REPORT OF THE DIRECTOR I919 5I 



a period of more rapid wasting there would be a more than average 

 rate of withdrawal in the lobate portions ; for these projections may be 

 likened to the dial hands of various gauges whose points swing over 

 a wide range of degrees when their centers are only slightly dis- 

 placed. A slight reduction and melting northward of the great bulk 

 of the main ice mass would bring about a much greater lineal with- 

 drawal of the attenuated, " sensitive " tips of the lobes, hence a more 

 rapid one, and such process would tend to keep the portion of the 

 trough in which the lobe had rested before comparatively free of 

 detritus deposits made directly from the ice. 



The Onondaga trough is a secondary and accentuated example of 

 the lobe basin of figure i. It corresponds most exactly with Lake 

 Thur in figure 2. Minor projections of larger lobes rested in both 

 hollows and their relatively rapid melting preserved the basin effect. 



Hanging valleys. Genetically, however, the Onondaga trough 

 is the result of ice erosion and it should be discussed as being of 

 such origin. Its form, as seen from both sides, is well illustrated by 

 plates 6 and 7, the former showing the east side, the latter the west 

 side, both opposite the village of Tully Valley. In plate 6 note the 

 level summit of the upland, comparatively little modified by glacial 

 erosion, and the over steepened slope of the Onondaga trough extend- 

 ing as a straight-l|ine cut through the area. Plate 7, the west side 

 opposite, shows the steep slope quite as clearly and illustrates 

 another characteristic effect due to the differential action of glacial 

 erosion, namely the creation of hanging valleys and the postglacial 

 development of rock gorges on their lips. The cross-section form 

 of the hanging valley and of the top of the postglacial gorge has 

 been outlined in ink on the photograph so that the reader may be 

 able to identify them clearly. The summit that appears in the dis- 

 tance in the center of the hanging valley depression is the hill, 1620 

 feet high, lying to the west of Maple Grove (Case P. O.) on the 

 topographic map. 



The thicker ice currents, moving freely through the valley chan- 

 nels with axes parallel to the general direction of the ice advance, 

 notably deepened, widened and straightened such depressions during 

 the course of the ice invasions, as is well illustrated by the Onondaga 

 valley. Preglacial valleys with courses lying athwart the direction of 

 the ice motion were, on the other hand, modified little if at all by the 

 ice scour. In general with the rest of the upland country they were 

 probably planed down somewhat but there was no concentration of 

 erosive effect along their lines, and they may even have escaped the 

 full erosion of the uplands ; for their furrows would tend to obstruct 

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