72 PROCEEDrNGS OF THE PRIXCETOX MEETING 



IteSCUSSIOX OF PRECEDIXG TWO PaPEBS 



Prof- H. L. Fairchild: Estimates for the life of Lake Ontario make it not 

 less than 10AKX> years. Before Ontario came into existence the basin was 

 covered by waters confluent with the sea. The uplift at the head of the Saint 

 Lawrence has not been less than 1^ feet With the slow movement altogether 

 probable for diastrophic movement, it seems likely that the marine interval 

 was fully as long as the life of Ontario. Before the marine episode the glacial 

 Lake Iroquois accomplished an effect c-omparable to that of Ontario. That 

 gives a total of SO.WtO years for the simplest history that is possible since the 

 inception of Iroquois. Of c-ourse this is a crude estimate and the years may 

 be vastly more. 



Prof. Fr-ajsk Carxey: Concerning Professor Wrights referenc-e to the age 

 of Plum Creek, stream erosion in a given time inrerval varies with several 

 factors. Even if it could be shown that the precipitation factor involved no 

 cycles in the lai>se of several thousand years, it is c-ertain that a tributary 

 stream can not cut a channel below that of its major. The resistant Berea 

 sandstone in the vicinity of Elyria. Ohio, forms a loc-al baselevel in the West 

 Branch of Blo<-k River, to which Plum Creek is tributary. Plum Creek can 

 not cut its channel below that of the West Branch at the junction ix»int : heuc-e 

 the comparatively slight amount of erosion acct)mplished by Plum Creek since 

 the extinction of Lake Maumee. The amount of erosion done in a certain 

 segment of a valley in a particular length of time is an unreliable unit with 

 which to estimate the time required for the making of the valley. 



Mr. Fraxk Leverett called attention to evidence from northern Minnesota 

 that the Keewatiu and Labrador centers of glaciation were each extensive at 

 the same time. This disposes of the suggestion of alternating glaciation in 

 these two c-enters made by Tyrrell in referenc-e to the Wisconsin glaciation. 

 Evidence is not so clear in referaice to earlier stages. 



Prof. R. D. Sausbuby called attention to the fact that the rate of erosion 

 has been greatly increased since cultivation of the soil has begun. Based on 

 present rates of erosion, estimates of time necessary for the total of post- 

 glacial erosion are likely to give much too small results. 



Prof. X. H. Wexchexx, called attention to the non-aj^lication of the dis- 

 cussion of the length of post- Wisconsin time to the question of the age of the 

 Don Valley leaves and of the Scarboro section, since they are much old«*. 

 The age either of the Plum Creek gorge or of the lakes that followed the 

 Wisconsin ice-epoch are only late episodes of a greater drama which began 

 at an earlier date. In the earlier portion of the great period, according to 

 Professor Col«nans photographs and his explanation, there were accumulated 

 four boulder clays indicating four ice-epochs prior to the Wisconsin epoch. 

 These boulder clays seem to be chronologically about parallel with four ice- 

 epochs which have been determined in the western c-ountry oc-curring sinc-e the 

 Kansas eiXK-h. namely. lowan. Illinois, etcetera, this being the most eastern 

 point, apparently, at which these minor ^)ochs have been detected by the 

 existence of actual boulder<lays earlier than the Wisconsin epoch. It is 

 probable that other traces of these earlier ic-e-sheets will be discovered as 

 time passes. 



