THE DISTRICT OF COLUMBIA 319 



The earth-matter cut out of the rills, gullies, ravines, and val- 

 leys is transported by the running water into the adjacent lake 

 or sea, where it is dropped, swept here and there by the waves, 

 and eventually built into sheets of sediment, or formations. So 

 long as land and sea maintain their relative position, the sedi- 

 ments are accumulated continuously and constitute a single 

 formation ; but if the earth-crust rises or sinks, the formation 

 changes : If the earth-crust rises, the ocean withdraws and sea- 

 bottom is converted into land to be sculptured into land-forms; 

 if it sinks, the ocean advances and sediments are laid down over 

 the land-forms sculptured by the running waters, and an uncon- 

 formity is produced. 



Thus in regions like the Coastal province there are two im- 

 portant classes of products, (a) land-forms, and (b) formations ; 

 and the unconformities separating the formations are old land- 

 surfaces. 



The development of the region is recorded in land-forms, forma- 

 tions, and unconformities produced in this way. 



THE LAND-FORMS 



Above the mouth of Rock creek, Potomac river flows in a 

 steep-bluffed gorge cut sharply in the Piedmont plateau; Rock 

 creek, too, occupies a narrow and rugged valley cut in a plain — 

 a plain so definite that the eye catches its continuity and fails 

 to note the valley save when near its brink. The lesser tribu- 

 taries of the Potomac and of Rock creek flow in narrower val- 

 leys, gorges, and ravines, each proportionate to the length and 

 strength of its stream. Thus the western part of the district is 

 a land of sharp-cut gorges and ravines, with rugged hills be- 

 tween ; while toward the main divides the waterways diminish 

 in depth and the surface becomes a gently undulating plateau. 

 And it is evident that each channel, great and small, was carved 

 by the great or small stream now occupying it ; it is evident, 

 too, that the channels are deep because this part of the land 

 stands high above the level of tide; and after a little study of 

 the steepness of the valley-sides, it is evident also that the period 

 of valley-cutting was not very long — for the steep slope is a sign 

 of rapid stream-work. 



Below Rock creek, Potomac river expands in a tidal estuary 

 flanked by moderately steep bluffs and lined with alluvium or 

 river-mud. Anacostia river occupies a similar but smaller 

 trough, relatively broad and shallow as that of the Potomac ; 



