322 GEOGRAPHIC DEVELOPMENT OF 



and on the hills toward Tenly ; and most of the broad divides 

 between the head-water ravines in the eastern part of the dis- 

 trict and still further eastward are floored with the deposit. 

 The structure of the deposit indicates that it was arranged by 

 waves and currents along the shore of a shallow ocean, stretch- 

 ing far northward and southward; and its uniformity indicates 

 that the deep valleys of the modern estuaries did not exist, and 

 that it was laid down on smooth sea-bottom, a former smooth 

 land-surface, before the post-Lafayette period of high level. It 

 is composed of materials which are either decomposed and thus 

 degraded chemically (the brown loam), or of great chemic ob- 

 duracy (the quartz and quartzite) ; and the simplest explana- 

 tion of its composition is that its materials were gathered by 

 swiftly flowing streams over a land which had long been sub- 

 jected to the action of chemical rather than mechanical agen- 

 cies — i. e., land lying low for a long period so that running water 

 was sluggish and impotent, while decomposition of the rocks 

 and soils went on apace. 



So the Lafayette formation tells of a time when the land was 

 low, so low that the Atlantic encroached be}^ond the longitude 

 of Washington ; it tells, too, of a seaward tilting of the Piedmont 

 whereby the streams were made swifter than before, so as to 

 tear up residuary soils and ancient quartz ledges. The dis- 

 tribution of the Lafayette indicates that it was originally a con- 

 tinuous mantle stretching from the Piedmont far seaward and 

 northward and southward throughout the Coastal plain ; but 

 that during the subsequent period of high level it was entirely 

 cut away along the larger and many of the smaller streams so 

 that it is now represented only by a series of remnants on the 

 higher divides. 



Thus, the Lafayette formation is a definite record of a great 

 subsidence and seaward tilting of the land ; and at the same 

 time it records a previous geographic condition during which 

 its materials were prepared by chemic processes, and a subse- 

 quent geographic condition during which most of its volume 

 was carried away by running waters. 



THE COMBINED RECORD OF LAND-FORMS AND FORMATIONS 



The margin of the Piedmont plateau reaching the district is 

 a land of fairly smooth contour, albeit trenched by gorges and 

 ravines, and its rocks yield red clays and quartz fragments on 

 decomposition ; and these conditions are in accord with the 



