Mat 3, 1907J 



SCIENCE 



705 



five or ten miles wide, gives good ground for 

 the inference that advanced maturity of ero- 

 sion had been reached before Sunderland de- 

 position began ; and this inference is confirmed 

 on reading a little later : " It is evident that 

 the valleys of the Potomac, Patuxent and 

 other large rivers as well as that of Ches- 

 apeake Bay existed [before Sunderland deposi- 

 tion], since the Sunderland formation, which 

 was deposited when this topography was sub- 

 merged, slopes toward all these depressions" 

 (p. 123). Yet the next sentence baffles the 

 reader by referring to the pre-Sunderland 

 valleys as 'gorges,' a term which suggests 

 something very different from valleys several 

 miles wide and only a few hundred feet deep, 

 with gentle lateral slopes of semi-consolidated 

 formations, and which implies a relatively 

 brief erosion interval. The reader's mind 

 would surely be more easily made up regard- 

 ing the writer's idea of the sequence of events, 

 if the important Lafayette-Sxmderland erosion 

 interval had been once for all effectively de- 

 scribed in technical language in an early 

 chapter, and afterwards referred to when 

 necessary in essentially the same language as 

 that first employed. 



The depth of the valleys eroded in the 

 Lafayette-Sunderland interval is believed by 

 Shattuck to have been less than that of the 

 present valleys; but the evidence presented in 

 favor of this conclusion does not seem to be 

 positive. That valleys were present when the 

 SiHiderland terrace deposits were formed is 

 shown by the fact that the deposits slope 

 "from the watersheds of the peninsulas of 

 southern Maryland toward Chesapeake Bay 

 on the one hand and the estuaries of the 

 Potomac and Patuxent rivers on the other" 

 (p. 115). But it is concluded that no valley 

 was cut so deep as it is now, because the 

 Sunderland formation " nowhere shows a tend- 

 ency to develop a thickness sufficient to fill 

 such a valley" (p. 123; see also p. 135). The 

 same is stated to be true of the other terraces 

 (p. 135). It is not altogether safe for one 

 who has but a slight acquaintance with the 

 district under discussion to oppose this con- 

 clusion; yet the argument by which it is sup- 

 ported does not appear to be fully convincing. 



The comparatively brief duration of marine 

 action at the several terrace levels would seem 

 to be compatible with the deposition of most 

 of the marine terrace sediments not far from 

 the shore line, particularly in view of the be- 

 lief that the scarps were presumably cut and 

 the terraces built for the most part during a 

 time of continued depression. Moreover, if 

 the main vaUeys had been significantly deep- 

 ened during the later inter-terrace intervals by 

 normal subaerial agencies under the leadership 

 of the larger rivers, the valleys must have also 

 been greatly widened during the same inter- 

 vals; and the time required for such widening 

 — a slow process — seems inconsistent with the 

 fairly good preservation of the first-made 

 (Sunderland) scarp. This leads to a consid- 

 eration of a third topic. 



3. The duration of the interterrace inter- 

 vals. The fact that the weathered Sunderland 

 scarp is now recognizable at all requires that 

 the entire duration of post-Sunderland time 

 can not have been a large part of a physio- 

 graphic cycle. Such a scarp, a minor feature 

 at the best, would be entirely destroyed by 

 general subaerial erosion in the passage of a 

 coastal plain from the stage of late youth to 

 that of early maturity, or from early maturity 

 to late maturity. Hence only a short dura- 

 tion can be allowed to each of the three 

 inter-terrace intervals of uplift and erosion, as 

 well as to each of the three intervals of scarj)- 

 cutting and terrace-depositing. The author 

 recognizes that these intervals are compara- 

 tively short, but he does not give sufficient 

 emphasis to the contrast between their short- 

 ness and the much greater length of the Lafay- 

 ette-Sunderland erosion interval. The work 

 done in cutting scarps and depositing terraces 

 is, however, very clearly presented; naturally 

 so, as this is the chief theme of the report. The 

 manner in which the later terrace deposits ex- 

 tend into the reentrants eroded in the margins 

 of the earlier terrace deposits gives good proof 

 that the post-Sunderland movements were os- 

 cillations; that is, repeated partial uplifts 

 separated by slight depressions ; and not mere- 

 ly pauses in a persistent uplift. The way in 

 which the marine sands have come to overlie 

 the fresh- or brackish-water bay-head clays is 



