The finding of large bowlders in fields are the deposit of icebergs that once floated over our country. The glaziers reached as far as Cincinnati, planing, grinding down, smoothing all rock surfaces, and excavating the basins of our great lakes. The retreat of the glaciers left clay and bowlders and a great inland sea of fresh water, filling basins, before occupied by ice, 500 feet above the present surface of Lake Erie. At a later period, by continental elevation or the removal of barriers to drainage, the water level was gradually depressed until the inland sea was reduced to the comparative insignificance of our great lakes.
The descent of the water by motion of the waves, cut the well-marked terraces and edges.
The term, Cincinnati Group, is now applied to the blue limestone series, and is an equivalent, in the geological nomenclature, to the Hudson Group of New York.
Its thickness is estimated at 1,000 feet. The line of upheaval passes from the south line of Tennessee, with a direction a little east of north, through Cincinnati to Lake Erie. Throughout its whole length the strata are raised in a distinct arch, from which they dip away, on the one side under the Alleghany coal-field, on the other beneath the coal basin of Indiana and Illinois.
The bearing of this axis of elevation is nearly parallel with that of the folds of the Alleghanies; that the date of its upheaval was subsequent to the carboniferous, and anterior to the Triassic period. The line, north of Cincinnati, extends from the Ohio River in a direction a little east of north to the lake shore, between Sandusky and Toledo. In consequence of the erosion, which all the region bordering the Cincinnati Arch has suffered, the line of the axis presents no conspicuous topographical features. About Cincinnati the summit of the arch has been much more deeply and extensively removed than farther north, yet this is still higher than its northern prolongation.
There is every reason to believe, therefore, that this was originally the highest part of that portion of the arch within Ohio, and, in common with the Blue Grass district of Kentucky, the blue limestone area about Cincinnati is the most elevated portion of the ridge; that which has been the longest above the sea level and suffered most from surface erosion. We find here a line or tract from which the strata dipped on both sides in opposite directions.