THE DISTRICT OF COLUMBIA 321 



vium with the Columbia deposits reveals certain minor differences 

 in the deposits, notably a larger proportion of brown loam and 

 a larger number and size of bowlders in the ancient one ; and 

 these differences suggest that during the Columbia period the 

 climate was colder than now, the bowlder-bearing ice-floes larger, 

 and the thaw freshets more destructive to soil than at present. 

 These features suffice to correlate the Columbia foimation with 

 the glacial deposits of northern United States. Thus the Colum- 

 bia formation records definitely a period during which the land 

 stood lower than now and the sea encroached further, and when 

 the climate was colder than now. Detailed study of the forma- 

 tion indicates that there were two epochs of depression of the 

 land, separated by a stage of elevation, the submergence during 

 the earlier period being much the greater. The earlier Columbia 

 deposits are found over the lower hills and uplands flanking the 

 Washington amphitheater up to 200 feet above tide ; the later 

 Columbia mantles Capitol hill and other portions of the amphi- 

 theater up to about 100 feet above tide. 



The distribution of the Columbia deposits is such as to indi- 

 cate that the great estuaries of Potomac and Anacostia rivers 

 and the narrower rock-bound gorge of the Potomac-from Great 

 Falls to its source were carved out in nearly their present form 

 before the Columbia period ; thus these great geographic feat- 

 ures record a pre-Columbia period during which the land stood 

 far above its present level so that the ocean retreated far beyond 

 the present shore-line, probably to the great submarine scarp 

 100 miles off shore. This period was one of great importance 

 in the development of the' district, though it has only recently 

 been defined through recognition of principles discovered during 

 researches in the district. At that time the entire Coastal plain 

 was land, so far elevated that rivers and brooks flowed swiftly 

 across it and down its slopes, producing characteristic land- 

 sculpture — a surface now represented in one of the strongest 

 unconformities in the Coastal plain. 



On some of the highest hills bounding the Washington am- 

 phitheater there is found a deposit of red clay and well-rounded 

 pebbles of quartz and quartzite somewhat resembling the Co- 

 lumbia, but differing in that the pebbles are harder and more 

 worn, and in that the deposit is more uniform and homogeneous ; 

 this is the Lafayette formation. Outcrops of the Lafayette are 

 found on Good Hope hill, in the uplands about Soldiers' Home, 



