Page:The Journal of geology (IA journalofgeology11893univ).pdf/150

being not more than thirty to forty feet. The outer moraine of the East White River lobe, after following an upland tract west of the West Whitewater northward for some distance, descends, near Cambridge City, into the valley of the West fork of West Whitewater, and after crossing this valley rises near Hagerstown onto elevated upland. The outer moraine of the Miami lobe also, in crossing Noland's fork, south-east of Cambridge City, descends into and is developed in the valley as well as on the bordering ridges. That the valleys were formed previous to the deposition of this moraine, there can be no doubt, and being made entirely in the drift, as noted above, they show clearly that their excavation must be confined to the interval between the deposition of the earliest drift sheet and that of this moraine. The amount of erosion is several times as great as that accomplished by the streams that have traversed this valley since the moraine was laid down. The size of the streams which formed them constitutes an important factor in determining the time required in this excavation. That these interglacial streams were not much larger than those now traversing this region, seems probable from the fact that within a few miles north from the sources of the present streams, the general slope of the country becomes northward, so that drainage would naturally be in that direction, instead of southward along the Whitewater. The erosion here displayed seems, therefore, to indicate the lapse of a longer interval between the deposition of the earliest drift sheet and that of the outer moraine of the later drift, than the time that has elapsed since the formation of that moraine. How much of this interval preceded the silt deposition, it is difficult to determine, because the outer moraine has concealed the silt. Light upon this question should be obtained upon careful study of the lower portion of the Whitewater valley and of other valleys lying within the silt-covered district and outside the moraine, but this line of study has not yet been undertaken.

Depression accompanied by silt deposition. The silt which is found on the upland outside this moraine, has been discussed at