THE MEDINA SHALE 



127 



underlies a plain about 7 miles broad, sloping northward from the foot 

 of the escarpment to the shore of lake Ontario and descending in that 

 distance 100 to 175 feet. If this plain were featureless, each traversing 

 stream would follow the shortest path to the lake and its course would 

 be normal to the lake shore. In fact, however, only a few streams take 

 this direct route, and the majorit}^ are deflected toward the east, running 

 northeastward instead of northward. On the accompanying map it will 

 be noted thai Johnson creek runs northeastward for 15 miles. Oak Orchard 



Figure 6. — Drainage Map in western New York. 



'The area represented measures 75 by 31 miles. The smooth lines are contours at 400, 600, and 800 

 feet above tide. The southern boundary of the Medina shale lies between the 400-foot and 600-foot 

 contours. Arrows show the direction of ice motion as recorded by strife and drumlins. 



for nearly the same distance, and most of the minor streams reaching the 

 lake between Johnson creek and Eighteenmile creek have northeasterly 

 courses. Other streams, like Sixmile and Hopkins creeks and the west 

 branch of IVelvemile, are made up of reaches alternating northward and 

 northeastward in direction, reminding one of the drainage lines among 

 folded rocks which alternately follow and cross the strike. The east branch 

 of Eighteenmile creek has for 7 miles a southwest course parallel to the 

 northeasterly courses of other creeks. 



Some of these peculiarities were noted by Hall, who suggested control 

 b}'' a system of rock fissures,* but this explanation is unsatisfactor}', as 

 the general trend of joints in the district is due east and west. 



In these days of accej^ted glacial theor3^ when the idea of topographic 

 modification by ice is familiar to all, glacial action readily suggests itself 

 in explanation of anomalies of drainage, and the working hypothesis 

 with which I entered the field was that of control by ridges of drift. It 

 was already known that a lobe or deep current of the ice-sheet followed 

 the Ontario basin from east to west, spreading southward in such a way 



* Geology of Fourth District, p. 435. 



