290 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



by a few terraces of strong scarps, or by a high terrace with one or two 

 lower ones beneath it ; and it is not uncommon to find at least one side 

 of a valley enclosed by a single scarp in which the whole descent is made 

 at once from the highest terrace plain to the lowest. If terracing had been 

 due to a general decrease in the volume of our rivers, stepping terraces 

 should be much more prevalent, and broad flood plains between the high 

 scarps of a single terrace on each side of the valley should be much more 

 rare than they are ; and when the whole descent from high terrace to flood 

 plain is made in a single scarp on one side of the valley, stepping ter- 

 races with broad treads should be well developed on the opposite side ; 

 but no such arrangement of terrace form can be said to prevail. De- 

 crease of river volume must therefore be at most a subordinate cause of 

 terracing, if, indeed, it is not as a rule a negligible factor in their 

 production. 



This conclusion seems to have been clearly in the mind of Adams, 

 state geologist of Vermont, who in 1846 wrote as follows: "The first 

 stage in the process in which the terraces originated, the deposition of 

 the materials, we have before referred to the older pleistocene. The 

 process of denudation must have next followed, when the rivers, cutting 

 down their channels through the drift barriers, lowered them gradually 

 above the barriers. Flowing through the level deposits of sand, they 

 must have formed serpentine channels, as rivers do now in alluvial 

 plains ; consequently by increasing the convexity of the bends, and then 

 cutting them off or wearing away their headlands and shifting their 

 beds, they would be meanwhile removing the greater part of the ma- 

 terials thus disturbed. By this process the greater portion of the orig- 

 inal plain must have been carried off, and it is not necessary to suppose 

 that the distance between opposite terraces is any indication of greater 

 magnitude of the river, but only of its shifting its channel" (1-45, 

 146). 



Terraces carved by Streams of Increasing Slope. When the 

 basin of an aggrading river system is slightly tilted it may be expected 

 that those streams whose slopes are decreasing will aggrade their valleys 

 more rapidly than before (unless their point of junction with a degrading 

 stream may be lowered more than their headwaters are depressed by 

 tilting) ; while those whose slopes are increasing will change their action 

 from aggrading to degrading. It is well known that New England has 

 suffered a differential elevation in postglacial time. The postglacial 

 clays of Lake Champlain and of southern Maine were deposited when the 

 sea stood three hundred feet or more above its present level. The clays 



