448 GEOLOGY. 



two of the three divisions do not differ notably from each other, but 

 their topographic and stratigraphic relations are such as to indicate 

 that a very considerable interval of erosion elapsed after the deposition 

 of the first, before the deposition of the second. The third subdi- 

 vision of the original Columbia formation is much younger than the 

 others; is, indeed, of last-glacial and post-glacial age. 



As originally defined, the Columbia formation was said to have a 

 fluvial and an extra-fluvial phase. Applied to the Atlantic coastal 

 plain, this subdivision means that along the valleys leading from the 

 mountains and the Piedmont plateau to the ocean, the Columbia for- 

 mation is thicker and composed of coarser and more heterogeneous 

 materials, than over the inter-stream areas. In the latter position 

 the formation is composed, in considerable part, of materials derived 

 from beds close at hand; in the former, it is composed of materials 

 from all parts of the drainage basin above the point of its occurrence. 

 In the valleys, the gravel, sand, and loam are more distinctly separated 

 from one another than in the inter-valley areas, and stratification is 

 more distinct. To the northward, the heterogeneity of composition 

 increases as the border of the glacial drift is approached. On the 

 whole, the formation thickens toward the coast, but is nowhere known 

 to attain great thickness. 



The oldest subdivision of the original Columbia formation is found 

 at higher levels than the second phase. In the principal valleys it 

 constitutes broad but often rude terraces, which rise up-stream. Thus 

 up the Potomac, the Susquehanna, the Delaware, and other valleys, 

 they rise to altitudes notably above those attained by the extra- 

 valley phase of the formation. 



In the type locality, the Low-level Columbia covers rock terraces 

 100 feet or so below the high-level phase of the series (Fig. 532). The 

 relations of the two subdivisions indicate that extensive erosion fol- 

 lowed the deposition of the high-level Columbia, and that the broad 

 valleys then developed were subsequently aggraded by sediments simi- 

 lar to those of the preceding epoch of deposition. The two deposits 

 are so nearly alike in composition that their separation is based chiefly 

 on their topographic relations. 



In areas of slight relief, the distinction between the high-level 

 and low-level phases of the Columbia is not always marked topo- 

 graphically, and the differentiation is then difficult or even impossible. 



