Final answer:
The size classes used to describe clastic rocks are based on their grain size: gravel, sand, silt, and clay. Grain size provides information about transport and depositional history. Geologists use various methods to determine grain size.
Step-by-step explanation:
The size classes used to describe clastic rocks are based on their grain size. Clastic sedimentary rocks are classified into four main size classes: gravel, sand, silt, and clay. Gravel consists of large fragments called cobbles and pebbles, while sand is made up of smaller grains. Silt and clay are even smaller particles.
The grain size of clastic rocks provides information about the transport and depositional history of the sediment. For example, well-sorted and rounded sand grains typically indicate long-distance transport, while poorly-sorted and angular grains suggest short-distance transport.
Geologists use various methods to determine the grain size of clastic rocks, including visual inspection and grain size analysis techniques such as sieving and sedimentation. These methods help in the classification and description of clastic rocks.