Patterns of deposition at the continental margin 303 



The slope is covered with a fine, uniform clay which has a median diameter of about 

 1 micron, and this texture continues across the bottom of the Sigsbcc I3ccp (Sti.ison, 

 1953). This has not always been the case, and, in fact, in the very recent past such 

 uniformity was not present as the cores indicate. For instance, a brown clay now 

 covers the central part of the eastern Gulf, while unconformably below it, at a depth 

 of only a few inches, hes Globigerina ooze. 



Our ignorance of bottom currents is profound, and any estimate of bottom veloci- 

 ties must in most cases be based on the grain size of the sediment. It can be taken for 

 granted, for instance, that clean, well sorted sands indicate bottom currents of half 

 a knot or better, conversely, silts and clays can only be deposited under lesser veloci- 

 ties. Waves and tides are the most obvious producers of bottom currents although 

 the competence of the former with increasing depth has recently been questioned 

 (DiETZ and Menard, 1951). But there can be no question as to the competence of 

 both over the shallower bottoms, and bottom currents from these two sources cer- 

 tainly have higher velocities in the Atlantic than in the Gulf. The winds of Atlantic 

 storms, except for hurricanes, have, by and large, higher velocities and a longer fetch, 

 and are therefore capable of building waves with longer periods. These in turn are 

 capable of moving sedimentary particles at greater depths, and in far greater quantities 

 along the strand, by beach drifting and strong longshore currents. The tidal range on 

 the open Gulf coast is small compared with the Atlantic (roughly 1-5 feet at Galveston, 

 as with 4-5 feet at Delaware Breakwater), and there must be a consequent reduction 

 in the velocities of the tidal currents. I am well aware that this is tantamount to 

 saying that we really don't know how sedimentary particles move over the deeper 

 bottoms; but move they do, as their texture shows. For all intents and purposes, 

 once they are out of sight their transport can only be inferred. There are few quanti- 

 tative data, and in this direction the oceanographer has so far largely drawn a blank. 



PRIMARY TERRACE CONSTRUCTION 



Every advance and retreat of the sea leaves its impression on the stratigraphic 

 record, and the numerous unconformities and facies changes exhibited by the over- 

 lapping lenses of the Coastal Plain formations are concrete evidence of the numerous 

 oscillations of the strandUne. A change of existing sea level, would, in fact, alter 

 the sedimentary distribution which has been described above. For instance, a deepen- 

 ing of the Atlantic shelf would once more make this platform an area of deposition, 

 as it has been many times before. 



Under the simplest conditions, during rising sea level, onlap means that the younger 

 deposits overlap the older in a shoreward direction. The nearshore sediments are 

 sandy or gravelly, while the offshore deposits resulting from the same sea-level, and 

 therefore of the same age, will be muddy. In vertical cross-section this eventually 

 results in an overlapping of shales on sandstones. It is a time of continuous accumu- 

 lation of sediment in a deepening sea, and the continental shelf grows in thickness 

 as well as in width, as successive layers are added to its surface as well as to its for- 

 ward slope. Conversely, during falling sea-level, offlap means that the younger 

 deposits overlap the older in a seaward direction. As the sea retreats, some of the 

 coarser deposits of the shoal water zone are transported seaward and overlap the 

 older and finer muds which had previously been laid down in deeper water. The 

 surface of the shelf is undergoing erosion, with a reduction in thickness, or perhaps 



