Silurian rocks.—During the Silurian period the sea of the Illinois region was an arm of the Gulf of Mexico, which shifted back and forth across the state. The most extensive outcrops of Silurian rocks arc two broad belts in the eastern and western parts of northern Illinois separated by a broad area of Ordo vician rocks in the north-central part of the state. The Niagara limestone is the principal rock formation of the Silurian system in Illinois. The Chicago Drainage Canal which extends from Chicago to Lockport, a distance of 2S miles, is cut in the Niagara limestone in much of its lower course. The traveler on the railroad running parallel with the canal may see a long ridge of rock fragments which have been excavated from the canal. The Niagara formation furnishes limestone for building, furnace flux, concrete, and road-making. Many quarries are found in Chicago and in the vicinity of Joliet.
Devonian rocks.—During much of the Devonian period, the Illinois region was largely dry land. The chief regions of Devonian rocks are in detached areas in several counties along the Mississippi River, including Rock Island, Calhoun, Jersey, Jackson, Union, and Alexander counties.
Mississippian rocks.—The Mississippian rocks are exposed in Illinois in a very long and relatively narrow belt extending from Mercer County southward along the Mississippi Valley to the Ozarks, then eastward to the Ohio River along the crest of the Ozark Ridge of Illinois. These rocks include sandstones, shales, and limestones. They furnish materials for building, lime, concrete, riprap, and Portland cement. A large Portland cement plant has been recently erected at Golconda, Pope County, where raw materials are furnished in abundance from the high bluffs of Missis sippian rocks overlooking the Ohio River. Veins of fluor spar, lead, and zinc occur in Mississip pian rocks of Hardin and Pope counties.
Pennsylvanian rocks. —The soil alone is the only resource of greater value to Illinois than the 240,000,000,000 tons of coal locked in the Penn sylvanian rocks or "Coal Measures" of the state.
This is a larger coal re serve than that held by any other state east of the Mississippi River. In addition to the coal, this system of rocks is rich in petroleum, building stone, and materials for Portland cement, building and paving brick, sewer pipe, pottery, and tile.
Pennsylvanian rocks consist of sandstones, limestones, shales, and thick layers of coal. They form the surface rocks of about three-fifths of the state, lying south of a line drawn from Rock Island to Joliet. After the close of the Pennsyl
vanian period, only the extreme southern counties were ever again beneath the sea.
The State Geological Survey has recognized sixteen differ ent coal seams, varying in thickness from one to nineteen feet, separated by layers of shale, sandstone, and limestone. The total thickness of the rocks of the Pennsylvanian system in Illinois is not less than 1,200 feet. Only a small fraction of the entire mass of the system consists of coal.
The Permian period.—The Permian period in the United States was accompanied by great land movements. The Appalachian Mountains to the east and the Ozark Plateau to the west were uplifted. The La Salle anticline in Illinois was further elevated, and the Ozark Ridge of southern Illinois, extending eastward from Union County to the Ohio River, was raised as a spur to the Ozark Dome of Missouri.
Molten rock moved upward through fissures in Pope, Hardin, and adjoining counties, and, if it did not overflow, it reached an elevation so high that subsequent erosion has exposed it in dikes. The numerous faults and small folds of southern Illinois were probably formed at this time. Beneath some of these low arches and domes, in the porous strata that are overlain by dense, impervious layers, the oil and gas deposits of Illinois have accumulated. While these folds were made during Permian time, the rocks that were folded belong to an earlier period, Pennsylvanian or Mississippian.
Cretaceous and Tertiary time.—The Illinois region has been dry land since the close of the Pennsylvanian period. During the Cretaceous period, in the latter part of the Mesozoic era, and during Tertiary time, in the early part of the Cenozoic era, a narrow strip of southern Illinois along the Ohio River in Pulaski, Massac, and Pope counties was submerged and received sediments of clay, sand, and gravel.
The Glacial period.—The latest event that greatly changed the surface of Illinois was the slow movement of enormous ice sheets over portions of the state at different times during the Glacial period, the last period of the Cenozoic era. The glacial drift covers the solid rock with a thick layer of mantle rock. The work of glaciation is discussed in some detail in the next chapter.
The geology of Illinois is strikingly presented in the Geologic Map of Illinois, published by the State Geological Survey, Urbana, Illinois.