Feat her stonhaugh s Geological Report. 117 



were reduced to their present level, the mud they brought 

 down was deposited at the point of their confluence ; this, since 

 the lowering of the streams, has become the triangular alluvial 

 deposite where Pittsburg is built, and which now occupies 

 almost the whole area. From this place the country descends 

 parallel with the Ohio river, whose banks of alternate sandstone, 

 shale, and limestone, are from 350 to 450 feet high, to Beaver, 

 in Ohio, where, in the valley of the Big Beaver river, several 

 coal veins are observed. The country now rises to Ravenna, 

 the summit-level of this part of Ohio, about 1,140 feet above 

 tide-water. Boulders and gravel of primary rocks are for the 

 first time found on the route here, and continue to increase in 

 proceeding westward to Cleaveland, Ohio, upon Lake Erie, 

 sixty-four miles distant. Here we have the evidence of a 

 lowering of level of the Western waters, a low rich alluvial 

 flat extending from the banks of the lake, about fifty feet 

 high, three miles east, to its ancient border. The same ap 

 pearance presents itself in various parts of the shore of this 

 lake, as well as on the shores of Lake Ontario. At Sandusky 

 regular beds of the carboniferous limestone, with its usual 

 fossils, are found. On this great level the formations change 

 no more for an immense distance in the line of my route. 

 On the approach to Detroit nothing is to be seen but a low 

 sedgy shore to the west, and a flat country to the east, con 

 sisting of sand and clay, without any sensible inequality of 

 surface, being the old lacustrine deposite, when the whole of 

 this region formed one large lake. This appears to have be 

 come dry land at the lowering of the level of the waters of 

 this continent, more than once already alluded to. At Fort 

 Gratiot, seventy-five miles from Detroit, finding some anodon- 

 tas on the shore of the St. Clair river, I had the curiosity to 

 dig into the sides and bottom of the bank of the river, about 

 thirty-feet high, where I found great quantities of unios, ana- 

 dontas, and numerous fresh-water shells enclosed in the clay ; 

 those near the level of the water were quite soft, but indu- 



