GREAT LAKES BASINS— SHEPARD 



273 



is the case. First, let us consider the earthquake distribution in 

 relation to glacial territory. If we place the great seismic belts and 

 the glaciated territory on the same world-map, we find that the two 

 hardly coincide at all (fig. 4). Further consideration shows that 

 both the Scandinavian center in Europe and the Labradorian and 

 Keewatin centers in America are related closely to ancient shields 

 where stability has long been developed. Nor are there notable 

 fault scarps and fault troughs developed in these regions. 



Now, if we consider the other areas of the world which contain 

 large basins, we find a very different picture. These areas are found 

 within earthquake belts in practically every case. The areas con- 

 tain a maze of fault scarps and all other indications of having been 

 active diastrophically in recent times. 



Figure 4.— Showing the lack of relation between earthquake belts and glaciated territory. 



The argument that there has been warping in glaciated territory 

 in postglacial times requires some consideration since this may rep- 

 resent the cause of the basins. It is generally agreed, however, that 

 this warping is the result of readjustment of the earth's crust fol- 

 lowing the release of the load of the continental glaciers. The com- 

 pilation of the data by Gutenberg ^° shows that, so far as the Great 

 Lakes area is concerned, there is a progressive rise from the southern 

 boundary of the lakes to the north. The principal effect of such a 

 tilt would be to decrease the size of the basins, and it certainly should 

 not be considered as an important factor in their production. 



MOVEMENT OF THE ICE 



The various opponents of the idea that ice excavation is important 

 have made much of the presence of deep basins transverse to the 

 general motion of ice. This is true of many of the fiords along the 

 various glaciated coasts of the world. It does not, however, dis- 



"> Gutenberg, B., Tilting due to glacial melting. Joum. Geol., vol. 41, p. 458, flg. 3, 1933. 

 31508—38 19 



