Geological features of Van Stadens are mostly of the arenaceous succession overlying the Kaan Formation and previously designated “upper pre-Cape” has since been assigned to the Sardinia Bay
Formation of the Table Mountain Group. The remainder, named Van Stadens Formation, comprises medium- to dark-grey cross-bedded quartzite, lenses of polymictic small-pebble conglomerate and subordinate phyllite Felspar is rather common in the two first-mentioned lithologies.
The geology of the Van Stadens catchments is primarily derived from rock of the mid-Palaeozoic Era that formed the Table Mountain Group of the Cape Supergroup.
The upper to middle catchment areas are characterised by high gradients indicative of steep gorges.
Along the river, nutrient concentrations are naturally low. This is a result of the steep topography that prevents human activity like farming which would disturb the natural processes.The Van Stadens Wild Flower Reserve situated upstream and this nature conservancy has kept the level of nutrient input low.
The Van Stadens River estuary is 0.52 km2 when the river mouth is closed and the water level is at maximum height.
Marine gravels, on marine terrace, lying in gullied surface of Table Mountain quartzites. The cobbles and pebbles have been beautifully rounded and sorted by wave action through a number of cycles, starting some 30 million years ago when the sea was right up to the mountain.
Geology groups found in the reserve are: Peninsula, Nanaga, Sardinia bay.