[Coastal Watershed Assessment Procedure Guidebook Table of Contents]
Appendix 11
Classification of bedrock geology
Because till is often derived from local bedrock, bedrock geology can often be used as a good indicator of till composition and texture in cases where no other information exists (e.g., surficial geology or terrain maps). Grouping of bedrock types used for the CWAP are shown below. Indicate with the appropriate numbers the predominant bedrock type in the watershed in Form 9. If two or more rock type groups occur in relatively equal proportions they can both be listed:
1. Intrusive–granite/granodiorite/diorite/monzonite/syenite/pegmatite
2. Ultramafic–gabbro/pyroxinite/hornblendite/serpentinite/dunite
3. Metamorphic–gneiss
4. Metamorphic–schist/phyllite/amphibolite/slate
5a. Sedimentary (Mesozoic or older)–sandstone/wacke/conglomerate/quartzite
5b. Sedimentary (Cenozoic or younger)–sandstone/wacke/conglomerate
6. Sedimentary–Limestone/dolostone (or dolomite)/marble
7. Sedimentary–siltstone/shale/mudstone
8. Volcanic (andesite or rhyolite)–flows/breccias/porphyries
9 . Volcanic (basalt)–flows/breccias/porphyries/greenstone
10. Volcanic–tuffs/pyroclastics/lahars
- Note:
- The preceding bedrock geology information may be confusing for non-geologists. You may wish to obtain the assistance of an individual with experience in geology.
Interpretation
The rock types are grouped below according to occurrence, similarities in resistance to erosion, weathering, and products of weathering:
- Rock types in groups 1, 3 and 8 are generally resistant to erosion. Colluvium derived from these rock types typically consist of large angular boulders. Soils and tills derived from these rock types are likely to be sandy to silty in texture, with abundant coarse fragments.
- Rock types in groups 2 and 9 contain minerals with high concentrations of iron and magnesium (mafic minerals). Although they are relatively resistant to physical erosion and can produce large blocky boulders of colluvium, they decompose readily through oxidation to form soils with high clay content. Tills derived from rock types of group 2 and 9 are likely to be fine-textured (silt or clay).
- Metamorphic rocks of group 4 (also called metasediments) are relatively erodible due to the presence of abundant parallel planes of weakness (foliation planes) that cut through the rock, and to the presence of micaceous minerals (such as biotite and muscovite) which weather to form clay. Colluvium derived from rock types of group 4 is generally small and platy. Till and soil derived from group 4 rock types are likely to be fine textured (silt to clay) and micaceous.
- Rock types in group 5a are relatively resistant to erosion, particularly where they are of Mesozoic age or older. Colluvium derived from these rock types often consists of large blocky boulders. Soils and tills derived from rocks of group 5a are generally coarse-textured (sandy). Younger sedimentary rocks (group 5b) are often poorly lithified and can be highly erodible. Such rocks are common in most of the Tertiary coal basins of interior B.C. Soils and tills derived from rocks of group 5b are likely to be coarse textured (sandy).
- Limestone, dolostone, and marble (group 6) are composed primarily of calcium carbonate. These rock types are generally resistant to physical erosion, commonly producing large blocky colluvium. However, these rock types are highly susceptible to chemical weathering and break down to produce fine-textured (silty), calcareous soils and tills.
- Group 7 rock types are characteristically fine grained and have abundant parallel planes of weakness (bedding planes) through them. Because of these physical characteristics, these rock types are generally susceptible to erosion. Colluvium produced from these rock types is typically small and platy. Till and soil derived from rocks of group 7 are typically fine textured (silt and clay).
- Rock types of group 10 are generally fine textured, easily erodible and highly susceptible to weathering. Volcanic tuffs, lahars and pyroclastic rocks of Eocene and Miocene age are very common in the Cariboo, Prince George and Kamloops Forest Regions. These volcanic rocks, in particular, are often altered to clay. Soils and tills derived from these rock are also generally clay-rich. Quaternary volcanic rocks of the Garibaldi volcanic belt (Vancouver Forest Region) and the Stikine volcanic belt (Prince Rupert Forest Region) are also included in this group.
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