452 



GEOLOGY. 



The origin of the Columbia and associated formations. — The origin 

 of the Columbia formal ion presents much the same problems as that 

 of the Lafayette, and is probably to be explained in much the same 

 way; that is, the series is looked upon as largely fluviatile and sub- 

 aerial, the result of land aggradation. The occasion for renewed depo- 

 sition on the Coastal Plain in the Quaternary period probably lay 



(1) partly in changes of gradient incident to crustal warpings, and 



(2) partly in the climate of the period. Renewed upward bowing 



Fig. 535. — Unconformable contact between the Columbia formation and the Potomac, 

 Washington. D. C. (Darton, U. S. Geol. Surv.) 



of the Appalachians and of the plateau to the east of them probably 

 stimulated the streams descending from them to increased erosion, 

 and the deposition of a part of their loads on the plain below was a 

 natural result. Under these circumstances, deposition would prob- 

 ably have extended up the valleys to altitudes considerably greater 

 than those of the plain where the principal deposition took place. 

 The poor assortment of the material, the common cross-bedding, the 

 numerous trifling unconformities, and the absence of fossils, all are 

 consistent with this interpretation. So also is another feature of 



