the Middle Atlantic Slope. 463 



on well-known phenomena of the respective regions, is not far 

 to seek : In the east the ice was thick and moved energetically, 

 ploughing up subjacent deposits and scoring subjacent rocks, 

 and quickly reached the line of equilibrium between growth 

 and waste corresponding to given temperature ; while in the 

 Mississippi Yalley the ice was but a third or a quarter so thick 

 and moved sluggishly, passing over hundreds of square miles 

 without removing the subjacent deposits or touching the sub- 

 jacent rocks, probably failed to reach its line of equilibrium 

 during the earlier, and certainly fell far short of it during the 

 later and briefer refrigeration. The two ice-boundaries cross 

 somewhere in Ohio. 



The History Recorded in the Columbia Formation. — The 

 geologic history recorded in the Columbia deposits and ter- 

 races and in the erosion and alteration which both have suf- 

 fered is almost wholly supplementary to that read by most 

 geologists in the later glacial deposits, and multiplies many 

 times th^ length of the Quaternary as commonly conceived. 

 Collectively the two series of deposits indicate that the Quater- 

 nary consisted of two and only two great epochs of cold (the 

 later comprising two or more sub-epochs) ; that these epochs 

 were separated by an interval three, five, or ten times as long 

 as the post-glacial interval ; that the earlier cold endured much 

 the longer ; that the earlier cold was the less intense and the 

 resulting ice sheet stopped short (in the Atlantic slope) of the 

 limit reached by the later; that the earlier glaciation was 

 accompanied by much the greater submergence, exceeding 

 400 feet at the mouth of the Hudson and extending 500 miles 

 southward, while that of the later reached but a tithe of that 

 depth or southing ; and that during the long interglacial inter- 

 val the condition of land and sea was much as at present. 



Moreover, as in the Potomac formation, geologic history is 

 recorded not only in the formation itself but in its relation to 

 the floor upon which it rests ; and the history read from the 

 deposits is thus materially supplemented. 



A remarkable topographic characteristic is displayed by the 

 Piedmont and Appalachian regions in the middle Atlantic 

 slope, which has only been interpreted — or indeed recognized 

 — within the decade. The entire area is but a gently undulating 

 plain, diversified throughout by deeply incised waterways and, in 

 the Appalachian zone, by bosses and ridges of obdurate strata 

 which are narrowed and truncated by erosion but not planed off. 

 The cross-section of the Susquehanna (fig. 1), with its gently 

 undulating plain bounded by mountains and dissected by a 

 steep bluffed gorge, is representative of the entire Appalachian 

 zone ; it is constantly repeated along each principal waterway 

 of that zone, and — save that the bounding mountains are ab- 



