22 GEOLOGY OF OHIO. 



Logan upon the same formation where conspicuously exposed on the 

 north shore of Lake Erie. 



The Erie clay immediately underlies the surface over a large part of 

 the northern half of the State. This is especially true of the counties 

 included in the Western Eeserve, and has given them the clay soil 

 which makes them the great dairy district of the West. In the north- 

 western portion of the State the Erie clay is very thick, frequently 100 

 feet, and continuous, but it is more generally covered with lacustrine 

 deposits than on the Reserve. In several of the north-western counties 

 it has been pierced by numerous wells, sunk to obtain water, and its 

 thickness and structure have been by this means very clearly revealed. 

 It is here found to have a thickness of 100 to 150 feet, to contain irregu- 

 lar sheets of sand, gravel, and bowlders, which are water-bearing, and 

 the source from which the artesian wells of this region are supplied with 

 their flow of water. It should also be said, that in this part of the State 

 the Erie clay contains more and larger bowlders than farther south. 

 These are, for the most part, fragments of crystalline rock, which have 

 come from the far north, with many others derived from the Silurian 

 and Devonian limestone, which outcrop in the lake region, north of 

 Ohio, but south and west of the Laurentian belt. Among the bowlders 

 contained in the Erie clay in this section, rolled masses of coal are not 

 unfrequently met with, and some of these, struck in boring, have given 

 rise to much hope of finding coal in the vicinity. It is hardly necessary 

 to say that such hopes will be fallacious, for this coal has undoubtedly 

 come from the coal-field of Michigan. In the Maumee valley the upper 

 portion of the Erie clay is often laminated, and its color is yellow where 

 exposed to atmospheric action. 



In the more easterly of the northern counties the Erie clay is gener- 

 ally thickly set with small fragments of shale, evidently derived from 

 the Huron and Erie shales excavated to form the basin of Lake Erie. On 

 weathered surfaces these fragments are exposed in great numbers, and the 

 clay is rendered yellow or brown by the oxidation of its iron. In recent 

 sections this change of colors is found to follow down all cracks in the 

 clay as far as atmospheric water penetrates, and where such joints are 

 numerous it is divided into irregular blocks, of which the central por- 

 tion will be blue or gray, the exterior brown. The best development 

 of the Erie clay in the northern part of the State is found in the old 

 valley of the Cuyahoga, which it fills from the bottom to a point some 

 60 feet above the lake-level, giving a total thickness of 280 feet. It 

 covers the highlands adjoining, however, rising to the height of 400 to 

 500 feet above the lake. It there has a thickness of from 10 to 30 feet. 



