Siltstone is a sedimentary rock that is very similar to mudstone, except that at least two thirds of the quartz grains from which it is comprised are relatively coarse. These vary between 4 and 6.25 µm in diameter, making it part way between a mudstone and a sandstone. It also has very small pore sizes, which is another good field indicator. Siltstones are typically formed in medium depth marine environments taht are deeper than those where sand is formed, but shallower than the ocean floor where fine clays are deposited.