Ministry of Environment Terrain

Texture of Surficial Materials

Texture refers to the size and roundness (degree of curvature of corners and edges) of the particles that constitute a surficial material, and to the sorting (uniformity of particle sizes) within the mass of sediment. On some terrain maps, the texture of organic sediments may be identified. Organic texture denotes the relative proportions of fibres (undecomposed remains of vegetation) and humus (decomposed material).

Texture is important because it influences many physical properties of surficial materials. It largely determines their permeability, compressibility, drainage characteristics, and stability and erodibility on steep slopes; it also influences the response of a sediment to freezing and thawing (frost heave susceptibility) and changes in moisture content (shrink-swell potential). For example, coarse textured materials such as gravel tend to be highly permeable, well drained, and non- compressible. Fine textured materials such as silt and clay have low permeability and are subject to frost heave; they may be plastic, highly compressible, and shrink and swell when dried and wetted; they commonly give rise to landslides and mudflows.

Thus texture gives some indication of the physical properties of a surficial material. It should always be considered when assessing the suitability of an area for development or any change in land use.

Description of Commonly Used Textural Terms and Map Symbols

Organic Terms

If more than one letter symbol is used to indicate the texture of a surficial material, the right hand letter indicates the dominant texture, and the left hand letter(s) the minor component(s). Thus sgFt is a fluvial terrace composed of sandy gravel; zsgFt indicates a small admixture of silt in the sandy gravel. If texture is omitted from a terrain unit symbol, then the map user should refer to the appropriate surficial material in the map legend for a general description of texture.