764 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



become advanced gives a greater thickness of rock above baselevel, and 

 will require a longer time for the cycle to become completed. If the 

 stages of uplift were equalh^ spaced, the first cycle of rejuvenated erosion 

 would advance farther, there being less rock to be removed. ISTevertheless 

 the whole of the younger series, if the erosion due to them is not well ad- 

 va,nced, must be embraced in far less time than that which led to the 

 development of the peneplain. 



Turning to the shore, the Pleistocene history shows that the aggregate 

 motion of uplift has been not merely discontinuous uplift, but that move- 

 ments of submergence took place between each emergence, and the land 

 at the present time is in a submcrgent phase, but not necessarily now in 

 movement. A submergent phase is felt as a stagnation of drainage and 

 marked alluviation on the valley floor for a short distance above the actual 

 drowning of the river mouth. Farther upstream, however, there is but 

 little change in the grade of the stream, and the headward erosion from 

 the previous intrenchments of the rivers continues. 



Thus, although the problem involves a number of factors, these do not 

 change the direct and simple interpretation of the significance of the 

 Pleistocene-Pliocene series of valley slopes. There have l)een aggregate 

 uplifts by movements which on the whole have increased in rapidity, and 

 notably so in the Pleistocene. Over the Atlantic coastal plain of the 

 United States the submergent phases of the Pleistocene movements are 

 represented by deposits more or less eroded, the older showing much 

 greater erosion than the younger. In Mar3dand, the Lafayette, late Plio- 

 cene in age, attains, on the low plateaus facing the sea, a maximum eleva- 

 tion of 500 feet. Up the stream valleys the river grades carry it to higher 

 elevations. The Lafayette Valley forms correspond to C of figure 2. The 

 next younger deposit is known as the Sunderland. It belongs to the older 

 Pleistocene and shows wave-cut terraces reaching to 220 feet in elevation. 

 Farther inland the valley profile corresponds to E of figure 2. The mid- 

 dle or later Pleistocene wave-cut terrace is the Wicomico, reaching to 100 

 feet in elevation. Its corresponding valley form is F. A number of gla- 

 cial geologists have estimated the time necessary to cut these valley pro- 

 files, and agree that it increases backward in geometrical progression. If 

 F was cut some tens of thousands of years ago, E was cut some hundreds 

 of thousands of years since, and C would appear to date back more than a 

 million years — possibly two million, possibly several million. McGee, in 

 fact, is inclined to estimate post-Lafayette time as from five to ten million 

 years in length.^^ 



The Pleistocene need not, however, be regarded as unique in showing 



" W J McGee : Note on the age of the eai-th. Science, vol. xxi, 1893, p. 309. 



