DEPOSITION ON CONTINENTAL SHELF AND SLOPE § 137 
nental shelf, “built terrace” by Gilbert and others, and ‘‘topset,”’ 
“foreset,” and “bottom-set beds”? by Chamberlin,’ who thus 
establishes the analogy of the continental shelf with deltas. 
Some clear statements have been made as to the origin of that 
portion of the continental shelf fringing eastern North America. 
Willis writes as follows: 
The plateau is composed of sands which are indeed fine near the eastern 
edge, yet are distinctly granular and incoherent. But soundings on the steep 
slope beyond the 1oo-fathom line have brought up very fine silt from the bank 
of which that slope is the surface, and this silt passes at its foot into globigerina 
ooze. The zone of transition from clean sand to silt is as sharp as the edge of 
the slope and is coincident with it. It is evident that the suspended mud 
which escapes beyond the estuaries and sounds of the littoral is swept out until 
the undertow expands over the edge of the escarpment, and is diffused in deep 
water; there the silt forms a great bank 10,000 feet high, with a slope of 3 to 8 
degrees, which has grown seaward during geological ages, and continues to 
expand as erosion continues on the land. 
The structure of this deposit can only be inferred, but it is worthy of 
consideration. The surface of accumulation, to which bedding planes are 
probably parallel, is inclined at a considerable angle, and traverse the bank 
from top to bottom obliquely to the vertical thickness. The direction of the 
growth is outward, not upward. The conditions of deposition are similar 
to those of a delta advancing into fresh water, and the structure of the deposits 
is probably similar to that shown by Gilbert for a fresh water delta.? 
The generalization by the same writer that ‘‘the ocean basins 
are now somewhat overfull . . . . not large enough to hold all the 
waters, which therefore extend over the margins of the continents,’ 
does not necessarily contradict the foregoing; but it omits to state 
that, though the growth may now be outward only, during the 
postulated overflow of the oceanic waters the shelf must have 
maintained itself by upward growth. 
Barrell, also, writes as follows: 
Ocean waves are known to have a perceptible effect to a depth of about 
too fathoms, planing away the shore and the higher parts of the bottom, 
carrying the products of fluviatile and marine erosion outward to deep water. 
tT. C. Chamberlin, ‘‘ Diastrophism and the Formative Processes. VI. Foreset 
Beds and Slope Deposits,” Jour. Geol., XXII (1914), 268-74. 
2 B. Willis, ‘‘ Conditions of Sedimentary Deposition,” Jour. Geol., I (1893), 497-08. 
3 B. Willis, ‘Principles of Paleogeography,”’ Science, N.S., XX XI (1910), 241-60 
(see p. 244). 
