Pleistocene Papers at the Malison Meetings. 179 
poraneous with adjacent moraines on the north, were then 
formed hy the filling of the valleys again to their hight and 
by subsequent stream channelling. The valley drift on the 
Beaver river in Pennsylvania and on Clark's creek, a tribu- 
tary of that river from the west about nine miles above its 
mouth, was described as supporting this view of the pregla- 
cial erosion of the whole depth of the rock valleys. In the 
Delaware valley the same history is thought to be recogniz- 
able, the great length of time needed for the rock erosion hav- 
ing been preglacial. 
Changes of Drainage in Rock River Basin in Illinois. Bv 
Fkank Leverett, Chicago, 111. The Rock river, in its course 
as the chief line of drainage in northwestern Illinois, lies 
wholly on the area of the earlier drift. Being turned out of 
its preglacial course by the drift deposits, it has in numerous 
places along its new route cut 50 to 125 feet into the rock 
formations. A computation of the volume of rock so eroded 
shows it to be equal approximately to a mass one mile square 
and 1,095 feet high ; and this erosion was accomplished be- 
fore the time of the Kettle moraine of Wisconsin, whose over- 
wash gravels run along the bottom of the valley and form 
terraces 50 feet above the river. Since the deposition of 
these gravels contemporaneous with the later drift, the river, 
which has a width of about 500 feet and descends about ten 
inches per mile, has removed a portion of them equal approx- 
imately to a mile square and 657 feet high. The large 
amount of cutting in the hard rock, and the smaller erosion of 
the soft gravel and sand, are considered as measures of the 
relative duration of the time subsequent to the deposition 
respectively of the earlier and the later drift. 
In discussion of the two foregoing papers. Prof. Ciiamhek- 
i.ix spoke of loops, called ox-hows, at the side of the Ohio val- 
ley, where a part of the ancient winding course of the valley 
has been cut down and shows the highest drift gravels lying 
on the rock of the old river bed high above the present channel. 
Prof. Salisbi in considered the relations of the upper early 
terrace gravels in the Delaware valley, referred to the Colum- 
bia formation, and the lower and late Trenton gravels to be 
demonstrative of a long interglacial epoch between their 
times of deposition. 
