368 W. J. Mc (fee— Three Formations of 



The first phase is bipartite, the upper division consisting of 

 massive or obscurely stratified brick clay, loam, and fine sand, 

 and the lower of stratified and cross laminated gravel and 

 coarse sand, containing abundant erratic bowlders ; while the 

 second consists of an indivisible bed of gravel, sand, clay, etc., 

 chiefly of local origin and thus varying from place to place 

 though tolerably homogeneous in each exposure. The -first 

 phase, too, is confined to limited altitudes, approximately con- 

 stant on each river but rising northward, while the second 

 occurs indiscriminately at the highest and lowest altitudes 

 within the Coastal plain, its thickness culminating at the lower 

 levels and along the coast. 



The Fluvial Phase. — The bipartite phase of the formation 

 is well developed along all of the larger rivers of the Middle 

 Atlantic slope, but most characteristically and extensively on 

 the Potomac, the Susquehanna, and the Delaware. 



The deposits on the Potomac. — "Washington lies within a 

 rudely triangular amphitheater opening southward, into which 

 the Potomac falls from the northwest and the Anacostia from 

 the northeast, the former passing from torrential to estuarine 

 condition and turning southward within the limits of the 

 city. The western side of the amphitheater is the Piedmont 

 escarpment, which south of the city is a terraced or irregular 

 slope rising to a somewhat undulating plain 200 to 425 feet 

 in altitude ; the eastern side is the line of bluffs overlook- 

 ing the Anacostia and rising into two broad terrace plains 

 175 and 275 feet in height respectively; and the northern con- 

 fine is the deeply ravined margin of a terrace 200 feet in alti- 

 tude stretching from the breach made by the Potomac in the 

 Piedmont escarpment directly eastward to the broader valley 

 of the Anacostia three or four miles above the confluence. 

 The floor of the amphitheater is a series of low terraces rising 

 from a few feet below to about 100 feet above tide, the most 

 conspicuous two being about 40 and 80 feet in altitude re- 

 spectively. To the southward the amphitheater opens into a 

 broad valley occupied partly by the Potomac estuary and 

 partly by a low but extended series of terraces, of which the 

 best developed members are about 20 and 40 feet above tide 

 respectively. 



Throughout this amphitheater the fluvial phase of the 

 Columbia formation is the prevailing superficial deposit up to 

 150 feet above tide ; except where manifestly eroded or buried 

 beneath modern alluvium, it is everywhere exposed ; all of the 

 lower and many of the higher terraces are built of it ; and it 

 unquestionably lines the estuaries of both the Potomac and the 

 Anacostia beneath the recent alluvium. The relation between 

 the deposit and the topographic configuration is striking, and 



