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The Three Cape Supergroups: A Growers Guide to Western Cape Geology
Your soil starts with the rock it came from. Three Supergroups define the Cape:
TABLE MOUNTAIN GROUP (440-400 million years):
- Piekenier: Conglomerate and sandstone
- Graafwater: Purple thin-bedded sandstone (Chapman's Peak)
- Peninsula: Coarse orthoquartzite, 2,000m thick — forms Table Mountain and Tygerberg Hills
- Pakhuis: Glacial mudstones — evidence of ancient ice
- Cederberg: Shale with marine fossils
- Nardouw: Coarse orthoquartzite, feldspathic top
BOKKEVELD GROUP (Devonian, ~400 million years):
Shale-rich marine sediments. Ceres and Traka Sub-Groups. Rich in brachiopods, trilobites, gastropods.
WITTEBERG GROUP (Late Devonian-Carboniferous):
Clay-rich, contains first fish fossils in SA.
What this means for growers:
- Sandstone soil: drains well, needs amending
- Shale soil: holds water and minerals, can be heavy
- Granite soil: the sweet spot — sandy loam, good drainage, high K, slightly acidic
Durbanville has all three. What is your substrate?