This map was created from published 1:100,000-scale geologic maps of New Castle and Kent Counties, and the most current knowledge about the surficial geology of Sussex County. Sussex County was compiled from published 1:24,000-scale geologic maps of various quadrangles and recent fieldwork. (The current version of Sussex County seen here is the same as published in RI76.)

The Delaware Geological Survey (DGS) published the surficial geology of the state of Delaware at a scale of 1:100,000 for New Castle and Kent counties (Ramsey, 2005, 2007). Maps at this scale are useful for viewing general geologic framework on a county-wide basis, determining the geology of watersheds, and recognizing the relationship of geology to county-wide environmental or land-use issues. These maps, when combined with subsurface geologic information, provide a basis for locating water supplies, mapping groundwater recharge areas, and protecting ground and surface water. Geologic maps are also used to identify geologic hazards, such as flood-prone areas, to identify sand and gravel resources, and for supporting state, county, and local land-use planning decisions. Portions of Sussex County have previously been mapped at a scale of 1:24,000. Field work is in progress in eastern Sussex County and will be complete in the near future.

GIS Methods Used to Create this Map

Heads-up digitizing was performed in ArcGIS from maps on which the geologist drew geologic unit boundaries. Geologic unit boundaries are determined through field interpretation of well and borehole data, aerial photographs, as well as contours from LiDAR. Data compilation methods included merging existing geologic attributes (Ramsey, 1993, 2001, and 2003; Andres and Ramsey, 1995; Schenck et al., 2000), clipping polygons to the state boundary, and extracting and merging the swamp/marsh attribute from the USGS National Hydrography Dataset (NHD). Existing geologic digital data and the NHD are both mapped at a scale of 1:24,000. These areas were designed to be viewed at a scale of 1:100,000 (polygons with an area less than 25,000 square meters were deleted). Base map data (not shown) were compiled for geographic location and editing purposes. Datasets included hydrography, transportation and boundaries for the state of Delaware.  Hydrography data were also generalized to a scale of 1:100,000 using the same method for generalizing the geologic data. A few deleted hydrography polygons were reintegrated into the dataset since they connected stream line data.

The State of Delaware is located within two physiographic provinces, the Appalachian Piedmont and the Atlantic Coastal Plain. Most of the state lies within the Coastal Plain; it is only the hills of northern New Castle County that lie within the Piedmont. Piedmont means foothills. Delaware's rolling hills, which rise to over 400 feet above sea level, are a part of the foothills of the Appalachian Mountains. The rocks at the surface in the Piedmont are old, deformed, metamorphic rocks that were once buried in the core of an ancient mountain range. This range formed early in a series of tectonic events that built the Appalachians between about 543 and 250 million years ago. During an early event, called the Taconic orogeny, an offshore chain of volcanoes collided with the ancient North American continental margin to push up a gigantic mountain range that was as tall as the Alps or the Rockies of today. Geologists date the Taconic orogeny between 470 and 440 million years ago. The Taconic orogeny is important to our understanding of the geology of Delaware, because during this event, the rocks of Delaware's Piedmont were deeply buried under miles of overlying rock and metamorphosed by heat from the underlying mantle. Since that time, rivers and streams have carried the erosional products, mostly sand, clay, and gravel, from the mountains onto the Atlantic Coastal Plain and continental shelf. As the mountains wear down, the buried rocks rebound and rise to the surface. Thus what we see in the Piedmont today are old, deformed, metamorphic rocks that were once buried deep within an ancient mountain range.

The oldest rocks in Delaware also preserve the history of an earlier mountain-building event called the Grenville orogeny. This event occurred approximately one billion years ago. Within Delaware's Piedmont, five distinct rock units can be recognized: (1) rocks of the volcanic arc (Wilmington Complex), (2) rocks formed from the mud and sand deposited in the deep ocean that existed between the volcanic arc and the ancient continental margin (Wissahickon Fm.), (3 & 4) rocks that were once sand and carbonates (calcite and dolomite) lying on the shallow shelf of the ancient continental margin (Setters Fm. and Cockeysville Marble), and (5) rocks of the ancient North American continent (Baltimore Gneiss). The names given to these units indicate the geographic area where they were first identified. Because of the total absence of fossils, determining the age of the Piedmont rocks has always been difficult. Age must be determined either by correlation with units elsewhere in the Piedmont, or by calculating radiometric ages from measurements of radioactive elements and their decay products (usually uranium-lead). Fall Line Delaware's Piedmont ends at the Fall Line where the metamorphic rocks dip under and disappear beneath the sediments of the Coastal Plain. The Fall Line roughly follows Kirkwood Highway, Route 2, across the state between Newark and Wilmington. Parallel to the Fall Line is a narrow zone where rapids and waterfalls are common. Delaware's early settlers built near the rapids using the energy generated from the falls to power their mills. Explorers and sea captains of the colonial period found the bays, rivers, and streams of the Coastal Plain navigable until they reached the fall zone. Here it was necessary to dock their ships, unload the cargo, and move it inland by rail or road. Many of the settlements that grew around these unloading sites later became large cities. Richmond, Washington, Baltimore, Wilmington, and Philadelphia are cities built around ports located along the fall zone. Atlantic Coastal Plain Delaware's Coastal Plain rises to about 100 feet above sea level. Its streams drain into the Delaware River or Bay, and for much of their length they are tidal. The Coastal Plain is made up of sediments, mostly silt, sand, and gravel, that have been eroded off the Piedmont and adjacent Appalachian Mountains. In cross section these sediments form a southeastward thickening wedge that increases from 0 feet at the Fall Line to over 10,000 feet along Delaware's coast. Offshore, on the continental shelf, the sediments become even thicker with reported thicknesses of 8 to 10 miles. In the 1970s and 1980s, exploratory drilling in this thick pile of sediments found no commercial deposits of gas or oil, although one noncommercial gas deposit was discovered. Underlying the sand, silt, and gravel of the Coastal Plain lie consolidated rocks that geologists refer to as the basement. While test drilling into the basement near the Fall Line, metamorphosed and deformed rocks similar to those of the Piedmont were extracted; therefore, the basement is probably a subsurface extension of the Piedmont.

The contact between the sediments of the Coastal Plain and the basement is called an unconformity. An unconformity represents an interval of time during which no sediments are preserved to record geological events. In the Coastal Plain, the oldest sedimentary rocks beneath Delaware's coast are of Late Jurassic to Early Cretaceous age (140-150 million years), and the basement rocks they overlie are of Paleozoic age, older than 245 million years. During the time represented by this unconformity, the rocks we see in the Piedmont today reached the earths surface as approximately 7 to 13 miles of overlying rock were removed by erosion allowing the buried rocks to rise to the surface in compensation. The oldest Coastal Plain sediments observed in Delaware are river-deposited sediments. These sediments were eroded from the Appalachian Mountains to the west, transported to the southeast by rivers, and deposited where the rivers met the ocean to form a delta. On top of the river sediments a sequence of marine silt and sand deposits records the rise and fall of the sea level many times during a period of over 80 million years, from the Late Cretaceous until the end of the Tertiary Period, about 2 million years ago. On top of all of these sediments is a thin veneer of young sand and gravel that was carried into Delaware by glacial outwash during the Ice Age. Glacial ice did not advance into Delaware, but melt-water pouring off the glacier fronts carried great quantities of sand, silt, and gravel over southern Pennsylvania and Delaware. Delaware's largest mineral resource is the sand and gravel deposited from the glacial outwash. Most of the information and figures obtained from SP20 Delaware Piedmont Geology

Sillimanite, a white to tan to green aluminum silicate, (Al₂SiO₅) occurs in high temperature, aluminum-rich metamorphic rocks. In Delaware, it is found in the Hoopes Reservoir and Brandywine Springs areas.

In 1977, the Delaware General Assembly, acting on a proposal by the Delaware Mineralogical Society, established sillimanite as the Delaware State Mineral. This act recognizes the geological and mineralogical significance of the large masses of this mineral found as boulders at Brandywine Springs, an occurrence that was recognized as important in the 6th (1892) edition of Dana's System of Mineralogy. The Brandywine Springs boulders are remarkable for their size and purity. The sillimanite has a fibrous texture reminiscent of wood and could potentially be cut into cabochon gems showing a chatoyant ("cat's eye") effect. Sillimanite is not mined as an ore or raw material In Delaware.

Sillimanite forms at temperatures greater than 550^oC, and its coarse grain size at Brandywine Springs indicates a prolonged period of high-temperature metamorphism of the rocks. These conditions are confirmed by the absence of muscovite and the occurrence of the pair sillimanite + K-feldspar (second sillimanite zone) in the schists/gneisses and by the presence of micropegmatites in the fold noses of the schists/gneisses, which are interpreted as partial melts of the rock under high-temperature conditions.

Delaware Facts and Symbols from Delaware.gov

Belemnite is the common name applied to an extinct order (Belemnoida) of mollusks belonging to the cephalopod class. Modern cephalopods include the squid, octopus, and pearly Nautilus. The belemnoid animal was most closely related to the squid as it had an internal shell covered by a leathery skin, tentacles that pointed forward, and a siphon that expelled water forward thus moving the animal backward by jet propulsion. The internal shell of the belemnoid was cone-shaped and divided into chambers that were gas-filled for maintaining buoyancy in the sea. The chambered shell had a blade-like forward extension that is seldom preserved as a fossil. The most common fossilized part of the internal shell is called the "guard" or "cigar" consisting of a massive, generally brown-colored, subcylindrical structure called the rostrum that encloses the chambered shell and extends to the rear where it tapers to a conical apex. The rostrum served as a counter-weight to the buoyancy provided by the chambered shell and also for protection of that delicate shell. Belemnoids reached their greatest abundance and diversity during the Jurassic and Cretaceous periods. On July 2, 1996, Belemnitella americana was named as the official fossil of Delaware. The Martin Luther King, Jr. Elementary School (Wilmington) third grade Quest students of Kathy Tidball suggested honoring the ancient and noble belemnite as our State fossil.(Delaware Code Title 29 § 314) Belemnites have been found abundantly in the exposures of the Mount Laurel Formation along the banks of the Chesapeake and Delaware Canal in Delaware, east of St. Georges. The fine-grained sands and silts of the Mount Laurel were deposited in a shallow sea during the Late Cretaceous time around 70 million years ago. The fossil belemnite species found here is Belemnitella americana. Sometimes, almost complete belemnite guards can be found, similar in size and shape to a pencil, pointed at one end, but flaring at the other end (if preserved) and partly hollow in the center where the chambered shell was located. Often, only rod-like broken sections of the brown rostrum are found. In Delaware, the best place to look for Belemnitella americana is in the dredge spoil piles on the north side of the Chesapeake and Delaware Canal, just west of St. Georges and also just east of the north side of the Reedy Point Bridge. For more information:

• Belemnites on Wikipedia
• Delaware Facts and Symbols from Delaware.gov

For many years, there has been a question in the minds of some Delawareans as to whether Delaware's highest elevation is Centreville or on Ebright Road.

The Delaware Geological Survey (DGS) at the University of Delaware, through its relationship to the National Geodetic Survey (NGS) has determined that the highest monumented spot in Delaware is located on Ebright Road, near the Pennsylvania state line. Ebright Road is north of Namaans Road, east of route 202.

The Ebright Road benchmark (NGS disk EBRIGHT AZI) was found to be 447.85 feet above sea level. Many people consider Centreville, Delaware to be the highest point in the state; however, a benchmark at Centreville has an elevation of only 445.58 feet making the Ebright Azimuth disk more than two feet higher.

Additional surveying by NGS and DGS personnel and 2007 LiDAR-derived contours indicate that areas just west of Ebright Road are at least two feet higher than the Ebright Azimuth benchmark elevation at around 450 feet. Therefore, according to DGS scientists, the highest actual elevation in Delaware is around 450 feet above sea level. These areas are shown in the map below. The areas within the 450 foot contour lines are highlighted in red.

It's a common misconception that Delaware's High Point at Ebright is the lowest of all US States' high points. This was based on a question in the popular trivia game Trivial Pursuit. In fact, Florida has the lowest high point at 345 feet (Britton Hill, on the Panhandle, near Lakewood.) Actually, Delaware would have the third lowest high point if Washington DC became a state (Fort Reno at 429 feet).

For more information:

• NGS official benchmark data sheet

• Ebright Azimuth Wikipedia entry
• Ebright Azimuth Wikimapia entry
• DelawareOnline: The Backstory on Delaware's Highest Point