222 THE POPULAR SCIENCE MONTHLY. 



hence they are frequently characterized by swamps or linear 

 lakes, especially near their heads. On the other hand, the 

 streams of the second class, surcharged with detritus from the 

 ice, frequently demanded a steeper grade than they found, and 

 therefore deposited much of their load, filling up their valleys 

 with broad flood plains of sand and gravel, such as are now 

 growing in front of the Greenland and Malaspina glaciers ; and 

 sometimes they may have transformed side streams into lakes, 

 such as now characterize the side streams of the Red River of 

 Louisiana. 



The record of the latter feature is truly a rarity in the past, 

 but it has been deciphered by Andrews as perceptible in some of 

 the lateral tributaries of the gravel-filled valleys of Ohio. The 

 following is Andrews's account. After describing the terraces of 

 sand and gravel derived from the glacial drift and occurring 

 along those streams whose sources lie in the " area of the general 

 drift," he says : " There is in the second district [southeastern 

 Ohio] another and very distinct system of terraces, found on 

 streams emptying into the larger streams bordered by true drift 

 terraces. They may be called backwater terraces. When in the 

 Ohio, Muskingum, Hocking, etc., rivers, the water in the drift era 

 stood eighty or ninety feet higher than at present, the backwater 

 would set back up all the tributaries. In this still water the sand 

 and sediment brought down these tributaries were deposited ; or, 

 in other words, the still-water areas were silted up, as mill ponds 

 often are. When afterward the main streams fell to their present 

 level these affluents cut through the backwater beds and carried 

 away much of the soft materials, but left in many places fringing 

 terraces which tell very plainly how they were formed. In these 

 backwater terraces we find no true drift sand or gravel. The 

 beds are entirely of home origin. Such terraces I have seen in 

 the Little Scioto River, above its junction with the Ohio at Scioto- 

 ville, on Duck Creek, and on the Little Muskingum River in 

 Washington County, and on Sunday Creek in Athens County. I 

 have no doubt they are to be found on a large number of 

 streams." (Geology of Ohio, ii, 1874, p. 444.) This record shows 

 a delicacy of observation and a skill in physical interpretation 

 that have impressed me as exceptional and admirable. 



Taking up again the comparison of the two classes of con- 

 strained streams, it is seen that the channels of the first class 

 were cut down to so gentle a grade by the filtered glacial streams 

 that they are now not infrequently found to be filling up, and 

 lakes are forming in them ; but the valleys of the second class 

 were filled so high by the gravels deposited from the surcharged 

 glacial streams that they are now being terraced, and the lakes 

 that were formed on their lateral tributaries are now discharged 



