Mudstones and siltstones, as their name implies, were originally very fine-grained muds (particles <0.004 mm across) or silts (0.004-0.063 mm across). They are among the most common type of sedimentary rock and may contain very well preserved fossils. This mudstone at Whitepark Bay is Lower Jurassic in age (about 195 million years ago); the large ammonite shows that it is a marine mudstone.