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.

Groundwater levels are basic information needed for evaluating water conditions and for basic and applied research. For these efforts, water levels are being measured statewide in wells completed in multiple aquifers. Some wells are measured for specific projects, such as the Coastal Aquifers Salinity Project and the Water Conditions program, while other wells are measured so that staff can maintain long-term records of groundwater levels for evaluation of trends.

Table contains summary data from wells having 4 or more water level observations.

Ground-water levels are basic information needed for evaluating water conditions and for basic and applied research. For these efforts, water levels are being measured statewide in wells completed in multiple aquifers. Some wells are measured for specific projects, such as the Coastal Aquifers Salinity Project and the Water Conditions program, while other wells are measured so that staff can maintain long term records of ground-water levels for evaluation of trends. Table contains summary data from wells having 100 or more water level observations.

Groundwater levels are basic information needed for evaluating water conditions and for basic and applied research. For these efforts, water levels from various aquifers are being measured statewide. Some wells are measured for specific reasons, such as for the Coastal Aquifers Salinity Project and the Water Conditions Report, while other wells are measured so that staff can maintain long-term records of groundwater levels for evaluation of trends.

This vector data set contains the rock unit polygons for the surficial geology in the Delaware Coastal Plain covered by DGS Geologic Map No. 16 (Fairmount and Rehoboth Beach quadrangles). The geologic history of the surficial units of the Fairmount and Rehoboth Beach quadrangles is that of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology reflects this complex history both onshore, in Rehoboth Bay, and offshore. Erosion during the late Pleistocene sea-level low stand and ongoing deposition offshore and in Rehoboth Bay during the Holocene rise in sea level represent the last of several cycles of erosion and deposition.

To facilitate the GIS community of Delaware and to release the geologic map of the Fairmount and Rehoboth Beach quadrangles with all cartographic elements (including geologic symbology, text, etc.) in a form usable in a GIS, we have released this digital coverage of DGS Geological Map 16. The update of earlier work and mapping of new units is important not only to geologists, but also to hydrologists who wish to understand the distribution of water resources, to engineers who need bedrock information during construction of roads and buildings, to government officials and agencies who are planning for residential and commercial growth, and to citizens who are curious about the bedrock under their homes. Formal names are assigned to all rock units according to the guidelines of the 1983 North American Stratigraphic Code (NACSN, 1983).

IS7 is a foldout brochure that briefly discusses the background and current activities of the DGS. Specifically, the following major programs are listed: Geology, Hydrology, Cartographic Information, Geologic Hazards, Seismograph Network, Outer Continental Shelf, Mineral Resources, Well Records and Sample Library, Publications, and Joint-funded Programs.

Delaware is not in a seismically active region but, even here, earthquakes occur because of sudden adjustments in the earth's crust. Even though such movements can be startling they yield important information about the behavior of the earth and the potential for future events. The Delaware Geological Survey continually monitors seismicity in the State because of both the scientific value of the information and to help address the concern of residents and other agencies over possible effects.

The Delaware Geological Survey Cartographic Information Center has made the DGS a focal point for questions concerning the availability of all types of aerial photography, thematic maps, planimetric maps, topographic maps, historic maps, LANDSAT imagery, space imagery, side looking aerial radar imagery (SLAR), and geodetic control. Sources of maps, charts, aerial photography, boundary mark information, and vertical geodetic control can be obtained for your area of interest within the State through the Center's computer-searchable data bases. In addition, through the Center's affiliation with the U. S. Geological Survey's National Cartographic Information Center (NCIC) in Reston, Virginia, we can locate all cartographic materials covering the United States produced by federal agencies.