins, kettles, floors of glacial spillways, and old channels and backswamps on floodplains and deltas.

Characteristics of the Physiographic Regions

For information regarding the geology of these physiographic regions and the relationships between rock type and weathering products, texture of surficial materials, and landforms, the reader is referred to Figure 1.3.1, Table 1.3.1 and the preceding "Geology" section.

Coast Mountains and Islands

This region consists of two parallel mountain belts (the discontinuous St. Elias - Insular Mountains and the Coast-Cascade Mountains) and the intervening, largely submerged Coastal Trough. In the mountain belts, summits are highest in the St. Elias Mountains (4600 to 5500 m), Coast Mountains (2500 to 4000 m), and the central part of Vancouver Island (2000 m). The mountain topography is extremely rugged, even in the relatively low Queen Charlotte Mountains (1100 m).

Terrain typical of intrusive igneous rocks and mountain glaciation, both alpine and overridden types, is especially well developed in the Coast and Cascade Mountains. Major linear structures have been excavated by glacial and fluvial processes to form striking alignments and grid-like patterns of valleys and fjords. Ice flowing westwards from the Interior Plateau and northern mountains excavated major low-level troughs across the Coast Mountains. These are now occupied by rivers such as the Taku, Stikine, Iskut, Nass, Skeena, Dean, Bella Coola and Homathko (Figure 1.3.2).

Glacial landforms dominate the St. Elias and Queen Charlotte Mountains. On many islands and coastal areas of the Insular belt, cirques occur at all elevations down to sea-level. Remnants of the Tertiary erosion surface occur as level or gently sloping ridge crests on both island and mainland along the Coastal Trough. The surface lies below 600 m elevation here, but it rises gradually eastwards and westwards towards the mountains. Erosion surface remnants and landforms of mountain glaciation (both alpine and overridden types) constitute much of the Vancouver Island landscape.

The St. Elias Mountains and parts of the northern and southern Coast Mountains still have extensive icefields and glaciers. Rivers draining from these areas have broad gravel floodplains that are free of vegetation and crossed by shifting, braided channels.

Within the mountains, thick drift deposits are restricted to the margins of the floors of major valleys and adjacent hillsides. Floodplains, and in some valleys river terraces, occupy the central part of the valley floor. On most slopes there are extensive bedrock outcrops and accumulations of rubbly colluvium. Relatively gentle mountain slopes may have a thin till mantle. Avalanching is the dominant geomorphic process operating today on steep slopes at intermediate and high elevations. Nivation, solifluction and other periglacial processes are locally important in the alpine areas. Neoglacial moraines occur adjacent to modern glaciers.

Within the lowlands and islands of the Coastal Trough, summit levels are generally below 600 m. Drift deposits are thickest and most continuous here, although there are some large areas of glacially abraded rock surfaces and outcrops of resistant rock types. A uniformly low glaciated rock plain (Milbanke Strandflat) borders the mainland coast along Hecate Strait. Marine drift and associated beach materials cover hilly areas and gentle slopes in the western part of the Fraser Lowland and other coastal areas (Figure 1.3.2). Till and fluvioglacial gravels also form extensive hilly and undulating areas in the lowlands and islands. Fluvial gravels and sands occur as terraces and floodplains along major rivers that cross the coastal lowlands. In Holocene time the Fraser River has formed a major floodplain and delta composed chiefly of sand and silt.

Interior Plateau

The Tertiary erosion surface, in places capped by lava flows, is the dominant landscape feature of this region. The degree of dissection of the plateau surface varies from place to place. In the northern two-thirds of the region (Fraser and Nechako Plateaus), large tracts are undissected and comprise flat to gently rolling terrain at elevations of 1200 to 1500 m.

The surface is interrupted however, by the entrenched Fraser River and its major tributaries which occupy channels that are 100 to 200 m lower than the plateau surface. Near Anahim Lake, several small mountain ranges (2500 m) are the dissected remnants of late-Miocene shield volcanoes. The Fraser Basin is a low-lying part of the plateau in the vicinity of Prince George, from 600 to 900 m a.s.l.

The southern part of the Interior Plateau (Thompson Plateau) is dissected by the Fraser and Thompson Rivers and their tributaries which flow in steep-sided valleys 600 to 900 m below the plateau surface. Extensive areas of rolling upland surface remain at 1200 to 1500 m, and in places higher hilly areas up to 2000 m are located upon relatively resistant bedrock.

Uplift of the Tertiary erosion surface was relatively great along the eastern margin of the Interior Plateau in the Shuswap, Quesnel and Okanagan Highlands. Subsequently, severe dissection has occurred here, producing relatively rugged terrain with gently sloping erosion surface remnants on ridge crests and summits. Summit levels lie between 1500 and 2000 m, and local relief of 600 to 900 m is typical. Glacial erosion produced rounded ridge crests and steep valley sides. Drift occurs on valley floors and on gentle slopes.

The surface of the whole Interior Plateau is mantled with drift which chiefly consists of drumlins and fluted till. Rock outcrops are not extensive on the plateau surface, although the plateau is bounded by lava cliffs or steep rocky slopes adjacent to the entrenched rivers. Eskers, kames and meltwater channels are numerous. Glaciolacustrine silts occur in many valleys and basins (Figure 1.3.2). Drift is over 100 m thick in some major valleys such as the Fraser and Okanagan. In many valleys this Pleistocene fill has been dissected and terraced during post-glacial downcutting by major rivers.

Columbia Mountains and Southern Rockies

This region comprises several parallel mountain belts and intervening valleys. The four mountain belts which together constitute the Columbia Mountains are lithologically and structurally complex (Table 1.3.1). In general, summit elevations range from 2500 to 3500 m, and are lowest south of latitude 50¼N. Where summits are high, the mountains are extremely rugged, and where deep valleys flank high peaks, local relief of over 2000 m is not uncommon. Ridges and peaks above 2000 to 2500 m were not overridden by ice and are serrated. Lower summits and crests are subdued and rounded and may have a thin covering of till. Drift is present on valley floors (along with fluvial materials) and on gentler mountain slopes at relatively low elevations. Steeper slopes consist of rock outcrops and rubbly colluvium. Avalanching occurs on steep valley sides at all elevations. Nivation is proceeding in alpine areas, and both active and relict periglacial features occur, such as patterned ground and rock glaciers. The modern glaciers are bounded by one or more strands of neoglacial moraine.

The Monashee, Selkirk and Purcell Mountains are separated by major glacially-enlarged trenches which now contain Arrow and Kootenay Lakes and part of the Columbia River. Drift materials, chiefly till and fluvioglacial gravels, are widespread on the floors and lower slopes of these valleys and alluvial terraces occur along rivers.

The Rocky Mountain Trench is a steep-sided structural depression which lies between mountains of strongly contrasting structures and rock types. It is largely floored with Quaternary sediments. Glacial deposits include till as drumlins, outwash terraces and glaciolacustrine silt. However, Holocene fluvial sediments occupy extensive areas and consist of terrace gravels and floodplain silts, sands and gravels.

In the southern Rocky Mountains, the topography reflects the structural control of underlying folded and faulted sedimentary rocks. Erosional landforms of alpine and valley glaciation such as cirques, troughs and horns are commonly asymmetric where they are cut in moderate to steeply dipping strata. The broadest troughs are located along zones of 'soft' rock. Summit elevations range up to 3600 m and local relief is typically 1200 to 1500 m. The distribution of drift in the Rockies is similar to that in the Columbia Mountains. However, rapid disintegration of the well jointed sedimentary rocks of the Rockies has given rise to much talus development, and to the formation of mantles of rubbly debris over bedrock slopes above timberline. Where bedding is nearly horizontal and along valley sides that parallel the strike of the bedrock, alternating cliff bands and shelves containing talus slopes are a typical landscape feature.

Northern and Central Plateaus and Mountains

This physiographic region consists of a miscellaneous collection of mountains, plateaus and plains, but some generalizations can be made. The plateaus (Yukon, Stikine, and small plateaus within mountain regions) are Tertiary erosion surface remnants displaying varying degrees of dissection by streams with various depths of entrenchment. Plateau surfaces are generally between 1500 and 1800 m, and flat or gently rolling. Where they are warped up to higher elevations, dissection has been more severe. Ice overrode virtually all plateau areas and produced drumlinoid and fluted topography in both bedrock and till. Drift is widespread, and deranged drainage and numerous lake basins provide sites for organic accumulation. Many centres of late-Tertiary and Quaternary vulcanism occur within the plateaus, especially on the Stikine Plateau where the shield volcano of Mt. Edziza (2700 m) and Level Mountain (2100 m), and other volcanic cones are located.

The mountain areas (Skeena, Cassiar and Omineca Mountains) are subdued by comparison with the Coast and southern Rocky Mountains. Summit elevations range between 1800 and 2700 m, and local relief is 900 to 1200 m. Surfaces below 1800 m were overridden by ice and consequently the lower summits are rounded and commonly bear a thin drift cover. Higher areas have all the features of alpine glaciation. Drift is widespread in broad valleys.

The Liard Plain and Nass Basin are areas of low relief and flat or gently rolling topography at elevations of 750 to 1000 m, and below 750 m respectively. There is extensive drift cover with numerous lake basins. Drumlins and fluted terrain in both drift and bedrock are common. Several major esker systems cross the Liard Plain.

The northern Rocky Mountains and Rocky Mountain Trench are essentially similar to their southern counterparts, although there are large areas of relatively subdued mountain topography, for example, in the Hart Ranges. In both the Rockies and the Rocky Mountain Foothills to the east, a strongly linear topography reflects geologic control. The foothills have landforms typical of dipping sedimentary strata with little glacial modification other than a drift cover.

Great Plains

This area comprises structurally controlled topography with mesas and cuestas, developed on flat-lying or gently dipping sandstones and shales. The sandstones are relatively resistant to erosion and underlie the uplands. Surfaces are generally flat to gently rolling at elevations of 900 to 1200 m on the Alberta Plateau, and below 600 m in the Fort Nelson Lowland. The Peace and Liard Rivers and their tributaries are incised, producing a low relief along valleys of 600 m in the plateau and 150 m in the lowland.

This region was glaciated chiefly by Keewatin ice from the east. Cordilleran ice from the Rocky mountains extended only a short distance eastwards beyond the Foothills (Figure 1.3.2). Drift was deposited over most of the region, although it is thin or absent from some uplands and ridge crests. Drumlins and fluted till plains are common throughout the area, and a variety of moraine features, pitted outwash and meltwater channels occur in the lowland. Large areas of outwash gravels and sands were deposited by meltwaters draining eastwards and northwards from the Cordilleran ice. These materials now occupy plateau margins adjacent to incised rivers, such as the Fort Nelson and the Hay. Lacustrine sediments of proglacial lakes occur in the Peace River basin and in parts of the Fort Nelson Lowland (Figure 1.3.2). They consist mainly of clay and silt, but grade locally to sands and deltaic gravels.

Further Reading

  • Alley, N.F., 1976. The palynology and palaeoclimatic significance of a dated core of Holocene peat, Okanagan Valley, southern British Columbia. Can. Jour. Earth Sciences 13, pp. 1131-1144.

    Describes changes of climate and vegetation that occurred in the dry interior of British Columbia during post-glacial time.

  • Armstrong, J.E., 1976. Quaternary geology, stratigraphic studies and revaluation of terrain inventory maps, Fraser Lowland, British Columbia. Geol. Surv. Can. Paper 75-1, Part A, pp. 377-380.

    An update and summary of Armstrong's work which covers absolute ages, stratigraphy and chronology of Quaternary sediments.

  • Armstrong, J.E. and W.L. Brown, 1954. Late Wisconsin marine drift and associated sediments of the Lower Fraser Valley, British Columbia, Canada. Bull. Geol. Assoc. Amer., 65, pp. 349-364.

    Describes in detail the fossiliferous, till-like stoney clays that cover much of the Lower Mainland. Several hypotheses for the origin of marine drift are discussed and illustrated.

  • Birkeland, P.W., 1974. Pedology, Weathering and Geomorphological Research. Oxford Univ. Press, 285 pp.

    Focuses on the use of soils and rock and mineral weathering to solve problems in Quaternary research that are common to the fields of pedology, geomorphology, sedimentology and stratigraphy.

  • Clague, J.J., 1976. Late Quaternary sea-level fluctuations, Pacific coast of Canada and adjacent areas. Geol. Surv. Can. Paper 75-1, Part C, pp. 17-21.

    Provides an excellent summary and discussion of previously published accounts.

  • Douglas, R.J.W. (Ed.), 1970. Geology and Economic Minerals of Canada. Econ. Geol. Rpt. No. 1, Dept. Energy, Mines and Resources, Canada.

    Major work on the geology of Canada. Includes useful chapter on Quaternary geology.

  • Fulton, R.J., 1969. Glacial lake history, southern Interior Plateau, British Columbia. Geol. Surv. Can. Paper 69-37, 14 pp.

    Describes pattern of deglaciations and related development of glacial lakes in the Nicola, Thompson and Okanagan basins.

  • Fulton, R.J., 1971. Radiocarbon geochronology of southern British Columbia. Geol. Surv. Can. Paper 71-37, 28 pp.

    A description of the radiocarbon-dated Quaternary history of southern British Columbia over the past 52,000 yrs.; includes a compilation of dates related to glaciations, post-glacial sea-levels, volcanic ash-fazes and post-glacial climate.

  • Fulton, R.J., 1975. Quaternary geology and geomorphology, Nicola-Vernon area, British Columbia. Geol. Surv. Can. Memoir 380, 50 pp.

    An account of Quaternary Landforms and sediments in area covered by NTS maps 92I/ SE, 92I/NE, 82L/NW, 82L/SW, and accompanied by surficial geology maps at 1:26,720.

  • Prest, V.K., D.R. Grant, and V.N. Rampton, 1967. Glacial Map of Canada. Geol. Surv. Can.

    1:5,000,000 map depicting glacial landforms.

  • Heusser, C.J., 1960. Late-Pleistocene environments of north Pacific North America. Amer. Geog. Soc. Spec. Pub. 35, 308 pp.

    Describes late-glacial and post-glacial climatic, and biotic changes that occurred in coastal regions as interpreted from peat and pollen stratigraphy.

  • Holland, S.S., 1964. Landforms of British Columbia, a physiographic outline. B.C. Dept. Mines and Pet. Res. Bull. 48, 138 pp.

    Physiographic regions are defined and described in terms of their topography, geology, landforms and glacial history.

  • Mathewes, R.W., 1973. A palynological study of postglacial vegetation changes in the University Research Forest, southwestern British Columbia. Can. Jour. Bot. 51, pp. 2085-2103.

    Pollen analysis is used to investigate post-glacial vegetation and climate changes.

  • Mathews, W.H., 1968. Geomorphology, southwestern British Columbia, pp. 18- 24 in Guidebook to Geological Field Trips in Southwestern British Columbia, ed. W.H. Matthew's, Dept. Geology, Univ. of British Columbia.

    A summary account of Tertiary and Quaternary landform development.

  • Nasmith, H., W.H. Mathews, and G.E. Rouse, 1967. Bridge River ash and some other recent ash beds in British Columbia. Can. Jour. Earth Science 4, pp. 163-170.

    Describes the distribution and time of eruption of post-glacial volcanic ashes.

  • Tipper, H.W., 1971. Glacial geomorphology and Pleistocene history of central British Columbia. Geol. Surv. Can. Bull. 196, 89 pp.

    Describes glacial landforms and Pleistocene history of area covered by NTS maps 92N, 920, 92P, 93B, 93C, 93F, 93G, 93J. Accompanied by maps of selected glacial features at 1:250,000.