Sandstones (5XX 6XX) and Pebbly Sandstones (7XX)
Sandstones are composed of grains easily visible to the naked eye. They are generally lighter in color compared to finer-grained rocks and not as heavy. Pebbly sandstones and conglomerates have a sandstone matrix supporting distinctly larger rounded or irregular pebbles of other minerals or rocks, including coal.
Some gray or reddish-brown sandstones are especially hard and will effervesce when HCl is applied directly to the rock or to a powder created by scratching the core with a knife blade. Proceed to Carbonate Cements to further classify these rocks.
Step 2: Determing the mineral composition (second digit)
Sandstones and pebbly sandstones without carbonate element are classified according to the mineral composition of the grains, and this is recorded in the second digit of the rock code. Go to the Sand Composition page to learn about sandstone composition.