DISCRIMINATIVE CRITERIA 201 



For the above reasons very precise determination of water levels, to the 

 exact foot, is usually impracticable, and is possible only by detailed and 

 intensive study over some distance, and even this is an average. Of 

 course, it is understood that deformation must be expected on lines that 

 cut the isobases. Thus it becomes necessary to discriminate clearly be- 

 tw^een beach variation due to variables of the shore and that due to land 

 tilting. 



The Map 



The map of isobases, figure 1, shows the total post-Glacial uplift, wliicli 

 is all of Wisconsin or post-Wisconsin time, except possibly territory west 

 of Ohio and Michigan. The map published as plate 10, in volume 27 of 

 tlie Bulletin of the Geological Society, is confirmed in essential features 

 by all subsequent study and constitutes the nucleus of the present map. 

 The chief modification of the former map is the slight curvature given 

 to the isobases in their extension over the greater area. For example, 

 Ottawa City here lies on the 700-foot isobase, while with the direct lines 

 of the former map it lay at about 730 feet. 



These isobasal lines are drawn in three forms or degrees of reliability. 

 Tlie heavy solid lines are quite positive^ being located by clear evidence 

 of the summit scale vel waters. The light solid lines are approximately 

 correct, based on suggestive data and their position being necessitated by 

 the relationship and control of the heavy lines. The broken lines are 

 more or less hypothetic, but the only lines which are wholly theoretic are 

 the extensions of the isobases of zero to 500 feet over Indiana, Illinois, 

 and Wisconsin. 



The map strongly indicates the causal relationship of the ice-caps to 

 the Pleistocene diastrophic land movement. The uplifted area shown by 

 the isobases is also the glaciated territory. The glaciers deployed on the 

 land to thin borders, but where the sea was reached the more rapid melt- 

 ing and erosion limited the extension of the ice-sheet. In other words, 

 on the land the ice extended farther from its gathering ground or alimen- 

 tation area than it did on the oceanic spaces. In consequence the flow 

 was more rapid and the surface gradient was steeper along the radii 

 toward the marine l)orders. 



In the Mississippi Basin glaciation extended to the Ohio Eiver, and, 

 judging from the apparent relation of land movement to glaciation over 

 tlie greater area, it would seem probable that this region should have 

 snfrered some movement. As the Mississippi Basin was occupied by icc- 

 sliecis antedating the more easterly Lahradorian glacier, and with greater 

 reach to the south and southwest, it is ])ossible that the early uidoadiiig 



XVI— Bull. Geo:.. Soc. Am.. Vol. 20. 1017 



