376 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



from following' their present course? And if a dam was produced, was it 

 maintained continuously throughout the entire period during which the 

 outer member of the morainic system was forming? 



That the ice sheet occupied this portion of the valley for a considera- 

 ble period is shown by the strength of the portion of the moraine developed 

 along the southeast side of the valley, and thait it would, while occupying 

 the moraine, present a serious obstruction to the flow of a stream along the 

 valley can scarcely be doubted, especially since the course of the stream did 

 not conform to the direction of ice movement but was almost at right angles 

 to it. The open valley (Beaver Creek), leading southward from the Mad 

 River plain to the Little Miami, to which attention has already been 

 directed, ma}- have been utilized because of an obstruction to the flow of 

 the water doAvn the valle}'. The obstruction may have been complete for 

 a brief period, but that it was not complete for the entire period required in 

 the formation of the moraine seems to be shown both by the small size of 

 the outlet through Beaver Creek Valley and bj- terraces along Mad and 

 Miami rivers. The portion of the Mad-Miami Valley under discussion is 

 occupied by a broad g-ravel plain similar to that along the upper portion of 

 Mad River and much broader than that along- the portion of the Great 

 Miami above the mouth of Mad River. This plain has, so fa,r as the writer 

 could detect, no trace of morainic features on it where the moraine crosses 

 Mad River above Dayton, and no well-defined morainic features at the 

 point where it crosses the Great Miami below Dayton, the only peculiarity 

 of the plain at the latter place being a series of channels and slight undula- 

 tions doubtfull}" referred to glacial action. So far as noted, the Mad River 

 plain has no terraces above the point where the moraine crosses that are 

 not present below that point. Indeed, tliere seems to be but one broad and 

 well-sustained plain higher than the present flood plain either above or 

 below the point where the moraine crosses, and this has a g-eneral elevation 

 of 30 to 40 feet a,bove the river. The great size of tlie gravel plain in the 

 vicinity of Dayton, when contrasted with the narrowness of the Beaver 

 Creek Valle}^, supports strongly the view that this portion of the valley 

 afforded a passage for the stream throughout much of the time the moraine 

 was forming. 



Mad River Valley was occupied by glacial waters only in the periods 

 during which the outer and middle members of the morainic system were 



