320 GEOGRAPHIC DEVELOPMENT OF 



and its bluffs rise to a moderately uniform plain in which the 

 trough is excavated. The lesser tributaries are estuaries toward 

 their mouths, but flow in steep-sided gorges and ravines much 

 like those of the Piedmont toward their sources ; while the di- 

 vides are broad, flat plains in which the drainage systems are 

 imperfectly developed. Thus the eastern portion of the district 

 is a land of steep-bluffed tidal estuaries, narrowing above into 

 gorges and ravines, with ill-drained expanses between. The 

 history recorded in these land-forms is a little more complex 

 than that recorded in the Piedmont : Since the valleys are pro- 

 portionate in size to their streams, it is evident that ail were cut 

 by the streams now occupying them ; since the head-water 

 ravines do not unite in the broad divide-plains, and since the 

 slopes are steep, it is evident that the land has not stood above 

 the ocean long enough to permit the drainage-systems to extend 

 themselves over the entire surface ; and since the larger valle} T s 

 are occupied by tide-water and lined with alluvium, it is evident 

 that the land formerly stood higher than now, and has since 

 subsided so far as to permit ocean-water to drown the larger 

 river-cut valle} 7 s. So the land forms of the district tell of certain 

 agencies and movements concerned in the development of the 

 district. 



THE FORMATIONS 



Washington is located in a triangular amphitheater opening 

 southward through its southern angle. This amphitheater is 

 lined with a peculiar deposit not found over the higher bound- 

 ing hills ; it is composed of brown loam or clay mixed with 

 sand, gravel, and bowlders. This is the Columbia formation. 

 It is generally coarser below and finer above, the upper portion 

 being used as brick-clay ; and in general it is coarser toward 

 the gateway in the wall of the amphitheater through which the 

 Potomac enters in the western part of the city, and finer in the 

 eastern and southern portions of the amphitheater. On com- 

 paring this deposit with the alluvium dredged out of the river- 

 bottom there is found so close similarity as to warrant the con- 

 clusion that both were produced by the same agency — that just 

 as the river is depositing the alluvium at the present time, espe- 

 cially during the spring freshets, so the Columbia formation 

 was deposited by the river during the freshets of past ages. This 

 conclusion involves the supposition that during the Columbia 

 period the land stood lower than now, so that the Potomac estu- 

 ary occupied the entire amphitheater. Comparison of the allu- 



