70 Cincinnaii Society of Natural History. 



Silurian appears as the surface rocks, existed as an island, from 

 the surface of which the winds and storms of countless centuries 

 grooved out and carried away, to the surrounding seas, material 

 for the formation of the Devonian and Carboniferous rocks; that, 

 with the growth of successive geological formations, our elevation 

 and area increased, finally becoming continental. 



During the successive geological periods in which were deposited 

 all the sedimentar)^ rocks, from the Silurian to the Cretaceous, the 

 topographical features appear to have been formed in what might 

 be termed a "normal condition." Evidently, the uplands were 

 comparatively level plateaus, upon which were the sources of 

 watercourses that united to form rivers, with their channels and 

 flood-plains. The ancient streams that carried to the sea the sedi- 

 ment which formed the Carboniferous rocks, and the immense 

 timber rafts which form our coal deposits, were undoubtedly the 

 engravers that marked out the lines which are now followed by 

 many watercourses of the present day. These ancient valleys of 

 erosion were deeper than the present river channels. It is fair to 

 presume that they were bounded by banks and bluffs more precip- 

 itous than those of the present day, for we find that the existing 

 topographical features have been modified by an agency operating 

 in a peculiar manner, subsequent to the deposition of the highest 

 rocks of the Tertiary formation. This was the "Ice Age," or 

 " Glacial Epoch,'' when our hills and valleys received their coating 

 of boulder clay, or drift. That the modifications were radical, 

 and that the graving tools of the Ice King were wonderfully effec- 

 tive, is evident from the most cursory examination. Water-courses 

 were in many places changed, and the ancient channels were silted 

 up to a depth of about forty feet, when the stream remained in 

 the same valley, so_ that the gravel bars of the ancient rivers are 

 found at and below the present low water level, and the ancient 

 flood-plain or bottom lands adjoining the streams are now found 

 but six to ten feet above our extreme low water level. 



Evidences of this condition of facts are so numerous as to 

 scarcely need citation It is illustrated by a section on page 427 

 of Volume I., "Geological Survey of Ohio," and confirmed by 

 nearly all the borings and excavations that have been made through 

 the drift deposits of the valleys. The essential fact to be noted in 

 this connection is that the gravel and boulders of the ancient river 

 channels consist largely of sandstone and granitic pebbles, while 

 the gravel beds of the more modern and upper terraces are 



