36 



THE PLIOCENE AND PLEISTOCENE DEPOSITS OF MARYLAND 



ran, beginning at the top, later Columbia, earlier Columbia, and 

 Lafayette. (Fig. 1^) 



Darton has published a number of geologic maps from time to time 

 in which these relations are depicted. In an early map of the Washing- 

 ton region which was published in 1891, he included only two formations 

 of gravel, the Lafayette and Columbia, but in the Washington folio which 

 appeared in 1901, three formations of gravel are portrayed, the earlier 

 and later Columbia and Lafayette, developed in distinct terraces, lying 

 one above the other. The ISTomini and Fredericksburg folios of the 

 Geologic Atlas of the United States convey his ideas regarding the se- 

 quence of gravel in portions of southern Maryland and eastern Virginia. 



WASHINGTON REGION 



EASTERN SHORE REGION 



LATER COLUN 



Fig. 1. — Diagram showing structure and distribution of surficial deposits in 

 the Middle Atlantic slope according to N. H. Darton. 



In these publications he distinguishes two gravels lying in distinct ter- 

 races, the Lafayette above, covering the divides and the Columbia beneath, 

 occupying the valleys. 



During the year 1898, while Darton's publications were still appear- 

 ing, there arose an interesting discussion regarding the age and origin 

 of certain clay lenses found at various places near tide level under one 

 of the lower terraces of the Columbia group. These lenses have been 

 found to contain vegetable remains such as cypress-stumps, logs, roots, 

 twigs, seeds, cones, leaves, etc., and animal remains including insect and 

 invertebrate fossils, the latter found especially at Wailes Bluff near the 

 mouth of the Potomac and at Langleys Bluff five miles south of Cedar 

 Point on the Bay shore. These last two localities were mentioned and 

 described long ago by Conrad. Mr. A. Bibbins, from the examination of 

 one of these clay deposits carrying cypress-stumps, near Bodkin Point 

 at the mouth of the Patapsco river, concluded that they were Raxitan in 

 age. Dr. Philip E. Uhler, after examining a number of similar deposits 

 farther up the Patapsco river near the head of Baltimore harbor, con- 



