Iron oxide
Fins will form typically in sedimentary rocks, mainly sandstone. Sandstone is bedded, which provides zones of weakness, along which weathering and erosion takes place. Iron oxide and calcium carbonate cemented sandstones produce fins. These are typically seen as weak cements, compared to the erosion resistant silica cement, but in arid and semi-arid regions they are much more resistant. This is due to the lack of water in these regions that would otherwise chemically weather iron oxide cements and calcium carbonate cements.