372 W. J. McGee — Three Formations of 



The same conclusion is reached \>y the converse process of 

 reasoning. At Washington the Potomac river passes from 

 fluviatic to estuarine condition, and the materials trans- 

 ported by the river proper are precipitated in the estuary. 

 The opportunities for examination of these sediments are lim- 

 ited, because they are seldom exposed above tide level (sub- 

 aerial alluvium being significantly absent along the fall line 

 margin of the Coastal plain) ; but the numerous borings made 

 in engineering operations indicate that the sub estuarine de- 

 posits opposite Washington consist predominantly of fine silt 

 or clay, and subordinately of sand and gravel, with occasional 

 pebbles and bowlders of considerable dimensions either scat- 

 tered or in beds. In brief, the deposits of the Potomac estuary 

 of the present at Washington differ from the upper division 

 of the fluvial phase of the Columbia formation only in the 

 larger proportion of silt and the smaller size of interspersed 

 bowlders ; and below Washington the modern estuarine de- 

 posits become progressively finer to and beyond Alexandria, 

 just as do the deposits of the Columbia formation. Now the 

 precise conditions of genesis of the modern sub-estuarine de- 

 posits are known : the silt is carried down the river and into 

 the estuary at all stages but most abundantly during freshets 

 to either settle immediately in the slack water or sweep back 

 and forth with the tide until flocculation and more gradual depo- 

 sition finally take place ; the fine sand is similarly transported 

 into the estuary and dropped toward its head ; most of the coarse 

 sand, gravel and pebbles are swept over the falls or collected in 

 the gorge of the Potomac by the raging torrent which the 

 river becomes during its freshet stages, and are quickly depos- 

 ited in the upper part of the estuary ; while the larger pebbles 

 and bowlders, together with some of the smaller, are gathered 

 along the river and floated into the estuary by the ice floes 

 with which the torrent is laden during spring freshets ; and the 

 distribution of the various materials, fine and coarse, is affected 

 by the strength of the currents, by local eddies and basins, and 

 by distance below the mouth of the gorge, while the area of 

 deposition is determined by present tide level. Were the land 

 in the vicinity of Washington to be elevated 150 feet, and 

 were this level to be maintained until the sub-estuarine deposits 

 now in process of formation were dissected by erosion, desic- 

 cated by draining, and decolored by oxidization, they would 

 unquestionably form a homologue of the upper member of the 

 Columbia formation, differing from it only in coarseness of 

 materials and in geographic extent ; and the modern deposit, 

 like the older, would rise upon the valley sides to the shore 

 line contour, and its surface would similarly form a broad ter- 

 race plain. 



