Page:The Journal of geology (IA journalofgeology11893univ).pdf/151

some length, in a recent paper.^[1] It need, therefore, be but briefly touched upon here. It is there shown that it forms a practically continuous sheet over the southern portion of the glaciated district in Ohio and Indiana, and extends to an undetermined distance over the unglaciated districts of Ohio. It also appears on the uplands in Kentucky, south-west of Cincinnati, and may appear further east in that state. It has been found along the margin of the glaciated district in Ohio as far north-east as the vicinity of Newark, but has not been observed farther north and east.

The thickness of this silt decreases in passing southward, especially on the interfluvial tracts of south-western Ohio and south-eastern Indiana, a fact which seems to indicate that its source was from the north rather than from flooded conditions of the Ohio river. From evidence gathered in the upper Mississippi region, it is thought to be the correlative of a sheet of glacial drift not exposed to view in these states, or at least not yet discovered. Its thickness in the northern part of the district, next to the outer moraine of the newer drift, is four to six feet or more while on the borders of the Ohio river it scarcely exceeds three feet, and in places is two feet or less. Wherever examined it is found to be thoroughly leached. This fact is thought to be of importance in showing great age, especially on the theory of the glacial origin of the silt, since glacial silts, as well as till, in regions underlain as this region is by limestone, contain a large amount of calcareous material.

The amount of depression involved in this subsidence is difficult to determine. As yet such data as have been discovered bearing upon the altitude of the land, either previous to or during the depression, are not precise, though it seems probable that the altitude was several hundred feet lower at the maximum of depression than at the present time, while before the depression the drainage appears to have been good, and we may suppose that the altitude was not much lower than at the present time.