FISHER: RIVER TERRACES. 
33 
on a strong rock sill on which corrosion is very slow. Any one or two> 
of these conditions may have weakened the ability of our New England 
rivers to- degrade. And they are essentially graded for long stretches 
and therefore devote their energy to swinging. The river-terrace 
topography such as is found along the Connecticut in the vicinity of 
Holyoke and Springfield suggests that possibly here the river has been 
prevented from degrading lower, and (in the absence of more control¬ 
ling rock ledges) has left high, strong escarpments on either side with 
a few low terraces in the more central portion of the valley. 
Were there no controlling elements, it would be surprising to find 
Fig. J.— Ideal terrace pattern; late stage. Absence of rock ledges has made 
it possible for river to wander broadly, removing the intermediate ter¬ 
races. Descent from the highest terrace to the flood plain is concen¬ 
trated in a single high scarp. Central portion of valley shows a few low 
basal terraces. 
preserved a series of eight stages in any such valley as that of the West 
River at Brattleboro. Evidently the factor which has made possible 
the preservation of the remnants of these terraces must be due to 
another and eminently important cause, viz., the rock ledges. 
Influence oj rock barriers upon the form and preservation of the West 
River terraces .— It has been stated that a free-swinging river in an 
aggraded valley will by degrading discover the rock prominences or 
reefs of its floor; and the outcrops of ledges so discovered will be con¬ 
stantly at lower levels and nearer the center of the valley. These 
