50 GEOLOGY OF OHIO. 



forty miles toward their sources. When they were united by the retiring of the 

 Lake, but slight cause was needed to turn them eastward along the level bottom of 

 the deserted channel; and they have now cut their beds so deeply in the Drift that 

 the highest freshets do not connect them with the Little river." 



TEEEACES AND BEACHES. 



The withdrawal of the water of the last submergence of the Drift took 

 place slowly, and its progress was marked by periods of rest, and, per- 

 haps, of recession. In these intervals, the terraces which line the banks 

 of so many of our rivers were formed, and hence this has been designated 

 the Terrace Epoch. Local and minor terraces are produced by streams 

 as they deepen their channels and swing from side to side in their val- 

 leys, but all the great and general valley terraces were formed by the 

 arrest in still water of the materials transported by water in motion ; 

 the still water having been that of lakes subsequently drained, or that 

 of the ocean withdrawn from inlets or arms which it once occupied. Ter- 

 races are also formed on the shores of the ocean and large lakes by shore 

 waves, which deeply notch the slopes upon which they beat. Should 

 the water level be depressed, or the land elevated, with intervals of sta- 

 bility, such terraces would constitute a series of steps, or benches, cut in 

 the superficial or rocky material of the shore. 



The terraces of the valleys of the Ohio and its tributaries have never 



been carefully studied, but it is well known that all the important 



streams of the Ohio system show them with more or less distinctness. 



In the Ohio valley are some strongly marked terraces, which rise high 



above the flood-plain, and are favorite Sites for residences and towns. 



Cincinnati is built on one of these. This has an average elevation of 108 



feet above low-water mark in the Ohio. According to Prof. Orton, this 



may be taken as a fair sample of them all : " It is composed of distinctly 



stratified gravel and sand of various degrees of fineness and purity. The 



gravel stones are all water-worn.. In weight they seldom reach ten 



pounds. The upper tributaries of the Ohio supplied the materials, in 



part; but a much larger proportion, in the vicinity of Cincinnati, is 



derived from the limestones of western Ohio and the crystalline rocks of 



Canada. Occasional seams of clay loam occur in the terraces, but seldom 



of tenacity enough to constitute reliable water-bearers. Less frequently 



met, but still constituting a noteworthy feature of the gravel terraces, 



are seams of bituminous coal in small, water-worn fragments.," It is 



scarcely necessary to say that the gravel terraces of the Ohio were once 



the bottom of this stream, then flowing 250 feet above its more ancient 



