POSTGLACIAL EROSION 161 



glacial" needs explanation, for much canyon-cutting was effected while 

 the ice-sheet still lingered on territory of the State. For example, the 

 Portage ravine of the Genesee began cutting while the ice-front was not 

 far away on the north. The Mount Morris ravine, the "High Banks," 

 was in the making while the ice covered Eochester, and the upper (south) 

 section of the Rochester Canyon was largely cut while Lake Iroquois 

 waters prevented the excavation of the lower part of the gorge. Cer- 

 tainly a large part of the erosional work in central and western New 

 York and the Hudson Valley occurred while the glacier still covered the 

 northern lowlands of the State, including the Champlain Valley. 



Glacial Time 



The first question commonly asked by the non-geologist is, "How long 

 ago?" We have to admit ignorance of any precise measure of geologic 

 time. Geologists have learned to think in millions of years, and they 

 are not greatly concerned with the precise duration of so short a period 

 as the glacial or postglacial episode. However, precise knowledge is de- 

 sirable and a yardstick of geologic time must be sought. All attempts 

 to use the present rate of canyon-cutting or cataract recession as an index 

 of time have failed, and no data yet discovered have much value. 



The history of the ice-front recession, with its long succession of lakes 

 and well developed river channels, compels the extension of our estimates 

 of the length of glacial time, and all studies in glacial geology have the 

 same result. 



If we take 10,000 years as a moderate estimate of the life of Lake 

 Ontario, then we must add an equal, and perhaps much greater, time for 

 the lifting of the basin out of the marine waters. Then we must allow 

 at least another 10,000 years for the duration of Lake Iroquois, and the 

 30,000 years carries us back only to the time when the ice-sheet was re- 

 moved from the western part of the State. This appears to be but a 

 minor portion of the time covered by the waning of the glacier, judging 

 from the maps and the known history preceding the initiation of Lake 

 Iroquois. 



If we assume 75,000 years as the time in the waning of the ice-sheet, 

 then we seem compelled to add an equal time for the invasion of the ice, 

 with some time in addition for the pause at the terminal moraine. Most 

 glacialists will probably agree that 150,000 years for the length of the 

 latest or Wisconsin ice epoch is a fair estimate, and back of this we have 

 the earlier and much longer Glacial and inter-Glacial epochs. The esti- 

 mates of those best qualified to judge of the length of Pleistocene time 

 are from 500,000 to 1,500,000 years. 



XI— Bull. Geol. Soc. Am., Vol. 24, 1912 



