THE COLCHESTER AND MACOMB rivers. Eastern Iowa lies chiefly in the basins of Turkey; there have been many shifts from deposition to erosion and Maquoketa, Wapsipinicon, Iowa, Skunk, and Des Moines from erosion to deposition. GENERAL rivers, all tributaries of the Mississippi. The pre-Cambrian formations are made up chiefly of igneous The Colchester and M£eomb qua^r^ggles are bounded by The average annual discharge of the Mississippi just above and metamorphic rocks having a complex structure. They parallels 40° 15' and 40° 30' and.^fi^dians-90'° 30' and 91° the mouth of the Missouri is about 125,000 second-feet and outcrop in the St. Francis Mountains of southeastern Missouri (and have a combined area of 455 square miles. They are in that of the Missouri at its mouth is about 100,000 second-feet.' and in parts of Wisconsin, Michigan, Minnesota, and Iowa, western Illinois (see fig. 1) and include parts of MeDonough, Computations based on the amount of mineral matter carried and doubtless underlie the entire province. Upon their deeply Hancock, and Schuyler counties. Macomb, the principal town by the upper Mississippi and on the size of its drainage basin eroded and planated surface rest all later formations the in the area, is in the northwestern part of. the shales, sandstones, limestones, and the relatively recent uncon­ quadrangle. solidated rocks. Lower and Middle Cambrian time seems not to be represented by strata in most of the region, but Upper Cambrian beds probably lie at or near the base of the unmetamorphosed sedi­ mentary rocks in a large part of it. The Upper Cambrian series is generally more than 1,000 feet thick and consists principally of sandstone and shale, limestone forming only a subordinate part of the whole. The Ordovician system consists largely of dolomite and limestone but generally includes a sandstone formation, the St. Peter, in its lower half and considerable shale near its top. SPKI J.acksonvill The Silurian system is made up chiefly of limestone and dolo­ «>RGAN mite and in places is thin or absent. The Devonian system in this general area is commonly thin and represented chiefly by limestone deposits; like the Silurian it is in places very thin or is absent. The Mississippian series of the Carboniferous system con­ sists of some shale at the base and of thick limestones and a little interbedded shale above. The highest group in the series, the Chester, includes considerable sandstone but is probably not present in northern and central Illinois and neighboring areas. The Pennsylvanian series of the Carbon­ FIGURE 1. Index map of central Illinois and parts of adjacent States. iferous system consists of interbedded shale and sandstone, The location of the Colchester and Macomb quadrangles (No. 208) is shown by the darker ruling. some limestone, and many coal beds. It is particularly impor­ Published folios describing other quadrangles, indicated by lighter ruling, are Tallula-Spring- fleld (188), Belleville-Breese (195). tant because it contains the only workable coal deposits in the central United' States, where it now lies in separate basins one occupying most of Illinois and part of western Indiana TOPOGRAPHY. « ^ and Kentucky; another most of southern Iowa, northern Mis­ In their general geographic and geologic relations the quad­ souri, and parts of Kansas, Oklahoma, Arkansas, and Texas; rangles form a part of the Glaciated Plains, a province char­ and a third a part of southern Michigan. acterized by a surface formed .largely of unconsolidated glacial There are no extensive Cretaceous deposits in or near drift left by the great ice sheets that invaded the region in the Illinois, and the only representatives of the Tertiary are bodies Pleistocene epoch. These plains lie between the Appalachian of gravel and sand scattered here and there on the divides. province on the southeast, the Ozark province on the south­ The Quaternary system covers a very large area. It con­ west, and the Great Plains on the west, and extend north­ DEVONIAN SILURIAN MIDDLEftUPPER CAMBRIAN Q LOWER PRE-CAMBRIAN sists of glacial, eolian, lacustrine, and fluviatile»deposits, almost ORDOVIC1AN ORDOVICIAN ward beyond the boundary of the United States. (See fig. 2.) wholly unconsolidated, which mantle the Glaciated Plains, Broadly considered, they include most of the United States FIGURE 2. Geologic map of Illinois and surrounding regions. Shows also effectually concealing the indurated rocks, except in some of physiographic provinces. north of Ohio and Missouri rivers, and their southern bound­ The indefinite boundary between the Ozark and. Appalachian provinces coincides approximately the valleys and in a few districts where outcrops of the older with the southeast boundary of Illinois. Geologic map copied from maps of North America rocks are plentiful. ary is the southern limit of glaciation. in U. S. Geol. Survey Prof. Paper 71,1911. Even before glaciation northern and central Illinois and Structure. The structure of most of the region is compara­ neighboring areas were not without many striking topographic show that the basin is being lowered by erosion at an average tively simple. Few complicated folds or faults lie at or near features. Glaciation reduced the relief, for it filled the valleys rate of 1 inch in about 1,000 years, and this is probably close its surface except in the southern tip of Illinois, where there with till and wholly or partly obliterated them. After each to the average rate in western Illinois and neighboring areas. are a number of east-west folds, which pass into faults on the ice invasion the drainage was forced to adapt itself to new In proportion to the area drained the Missouri carries a much east. The principal structural features of the region are the conditions and to begin the excavation of many new valleys larger amount of mineral matter but derives most of it from the following: on the poorly drained plains. Large parts of areas reached by western part of its basin. 1. A basin that occupies southwestern Indiana and most of comparatively recent ice sheets are still imperfectly drained and Illinois. It is shaped much like the bowl of a spoon, its deep­ CLIMATE AND VEGETATION. contain broad, flat divides that }ie between shallow valleys. est parts being along an axis extending from the southwest These features may be seen in most of Illinois, including the The climate of the Colchester and Macomb quadrangles and corner of Indiana to north-central Illinois. Colchester and Macomb quadrangles. Southern Iowa and neighboring areas is of the continental type the type char­ 2. A still broader basin that occupies southern Iowa, northern Missouri were reached by the earlier glaciers only, acteristic of the interiors of continents in the Temperate Zone. northern Missouri, and neighboring areas on the west and so that erosion has had sufficient time to excavate many val­ The temperature is below or just above the freezing point southwest. In southern Iowa the beds dip southwest toward leys and to narrow the divides. An area of about 10,000 during the greater part of winter, and is uncomfortably hot its center and in northern Missouri west and northwest. square miles in the northwest corner of Illinois and adjacent during short periods in summer. The chief farm crop is corn, 3. A broad arch that affects Wisconsin and Minnesota. parts of Wisconsin and Iowa was not glaciated and lacks some but wheat, oats, rye, and similar crops can be raised easily, 4. A dome in the Ozark region that passes northward into a of the characteristics of neighboring glaciated sections, though and grasses of various types grow rapidly and fairly luxuri­ low, broad anticline. This anticline continues to southeastern it is considered part of the Glaciated Plains because much of antly. The rainfall is nearly 40 inches a year; more than Iowa, the axis lying close to the Mississippi, and separates the its history is similar to that of surrounding territory. half of which falls during the growing season, supplying ample two basins just mentioned. The general altitude of the Glaciated Plains is 500 t6 1,500 moisture except during unusual droughts. Most of the upland Each of the two basins contains a great coal field, and these feet above sea level, but the local relief in few places exceeds and part of the alluvial bottom land is open prairie under fields are known respectively as the eastern and western regions 400 feet and is commonly much less, especially in most of cultivation. The woodland tracts are confined mainly to the of the Interior province. Around each basin the strata crop Illinois. The valley of Mississippi River is one of the prin­ sides of the valleys and, though not extensive, are valuable for out in concentric belts, the youngest forming the inner belts cipal features of the region ; for the stream flows between flat local fuel supplies, fence posts, and similar purposes. and the oldest the outer border. Beds that lie 1,000 feet above bottom lands that are generally 3 to 6 miles wide and are sea level in northern Illinois are more than 3,000 feet below GEOLOGY. bounded by bluffs 200 to 400 feet high. , sea level in the south-central part of that State, and if all A large part of Illinois is drained by Bock, Illinois, Kas- Stratigraphy. The rocks underlying Illinois and neighbor­ the beds found in Illinois were extended to the north the kaskia, and Big Muddy rivers, which flow southwestward to the ing areas are of many varieties and range in age from upper ones would be several thousand feet above sea level Mississippi. Northern Missouri is drained by tributaries of pre-Cambrian to Recent.