The geology and hydrology of the area between Wrangle Hill and Delaware City, Delaware, have been the focus of numerous studies since the 1950s because of the importance of the local groundwater supply and the potential environmental impact of industrial activity. In this report, 490 boreholes from six decades of drilling provide dense coverage, allowing detailed characterization of the subsurface geologic framework that controls groundwater occurrence and flow.

The region contains a lower section of tabular Cretaceous strata (Potomac, Merchantville, Englishtown, Marshalltown,and Mount Laurel Formations in ascending order) and a more stratigraphically complex upper section of Pleistocene-to-modern units (Columbia, Lynch Heights, and Scotts Corners Formations, latest Pleistocene and Holocene surficial sediments and estuarine deposits). The lowermost Potomac Formation is a mosaic of alluvial facies and includes fluvial channel sands that function as confined aquifer beds; however, the distribution of aquifer-quality sand within the formation is extremely heterogeneous. The Merchantville Formation serves as the most significant confining layer. The Columbia Formation is predominantly sand and functions as an unconfined aquifer over much of the study area.

To delineate the distribution and character of the subsurface formations, densely spaced structural-stratigraphic cross sections were constructed and structural contour maps were created for the top of the Potomac Formation and base of the Columbia Formation. The Cretaceous formations form a series of relatively parallel strata that dip gently (0.4 degrees) to the southeast. These formations are progressively truncated to the north by more flatly dipping Quaternary sediments, except in a narrow north-south oriented belt on the east side of the study area where the deeply incised Reybold paleochannel eroded into the Potomac Formation.

The Reybold paleochannel is one of the most significant geological features in the study area. It is a relatively narrow sandfilled trough defined by deep incision at the base of the Columbia Formation. It reaches depths of more than 110 ft below sea level with a width as narrow as 1,500 ft. It is interpreted to be the result of scour by the sudden release of powerful floodwaters from the north associated with one or more Pleistocene deglaciations. Where the Reybold paleochannel cuts through the Merchantville confining layer, a potential pathway exists for hydrological communication between Columbia and Potomac aquifer sands.

East of the paleochannel, multiple cut-and-fill units within the Pleistocene to Holocene section create a complex geologic framework. The Lynch Heights and Scotts Corners Formations were deposited along the paleo-Delaware River in the late Pleistocene and are commonly eroded into the older Pleistocene Columbia Formation. They are associated with scarps and terraces that represent several generations of sea-level-driven Pleistocene cut-and-fill. They, in turn, have been locally eroded and covered by Holocene marsh and swamp deposits. The Lynch Heights and Scotts Corners Formations include sands that are unconfined aquifers but complicated geometries and short-distance facies changes make their configuration more complex than that of the Columbia Formation.

This map shows the saturated thickness of the water-table aquifer. This aquifer consists of the deposits of the Columbia Formation and those portions of the Magothy and Englishtown-Mt. Laurel formations, and Rancocas Group that are hydraulically connected with the Columbia deposits (see Groot, Demicco, and Cherry, 1983). For example, large, saturated thicknesses in the zone trending northeast-southwest near Townsend reflect the addition of the sands of the Rancocas Group to the total thickness of the sands of the overlying Columbia Formation.

In recent years there has been a renewed interest by both amateur and professional paleontologists in the rich upper Cretaceous exposures along the Chesapeake and Delaware Canal, Delaware (Fig. 1). Large quantities of fossil material, mostly clams, oysters, and snails have been collected as a result of this activity. Recent dredging (1978, 1981) by the United States Army Corps of Engineers has helped expose a rich vertebrate fossil assemblage. It includes representatives from the classes Reptilia, Osteichthyes, and Chondrichthyes. An extensive literature search has revealed that a wealth of information exists which would aid in the identification of the vertebrate fossils of Delaware.

The Delaware Geological Survey, in cooperation with the U. S. Bureau of Mines, has investigated glauconite-bearing greensand deposits in Delaware for several years. The purpose of this effort is to find possible practical uses for this potentially important mineral resource. This report briefly describes the preliminary results of one phase of the study: application of greensands to the purification of industrial waste waters.

This report is a compilation of four papers presented by DGS staff members at the Baltimore Meeting of the Northeastern Section of the Geological Society of America, March, 1974.

This Bulletin presents the subsurface stratigraphy of the post-Potomac Cretaceous and Tertiary rocks of the Atlantic Coastal Plain of central Delaware, between the Chesapeake and Delaware (C & D) Canal and Dover. Geophysical log correlations supported by biostratigraphic and lithologic data from boreholes in Delaware and nearby New Jersey provide the basis for the report. The stratigraphic framework presented here is important for identifying subsurface stratigraphic units penetrated by the numerous boreholes in this part of Delaware, particularly those rock units that serve as aquifers, because such knowledge allows for better prediction at ground-water movement and availability. Also, accurate stratigraphy is a prerequisite for interpreting the geologic history of the rocks and for the construction of maps that depict the structure and thickness of each unit.

Geology and hydrology of the Middletown-Odessa Area, Delaware.

Geology and hydrology of the Chesapeake and Delaware Canal Area, Delaware. There are 2 sheets in this series.

This map shows the surficial geology of New Castle County, Delaware at a scale of 1:100,000. Maps at this scale are useful for viewing the general geologic framework on a county-wide basis, determining the geology of watersheds, and recognizing the relationship of geology to regional or county-wide environmental or land-use issues. This map, when combined with the subsurface geologic information, provides a basis for locating water supplies, mapping ground-water recharge areas, and protecting ground and surface water. Geologic maps are also used to identify geologic hazards, such as sinkholes and flood-prone areas, to identify sand and gravel resources, and for supporting state, county, and local land-use and planning decisions.

Geology map showing the sub-surface geology of the Chesapeake and Delaware Canal Area, Delaware.

This dataset contains the geologic polygons used for the creation of DGS Geologic Map 13. This dataset shows the surficial geology of New Castle County, Delaware, at a scale of 1:100,000.