 At the top of the river there is the Devonian Quartzite (Metamorphic), in the middle of the river there is the Permo-Triassic Marl (Sedimentary) and the lower part of the river consists of Jurassic Limestone (Sedimentary).The first two rock types are impermeable meaning there is little drainage for the river but the river depth becomes smaller as the water flows over the permeable limestone which will allow water to pass through.Quartzite is also very resistant to chemical weathering and can form ridges.