About 450,000,000 years ago, a shallow ocean covered Illinois. Its waves
and currents carried clean white sand and deposited it as curving
beaches, sand bars, and dunes. This sand differed from many sands in
that it was composed almost exclusively of grains of the mineral quartz
instead of being a mixture of quartz and other minerals.
Quartz is composed of silica (SiO₂), and sands such as the ancient
Illinois sand that are composed of quartz are known as silica sands.
Quartz is very hard and will scratch glass and some steel. Perfect
quartz crystals, which are rare, are longer than they are thick and end
in pyramids. Probably not many grains of the ancient sand came from
perfect crystals; they more likely resulted from the decaying and
breaking down of rocks such as granite, which are mixtures of quartz
grains and other mineral particles.
The quartz grains probably did not come directly from their source to
Illinois. Instead, it is likely they first were deposited elsewhere and
formed into sandstone. That sandstone was subsequently broken down by
weathering agents and the grains transported to the ancient Illinois sea
by streams.
As a result of the erosive action of the agents that transported them,
many of the originally angular grains, particularly the coarser ones,
were rounded and their surfaces dulled like that of frosted glass (fig.
13). Consequently, they appear white, although they actually are
colorless.
Since the ancient sea deposited its silica sand, other seas have covered
Illinois at various times and each has left deposits of sand, mud, or
limy materials. The silica sand thus was buried by hundreds of feet of
other sediments and became sandstone. This sandstone is called the St.
Peter Sandstone. It is named from the St. Peter River, now the Minnesota
River, in Minnesota where the sandstone was first described and named by
geologists. The overlying deposits also were consolidated into rock.
St. Peter Sandstone is exposed at the surface at many places in northern
Illinois and in one small area in the western part of the state. The
sandstone exposed in northern Illinois generally varies from 125 to 300
feet thick. The fact that it crops out at the surface indicates that the
materials that formerly covered it have been removed.
The uncovering was not a single, simple event but rather a series of
events that took place at various times during the many years since the
St. Peter sand was deposited. Among these was the up-bowing of the rocks
of central northern Illinois into a broad arch. Streams then began to
cut across the arched rock, slowly but persistently stripping away the
top layers until the core of the arch was laid bare. Among the rocks
thus exposed was the St. Peter Sandstone, which may be seen in northern
Illinois in the valleys and tributaries of the Rock River near Dixon and
Oregon and along the Illinois and Fox Rivers and some of their
tributaries near Ottawa, Wedron, Millington, and Troy Grove. The St.
Peter Sandstone at Starved Rock and Matthiessen state parks near LaSalle
and along the highway between Dixon and Oregon is eroded into scenic
bluffs and canyons.
[Illustration: Figure 13—Enlarged photograph of St. Peter sand
showing the rounded and frosted character of the grains.]