the Middle Atlantic Slope. 373 



The altitude of the old delta now exposed as the Columbia 

 formation indicates submergence of about 150 feet during the 

 period, of its deposition ; and such submergence is attested not 

 only by the deposits but by an extensive system of terraces. 

 The Columbia formation itself forms, within the Washington 

 amphitheater, two distinct terrace plains, modified by erosion 

 and culture yet each miles in extent, together with several 

 others of less area ; the upper level of the dep.osit is marked 

 by broad shore lines on both sides of the head of the estuary 

 and by a rock shelf in the gorge of the Potomac half a mile in 

 average width and fifteen miles long ; southwest of Washing- 

 ton there is a wave-fashioned plain, 220 feet above tide, which 

 is more than twenty-five square miles in area and so little 

 modified by erosion that considerable tracts are imperfectly 

 drained ; a more deeply ravined plain of like altitude five 

 square miles in area forms the marginal portion of the Pied- 

 mont plateau to the northward, of Washington ; beyond the 

 Anacostia there are equally distinct terrace plains, that of 175 

 feet above tide at St. Elizabeth's Insane Asylum being so imper- 

 fectly invaded by erosion and so level to the very verge of the 

 river bluffs that drainage is imperfect over fully a square mile of 

 its area ; and in many other localities, and at all altitudes up to 

 250 feet or more, broad terraces abound. The extensive ter- 

 racing of the tract gives origin to a striking topography of plains 

 and scarps, through which profiles, drawn in any direction, ex- 

 hibit characteristic combinations of horizontal lines and steep 

 slopes. Independently of the deposits, the terraces and shore 

 lines in the Washington amphitheater prove submergence of 

 the land to a depth of over 250 feet — the deposits at the high- 

 est levels representing rather the interfluvial than the fluvial 

 phase of the Columbia formation. 



While nothing more than comparatively brief submergence 

 of 150 or more feet was required to produce the upper member 

 of the Columbia formation, other conditions were required to 

 produce the coarse lower division, which differs materially in 

 composition from the sediments now laid down in the Potomac 

 estuary ; but since the abundance and size of the pebbles and 

 bowlders now swept into the estuary are determined by the 

 amount and thickness of the ice floated into it during the 

 spring freshets, it is evident that the chief additional condition 

 required for the deposition of the coarse materials of the older 

 formation was diminution of temperature and consequent in- 

 crease in floe transportation with, perhaps, concurrent strength- 

 ening of fluvial currents. It might accordingly be safely in- 

 ferred from the phenomena of this tract alone that the lower 

 member of the formation was deposited during a period of low 

 temperature. The refrigeration thus suggested by the depos- 



