SANDSTONE is a clastic sedimentary rock consisting of sand-sized grains
(one sixty-fourth to one-sixteenth inch in diameter) held together by a
cementing material. As sandstones become finer grained they grade into
siltstones; as they become coarser grained they grade into conglomerate.
The shape of sand grains in sandstones ranges from rounded to angular.
Quartz is the dominant mineral in sandstone, but other rock grains and
mineral grains (especially chalcedony, feldspar, muscovite, hornblende,
magnetite, or garnet) generally are present.
Sandstones are commonly cemented by carbonates, silica, iron oxides, or
clays. Most sandstones are a shade of gray or brown, but the color may
vary from gray or white to yellow, brown, or red. The color probably
depends on the type of cement, the amount of organic material present,
and the amount and degree of oxidation of iron in the rock.
Durability of sandstones depends largely on the character of the cement.
Some sandstones crumble easily, but others, especially those cemented by
iron oxides or silica, are tough and durable. Sandstones break around
the grains, giving the broken surface a granular appearance.
Sandstone crops out in many places throughout much of the state. In
LaSalle and Ogle Counties, large tonnages of sand are mined from the St.
Peter Sandstone and sold for a variety of uses, including abrasive sand,
molding sand, and sand for making glass. In extreme southern Illinois
attractively colored sandstones have been quarried for building stone.