Which type of rock is made mostly out clay?

The type of rock that is made mostly out of clay is called mudstone. Mudstone is a fine-grained sedimentary rock that is composed mainly of clay minerals. It is formed when clay particles are deposited in a body of water and then compacted and cemented together over time. Mudstone is typically soft and crumbly, and it can be various colors, including red, brown, gray, and green.