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"" f, f SURVEY, , DELAWARE GEOLOGICAL PREPARED BY , NEWARK, IN CO­ UNIVERSITY OF DELAWARE PLAN­ OPERATION WITH THE DELAWARE. STATE NING OFFICE, DOVER 75° 45' 38°30' + . PICKETT, APRIL 1976 REVISED BY THOMAS E JORDAN FROM NENAD SPOLJARIC AND ROBERT R AUGUST 1966 . • ..... I ~ ~ ! i Q ~ ~ GEOLOGIC OF DELAWARE ~ -~ « i 15 g S ! , 0 ~ ! ~ I Severe metamorphism has obscured the exact age relationships I of the rocks of the Piedmont. This regional metamorphism is a 1 1 product of crustal upheavals which built the Appalachian Mountain ~ 1 .. '0 1 System, The major events in the evolution of these rocks 0 ;;; .. 0 1 occurred between 500 and 200 million ago . 8.. I" s H 1 ~ I .. 1 A great period of , of which there is no record in Delaware, .. ~ 'N ~ :. !' I passed before the deposition of the oldest sediments of the c I Coastal Plain, the Potomac Formation, during the latter part of s I" ~ ~ / Early Cretaceous time, about 120 million years ago. Streams ~ / transported clays with interbedded sands from the Appalachians ~ .:: 1 which lay to the northwest. This process continued into Late .. E ~ 1 Cretaceous time and built a wedge with a thickness of about "z ~ -: I 4,000 feet in southern Delaware. A small unconformity, or Ill\! - 1 ::I i I period of nondeposition, separates the Potomac from the over­ KI lying Magothy Formation. The white sands and Iignitic black d -, silts of the Magothy form a distinctive marker indicating the ii I 1 transition from the older sediments to the later marine deposits. I 1 I I The sea now encroached deeply upon the land and remained I over most of Delaware until at least Middle Eocene time; a 1 sequence of varied marine sedimentary rocks was deposited I I essentially continuously during this interval. The oldest of I these sediments form the Matawan Group, consisting of the I Merchantville, Englishtown and Marshalltown Formations. None I I of these persist as distinct entities far into the subsurface, ,I ! I and so the Matawan is relegated to formational status at depth, ::J, as is shown on the cross-section. Above the Matawan is the ~I Monmouth Formation, called Mt. Laurel at the surface. These Cretaceous formations are exposed in the Chesapeake and Delaware Canal and are fossiliferous.

In the northern half of the Delaware Coastal Plain the time E "iJ boundary between the Cretaceous and Tertiary (ca. 63 million years) is located within the Hornerstown Formation. Farther south, in the SUbsurface, the upper part of the Monmouth Formation, the Cretaceous to Eocene Rancocas Group, and the Nanjemoy Formation (subsurface only), grade into a thick clay-silt mass called the Pamunkey Formation (Unit A). The interrelationships of these rocks in the subsurface may be seen on the cross-section. The subsurface Piney Point Formation, a quartz-glauconite sand, which is the youngest of the Eocene units is, in part, the time equivalent of the Pamunkey and Nanjemoy Formations, and is, in part, younger. All of these '.~ _~,,...... ,..,o marine sediments are truncated by a major unconformity; no ~ Oligocene deposits are . During the Miocene, the sea returned to cover the State, "e !o j ~ and deposition of sediment resumed. A sequence of three silts oz ..>­ separated by two sand intervals forms most of the Chesapeake i= " u '" Group. Additional sands and silts are added toward the southeast ~~!'::'-__---- l:l • where the Chesapeake attains its maximum thickness in Delaware of over 1,000 feet. Sands within the Chesapeake supply impor­ tant amounts of water and are named from oldest to youngest, the Cheswold, Frederica, Manokin and Pocomoke aquifers. No sediments of proven Pliocene age are known from the Coastal Plain and the Chesapeake sediments are beveled by another major erosional unconformity. .. During Pleistocene time, the advance and retreat of the 0: continental glaciers brought about profound changes in sea level and in the streams which drained into Delaware. The Columbia ~ Formation, consisting mostly of coarse sand with gravel, was i deposited on the stream-channeled surface formed by the truncated edges of the Cretaceous and Tertiary beds and thus is a ~ sheet or irregular thickness covering much of the Coastal Plain. ~ During a later period of higher-than-present sea level, the sea reworked these continental deposits in the southern part of the z o ; State where they may be subdivided into the Stavtonville unit, 0;:: ~ ;::0 0 Omar and Beaverdam (and probably other) Formations (Jordan, :E'""' .. ~ 1974). Holocene (Recent) sea level rise has resulted in marsh .. , .. filling adjacent to the coast. Total maximum thickness of all x'" 0'" ~ ",0 coastal plain units is about 8,000 feet (at Fenwick Island) 0: o ~ ~ According to the U.S. Bureau of Mines Minerals Yearbook, Delaware produces over 3.5 million dollars worth of sand, gravel, and brick clay annually. Greensand (Hornerstown Fm.l has the potential for use in waste water treatment. o Q Z ..~ ~ ~