Rock of the week 3 – Sedimentary rocks – Mudstone First of all, what’s a sedimentary rock? A sedimentary rock is made up of sediments that have accumulated in a ‘basin’ over geological time to form a rock. These can be made up of clay, silt, sand, gravel, cobble or even boulder sized particles. What’s mudstone? Mudstone is a type of fine-grained sedimentary rock primarily composed of clay- and silt-sized particles. It can be dark, grey, dark grey, brown, green, purple and even red depending on t