114 Frank Carney 



fall in level before the great drop which converted its marginal parts 

 into a lake plain. 



DRAINAGE CYCLES 



The Oberlin quadrangle shows concisely the relationship that 

 arises from succeeding base-levels each of which iniierrupted a nor- 

 mal development of drainage lines ; it also shows the influence that 

 formations of simple structure and varying hardness have on the 

 growth of valleys, a factor that may be more effective relatively 

 than time in producing the several Scages in the cycle of erosion. 

 The erosion cycles here are not as simple as they would be if each 

 had commenced on a normal consequent surface. Preglacially the re- 

 gion had been subject long to weathering; its rock structure is such 

 that the area has preserved this older erosion pattern; to a slight 

 extent only did the distribution of glacial drift change this former 

 topography. 



The Maiunee base-lei'd. — The definite level of the upper Mau- 

 mee in this region must have been preceded by a minor body of 

 water occupying the depression indicated now by the branches of 

 the Black river. This condition, however, did not endure long, as 

 no shorelines marking the lake have been found. 



The Maumee shoreline has an altitude of about 770 feet. 

 South of this the surface configuration is influenced by the Berea 

 sandstone, over which is a covering of shale so thin that a slighi: 

 amount of channeling places the streams on the more resistant rock. 

 The Berea here is heavy and has a mild dip to the south. The 

 northward slope of the preglacial surface was gentle. 



The amount of slope for several miles south of lake Maumee 

 was -so slight that the drainage pattern formed was but little more 

 developed than a true consequent surface would give. The fall of 

 these streams was so gentle that the beach diverted them towards 

 particular points, at which they cut the ridge when the Maumee 

 level declined. At Laporte the streams from east and west con- 

 verge and cross the beach ridge. This branch of the Black river 

 has not made much of a channel in the Berea sandstone : north of 

 its intersection with the Lorain and W^heeling railway, glacial drift 

 forms its banks; Sf)uth of this point, it has cut slightly int(^ the 

 Berea. 



The West Branch apparently is following an earlier \alley 



