INTERGLACIAL GORGES 6 1 



Should a tributary gorge have been incompletely filled with 

 drift, its stream would begin the task of clearing out the drift and 

 of cutting a second gorge in the bottom of the earlier one. Should 

 it happen, however, that the drift filling of a gorge was so complete 

 that the stream on reoccupying its old valley and taking a conse- 

 quent course found lower ground to one side, a new and independent 

 gorge would be cut. Should the consequent course induced by the 

 drift topography correspond in part with that of the old gorge and 

 differ from it in part, the stream would cut a new gorge where it 

 found itself out of the old channel, and merely clear out the old 

 gorge where the two courses were in coincidence. 



Parts of the old drift-filled gorge unoccupied by the stream 

 would be preserved and would remain indefinitely as fossil gorges 

 hanging above the level of the main valley bottom. 



If an interval of deglaciation sufficiently long ensued, the tribu- 

 tary streams would again bring their beds down to grade. A third 

 epoch of glaciation would bring about a repetition of events with 

 the possible formation of still another gorge in the hanging valley 

 bottom. The number of distinct gorges should, then, give at least 

 the minimum number of glacial invasions which the region had 

 suffered. It would not necessarily give the maximum, for during 

 one or more of the intervals the stream might have merely re- 

 excavated one of the older gorges. 



The relative width of gorges cut during different intervals 

 should, other things being equal, give a rough measure of the rela- 

 tive length of the interglacial intervals which they represent. A 

 comparison of the width of the older gorges with that of the post- 

 glacial gorge should, in the same way, give a rough measure of the 

 length of the interglacial interval as compared with postglacial 

 time. This would be true, of course, only providing the older 

 gorges were occupied by streams during only one interval. 



The criterion outhned above for the determination of repeated 

 ice invasions is considered to have advantages over others in that 

 the possibility of minor halts and readvances of the glacier being 

 interpreted as distinct glacial epochs is reduced to a minimum, for 

 streams require time for the excavation of gorges. If an interglacial 

 gorge is as large or larger than the postglacial gorge of the same 



