ABSTRACTS 



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GEOLOGY. — Geology and mineral resources of the Peoria quadrangle, 

 Illinois. J. A. Udden. U. S. Geological Survej^ Bulletin 506. Pp. 

 103, wdth maps, views, and sections. 1912.^ 



The surface of most of the Peoria quadrangle is formed by the upland 

 divided by the allu\'ial-filled valley of the Illinois River. The major 

 part of this upland consists of flat interstream areas which mark a plain 

 in most cases dipping gently to the south. Numerous water courses 

 have dissected the upland, especially in that part l>ing northwest of 

 the Illinois River. Some of these valley cuttings have reached bed 

 rock, which is of Pennsylvanian age. In this series two conformable for- 

 mations are recognized, a lower consisting of shales, sandstones, and coal 

 beds, and an upper made up chiefly of limestone. These are the only 

 hard rock formations exposed in the quadrangle, but well borings have 

 penetrated older strata to a depth of 1.300 feet. By these borings the 

 presence of Ordo\'ician, Silurian, Devonian, and Mississippian formations 

 have been revealed. The oldest formation penetrated is the St. Peter 

 sandstone (Ordovician) . 



During the greater part of the Tertiary period the Peoria quadrangle 

 was probably a land area. Some gravel which overlies bedrock under 

 the glacial drift at one locality may represent a residuum of surficial de- 

 posits of early Pleistocene or late Tertiary time, preceding the glacial 

 epoch. 



Erosion in preglacial time had produced a land surface which cUffered 

 considerably from the present topography. Since that time the land 

 has been rebuilt by glaciation, the general effect of which has been to 

 reduce the relief. The old land surface has undergone some changes by 

 postglacial agencies, but these are small. 



» See also, this Journal, 2 : 440. 1912. 



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