The Cambrian system of rocks is at the base of the Paleo zoic group of rocks in Illinois. Cambrian rocks are not found at the surface in the state, but they have been entered by numerous deep wells, none of which has penetrated through the system.
Small areas of southcn Illinois con tain rock materials formed in the Mesozoic era, a later era than the Paleo zoic, and all the extensive glacial de posits of the state were formed during the Cenozoic era, which is later than Mesozoic and includes the present.
Folds and faults.—The rock layers in Illinois approach the horizontal in general position, but at places they form definite folds, show slight dips from the horizontal, or present faults of con siderable magnitude. The best-known rock fold in Illinois is the La Salle anti cline which enters the state from Wis consin into Stephenson County and extends in a southeasterly direction across the state. It crosses Rock River at Grand Detour in Lee County, and the Illinois River at Split Rock between La Salle and Utica. Farther south the La Salle anticline disappears at the surface, but it seems to continue as an underground structure, and in the main oil fields of southeastern Illinois it forms the rock structure necessary for accu mutations of oil, at a depth of several hundred feet below the surface.
The eastern interior coal field, occupying about three-fifths of Illinois and portions of Indiana and Kentucky, is a broad, shallow, synclinal fold dipping imperceptibly from all sides toward the center.
Faults, formed by the breaking and slipping of rock layers, are found in western and southern parts of the state. A fault crosses the Mississippi and Illinois rivers in Calhoun and Jersey coun ties. The slipping of the rocks along this fault amounts to a vertical displacement of 720 feet. Some faults occur in Pope County, and they are extremely numerous in Hardin County where the fluor spar mines are closely associated with the faults. Other folds and faults than those men tioned are found in the state.
Cambrian cocks.— During the Cambrian period, the sea covered Illinois, and great quan tities of sand were washed from the sur rounding lands into the sea. The rock formed
from this thick layer of sand is known as the Potsdam sand stone. It underlies the entire state. It is not found at the surface in Illinois, but outcrops in Wisconsin. It forms a reservoir for water, and the deep wells of the northern part of the state which penetrate it have an abundant supply of water.
Ordovician rocks.—During the early part of the Ordovician period, the seas which covered Illinois furnished conditions favorable for the growth of animals having shells of lime. The shells were so abundant that they formed thick beds of limestone known as the Prairie du Chien group or the "Lower Magnesian limestone." These are the oldest rocks exposed in Illinois. They are brought to the surface along the La Salle anticline in the vicinity of Rock and Illinois rivers. These rocks have long been used at Utica, La Salle County, for making cement.
After the deposition of the Prairie du Chien group of rocks, the Illinois region was uplifted and became dry land. Later, the sea again occupied the region, and great quantities of sand, washed in from surrounding lands, formed the St. Peter sand stone. St. Peter sandstone forms the bluffs of the Illinois and Fox rivers in La Salle and Kendall counties, the bluffs of Rock River in Lee and Ogle counties, and it is found on the north side of the fault line in Calhoun County. It furnishes sand for glass-making, molding, and building. The Federal Plate Glass Works at Ottawa are located on St. Peter sand stone which may be used as the chief raw mate rial for the factory. The St. Peter sandstone is a great reservoir of water for artesian wells.
After the St. Peter sands were deposited, conditions became favor able again for the growth of shell-forming animals.
The Galena and Platte ville limestones, formed at this time, are the surface rocks of north central and northwestern Illinois, extending into Wisconsin. Veins of lead and zinc ores occur in these rocks in Jo Daviess and Stephenson counties in Illinois, and in southwestern Wis consin. The lead and zinc mines of this formation were of great importance prior to the opening of richer deposits in the Joplin, Missouri, district.