Complex sentence: They are the remains of a cap of tertiary sandstone which once covered much of southern England, a dense, hard rock created from sand bound by a silica cement, making it a kind of silicified sandstone.
Split into simpler sentences: They are the remains of a cap of tertiary sandstone which once covered much of southern England. It is a dense, hard rock created from sand bound by a silica cement, making it a kind of silicified sandstone.