and hydrologic units on land, are present within the upper 122 meters 

 of the shelf subbottom. The presumed Tertiary-Quaternary unconformity 

 is defined by a strong acoustic reflector; near the shore it lies at 

 about 31 meters below MLW and it drops over 61 meters, 12.9 kilometers 

 (8 miles) offshore. 



Buried channels are common to the sea floor of the entire region, 

 and in some instances are present in underlying Tertiary strata. In 

 the Delaware Bay entrance, there are numerous buried channels, most of 

 which reach more than 46 meters below sea level; many of these channels 

 do not coincide with present or historical Delaware channels. Seismic 

 reflection data indicate that at least some of the channels filled later- 

 ally from both the New Jersey and Delaware shelves. On the Maryland 

 inner shelf, numerous small channels, many of which are alined with one 

 another with existing onshore drainage, or with historical inlet sites, 

 lie within 12.6 kilometers (7 nautical miles) of the coast. These 

 channels display a linear relationship between maximum thalweg depth 

 and distance from shore that is applicable in engineering studies. 



The upper 6.1 .meters of the inner shelf sampled by cores consists 

 of terrigenous sands and silts derived from the adjacent Coastal Plain 

 and Piedmont provinces. Four major sediment types are recognized--three 

 are subarkosic arenites varying only in a modal grain size and sorting, 

 and the fourth is a slightly sandy mud. A fifth category comprises 

 those sediments that rarely occur throughout the region, such as peats 

 and iron-stained gravels. Based on fauna, lithology, and stratigraphic 

 position, the environments of deposition represented on the shallow 

 shelf are determined to be modern marine, back barrier, lagoonal, and 

 fluvial. Gray-brown, fine to coarse, well -sorted quartz sand is the 

 dominant lithology on the surface. It decreases in relative abundance 

 with depth. Increases in sand thickness occur locally in shoal areas 

 and correspond directly with topographic relief. The sand overlies 

 poorly sorted, fine sands and muds remnant from Holocene back-barrier 

 and lagoonal deposition which are periodically exposed and eroded on the 

 sea floor. 



The marked similarities in geometry and sediment relationships of 

 all linear shoals on the shelf show them to be genetically related. They 

 are typically 6.1 to 9.1 meters high, 6.4 to 12.9 kilometers (4 to 8 

 miles) long by about 1.6 kilometers (1 mile) wide, oriented north- 

 northeast, and strike an angle of 5° to 30° with the coast. Side slopes 

 are about 1° to 2°, and seaward slopes tend to be steeper than landward 

 ones, except along the shoreface. Individual shoals commonly display a 

 progressive south-to-north change in shape from a well-defined, relatively 

 narrow, single-crested form to a broader, multicrested or furrowed shape. 

 This axial trend is inherited from the shoreface origin of these shoals 

 where growth and bifuraction of these features always occur at the north- 

 ern end. Shoal sand unconformably overlies truncated Holocene lagoonal 

 muds, or in some cases, older, relict, coastal deposits. Coarser con- 

 stituents of the mud unit are incorporated into the base of migrating 

 shoals . 



104 



