in the Pleistocene and Post -Pleistocene. 9 



that existing at present. Still farther out there are probably 

 terraces cut still lower. But such older steps must be com- 

 monly mantled over by the terraces of later construction, their 

 forms would tend to become smoothed out and they are not 

 developed by sufficient soundings to permit as yet of a detailed 

 study. In regions where rivers do not bring much sediment 

 from the land, however, such fossil terraces may be best pre- 

 served. For the study of the negative phases of oceanic 

 movements, — that side of the rhythms of which, as previously 

 pointed out, so little is known, — they should be of high 

 importance and be best developed where the evidence given by 

 drowned river valleys is wanting. 



Pliocene and Pleistocene Marine Terraces. 



The second line of investigation which has led the writer 

 toward the problem of recent movements was in connection 

 with areal geologic work in southern New England. An 

 examination of the topography indicated that there were in 

 this region a series of descending baselevels, more than could 

 be fitted to the so-called Cretaceous, early Tertiary, and late 

 Tertiary baselevels. The necessity of determining the number, 

 the slope, and the age of these, as a key to the post- Jurassic 

 history of the region led to a development of methods the results 

 of which have been published in abstract.* Each baselevel 

 would be recorded in the interior by surfaces of subaerial 

 denudation, on the seaward side by surfaces of marine plana- 

 tion. The latter were cut as benches across the harder rocks 

 and are consequently better preserved than the former. They 

 were most strongly developed at the maximum elevation of the 

 sea during each oscillation. A method of projected profiles 

 restores these ancient levels and hides the later dissection 

 carried on by subaerial agencies. The length of the several 

 profiles of marine planation is related to the duration of the 

 baselevels ; their difference in elevation gives, on the other 

 hand, the amount of elevation of the land as the result of each 

 cycle of motion. It is thought that the Goshen level, attain- 

 ing an elevation of 1380 feet in northwestern Connecticut, 

 dates from probably the earlier Pliocene. The terraces below, 

 facing the sea, descend by steps which are a little over 200 feet 

 apart in elevation and many miles broad. The series resembles 

 a flight of stairs, but one in which the rises and treads are both 

 gently sloping and the whole so dismantled by subaerial erosion 

 that what the eye sees in viewing the landscape is almost 

 wholly the dissection due to later cycles of destruction. The 



* Piedmont Terraces of the Northern Appalachians and their Mode of 

 Origin ; Post-Jurassic History of the Northern Appalachians, Bull. Geol. 

 Soc. Am., vol. xxiv, pp. 688-696, 1913. 



