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THE JOURNAL OF GEOLOGY. 



Washington and Baltimore region. The age of the high tilting of 

 the Tertiary peneplain in the James river region in the Piedmont 

 province has not been determined, and a study of these gravel 

 terraces should throw much light on its history. 



Following later Columbia deposition the uplift continued, but 

 without much tilting. The entire Coastal Plain region was finally 

 lifted from ioo to 150 feet above tide water, and deep channels 

 cut through the later Columbia terraces. The drainage conditions 

 at this stage are represented in figure 5 and in section 4, figure 3. 



PAMLTNKEY 



CHESAPEAKE: 





SFA LEVEL 



W 



Figure 6. Diagram of Cenozoic oscillations of land and water in the Middle 

 Atlantic Coastal Plain region. A. Earlier Columbia. B. Later Columbia, C. The 

 conditions east and south of the area of inter-Columbia emergence. 



This was the maximum degree of uplift, and was of short 

 duration, for the valleys were not greatly widened, and they had 

 a relatively steep inclination seaward. It was followed by sub- 

 sidence, which is still in progress, and has resulted in the flood- 

 ing by tide water of the larger Post-Columbia valleys of the 

 Coastal Plain region. The total rate of this subsidence is not 

 known, but on the coast of New Jersey its present rate has been 

 estimated by Professor Cook at two feet per century. In many 

 districts the incursion of tide water is kept pace with and even 

 slightly exceeded by the deposition of silt and other detritus. 

 These accumulations, in themselves, indicate subsidence, for the 

 deposition is mainly due to slackening of currents in the shal- 

 lower channels, due to tide-water incursion. Marsh growth 

 keeps pace with subsidence in many portions of the region. 



In the following diagram an attempt has been made to repre- 

 sent the nature of the oscillations of the Coastal Plain region, 

 but, owing to the meagerness of quantitative data, is not very 

 satisfactory. 



