Shale, which is the most common form of sedimentary rock, is a mineral that started out as mud or clay and was then compacted by the weight of rock above it. It typically results from the deposition of sediments in marine or freshwater environments where the water is still or very slow moving. They are fissile materials — this means that they split easily into layers. Where the rock is very dark or black, such as is common with Paleozoic and Mesozoic rocks, it is an indicator that the formation conditions were anoxic, such as those found in stagnant pools. Some shale strata contains well preserved fossils, and when good specimens are found, they can be sought after by collectors.