OBSERVATIONS ON SUPPOSED GLACIAL DRIFT. 261 



These are all roughly parallel to one another, and are many 

 iiundred miles in length. The lowest, the Niagara, varies from 

 600 feet to 1300 feet above the sea ; the second, west of Lake Win- 

 nipeg, from 1600 feet to 2000 ; the third, the Grand Coteau de 

 Missouri, from 2000 to 3000 feet and more above the ocean (see fig. 

 3.) They have all easterly, north-easterly, or northerly aspects, in 

 relatively different parts of their lengths^* and appear to have a 

 common origin. If it can be shown conclusively, as Mr. "Whitney 

 believes, that the driftless area in Wisconsin has never been over- 

 flowed, these escarpments, as well as those of their great outliers in 

 the "far West," can only be due to the same agent which excavated 

 the basins of the great American lakes. 



The symmetrical escarpments of the Grand Coteau de Missouri, 

 the E-iding Mountain and its prolongations, and portions of the 

 Niagara escarpments, are probably the result, to a large extent, of 

 ihe action of glacial rivers undermining and washing away the soft 

 strata of the sedimentary rocks, and excavating in advance of the 

 glacial mass itself ; and they represent difierent and closely succeed- 

 ing glacial periods (the Missouri escarpment being older than that 

 of the Eiding Mountain), with, however, a distinct geological 

 interval between them. The close proximity of the isothermal curves 

 in these latitudes to the general direction of the escarpments of the 

 Grand Coteau and Riding Mountain is a very interesting and im- 

 portant feature in connexion with the cause which produced them. 



§ 7. Conclusion. 



The opinion that many of the phenomena attending the surface- 

 geology of a large portion of North America were caused by glacial 

 ice, appears to be gradually gaining ground among American geolo- 

 •. gists. First enunciated by Professor Louis Agassiz,t it received 

 the sanction, wholly or in part, of some well-known geologists. In 

 a recent paper by Dr. Newberry, it is stated that " in this * glacial 

 epoch' all the Lake-country was covered with ice, by which the 

 rocky surface was planed down and furrowed, and left precisely in 

 the condition of that beneath modern moving glaciers in mountain- 

 valleys "J. 



• The western exception at Cyprfes Hills has been already noticed. Here the flanks of the 

 Socky Mountains are approached, 

 t Lake Superior: its Physical Character, Vegetation, and Animals, &c. 1850. 

 4 Notes on the Surface-geology of the Basin of the Great Lakes. 



