THE HIGH CONTINENTAL ELEVATION PRECEDING THE 



PLEISTOCENE PERIOD. 



BY PROFESSOR J. W. SPENCER, A. M., PH. D., F. G. S. (l. & A.), 

 STATE GEOLOGIST OF GEORGIA. 



If, in the growth of the American continent, the moulding of the land 

 features had not largely depended upon its projection above the sea, favor- 

 ing or retarding the action of rains and rivers in sculpturing its surface, 

 there would be little interest as to what was its relative height, before the 

 commencement of the Pleistocene period. But w-e find valleys vastly greater 

 than the meteoric agents could have produced under existing conditions. 

 Thus, there are not only deep canons, but also vast depressions, descending 

 to levels far below the sea, which are now filled with the earlier drift ac- 

 cumulations, or form channels submerged beneath ocean waves, or constitute 

 basins occupied by lakes. Hence, in the study of the drift itself, in the 

 investigation of the lake history, or in the research upon the growth of 

 modern rivers, we necessarily inquire what was the altitude of the continent 

 that would permit of the mouldings and channelings of the original rock 

 surfaces. 



Following the period of high continental elevation, the geologist sees in 

 the valleys and old channels, still below the level of the sea, and in the high 

 level beaches, an extensive submergence, succeeded by a re-elevation, but 

 not to the original height, when the continent was being chiseled out by the 

 ancient rivers. That this re-elevation is still going on is shown by the north- 

 ward tilting of the comparatively recent marine accumulations along the 

 St. Lawrence valley and Gulf coast, and the raised beaches in the lake region, 

 as well as by the shoaling of the waters of Hudson's Bay during the present 

 period of observation. 



As general statements do not satisfy investigation, it becomes necessary 

 to search for definite measurements of the former height of the continent 

 among the archives of the geological past. Let us first seek for the testimony 

 recorded by the Mississippi river. 



For the distance of eleven hundred miles, measured in a direct line, above 

 the mouth of the " Father of Waters," the modern valley is merely main- 

 taining its own size, or more generally is being slowly filled by the deposition 

 of river alluvium upon its floor. There are only two exceptions, of a few 

 miles each, where the river is scouring out the rocky floor, and these are 

 over barriers recently exposed there during changes of the Pleistocene 



IX— Bull. Geol. Soc. Am., Vol. 1, 1889. (65) 



