The type of rock has several factors that influence how easily it is eroded by rivers:
1. Hardness: Softer rocks like sandstone, shale and mudstone erode more easily than hard rocks like granite, basalt and slate. Rivers can carve channels and form canyons more rapidly in soft rocks.
2. Grain size: Rocks with larger grain sizes, like conglomerate, tend to be stronger and more resistant to erosion. Finer-grained rocks are weaker and more easily eroded.
3. Cementation: Strongly cemented rocks, where the mineral grains are firmly bonded together, erode at a slower rate than loosely cemented or unconsolidated rocks. Limestone is an example of a cemented rock, while shale has loosely bound mineral grains.
4. Fracturing: Rocks that are highly fractured, jointed or faulted erode more quickly than massive unfractured rocks. Water can penetrate deep into cracked rocks, accelerating erosion.
5. Composition: Quartz-rich rocks like sandstone and quartzite tend to be more resistant to weathering and erosion. Limestone, dolomite and other carbonate rocks erode very easily due to dissolution in acidic water. Clay-rich rocks are also weak and prone to erosion.
6. Layering: Rocks with easily erodible layers, like shale, sandstone and limestone, erode faster than massive homogenous rocks. Rivers can work their way down along layer boundaries.
7. Weathering: Pre-existing weathering and damage makes rocks more susceptible to erosion. Fresh, unweathered rocks will naturally resist erosion better than weathered, altered rocks.
All of these factors ultimately determine how fast or slowly a river can erode a given type of rock. Harder, finer-grained, weakly cemented rocks will erode quickly, while softer, coarser-grained, strongly cemented rocks can resist erosion far more effectively.