704 S. R. CAPPS AND E. D. K. LEFFINGWELL 



swollen streams, and might well have caused the formation of the plain 

 from which these terraces were subsequently developed by erosion. 

 Somewhat careful search fails to show glacial striae on the stones of 

 the gravel, but striae were hardly to be expected even if the gravels 

 be of fluvio-glacial origin. 



Another fact which seems to favor the glacio-fluvial origin of these 

 terraces is found in the relation of the Twin Lakes and Clear Creek 

 moraines to the valley. At the time of the last glaciation the ice from 

 the west pushed across, or nearly across, the Arkansas valley at 

 these places, and must have blocked the valley to some extent. Now, 

 the older drift shows that during the earlier glacial epoch the ice was 

 even more extensive, and must also have obstructed the valley at these 

 points. This would have given the main stream a lower gradient, if 

 it did not dam it altogether, and so favored the deposition of the 

 gravels above. If the valley was effectually dammed, lakes would 

 have existed; but the main body of gravel does not appear to have 

 been deposited while this condition existed, if it existed at all. Fur- 

 thermore, it is not apparent that the dam could have been high 

 enough at any time to hold the water up to the level of the high ter- 

 races. 



That the high terraces are of great age is plainly shown by the 

 oxidized and decayed condition of their materials, as seen in a number 

 of fresh sections, especially about Malta. Again, the small streams 

 from the mountains have cut valleys deep into the terraces, valleys 

 which are much larger than those cut by similar streams in the late 

 glacial drift. 



Low terraces. — Outside of, and below, the high-terrace level there 

 sometimes occurs a set of low terraces which bear the same relation 

 to the last glacial moraines as the high terraces do to the older drift. 

 They connect with the new moraines at their upper ends, and their 

 materials are of the same lithological character as those found in the 

 new drift. The physical condition of the material of these terraces 

 corresponds to that of the new drift, and they have suffered an amount 

 of erosion comparable to that which has effected the new moraines. 

 The few available sections in the low terraces fail to show any striated 

 stones, although plainly formed from the out wash of the new drift. 



The relations of the low terraces to the new drift and of the high 



