DETRITAL ROCKS -- GRAVEL
Detrital rocks made of gravel-sized particles (> 2 mm, but often with large pebbles, cobbles, and boulders) indicate a transporting mechanism with sufficient energy to move such large grains. Not many depositional environments are fed by such strong transport systems -- glacial ice, very swift rivers, and gravity are the most common culprits.
When studying gravels we find it easy to determine the rough distance that the particles have been transported because such large particles hit each other, and hit other objects, with such force that their corners tend to round off very quickly. Detrital rocks made of gravel particles with sharp corners therefore indicate a very short transport distance from the weathering site. Such rocks are named BRECCIA, and the photograph shows a good example. Notice the angular corners of the grain indicated by the red arrows, as well as other grains. (The coin is 1 inch or 26 mm across.)
Even very short distances of transport begin to round the edges and corners of gravel, so most rocks of this grain size have rounded grains instead of angular ones. Such rocks are called CONGLOMERATE. The next photograph shows an example.
