113 113 9 9 9 TR 9 TR 9 30 30 30 5 5 5 Redden State Forest Redden State Forest Burton Tract Doe Bridge Nature Preserve Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Cem Georgetown Georgetown Georgetown Layton Park ZOAR RD CEDAR LN WOOD BRANCH RD ZOAR RD HOLLYVILLE RD MOUNT JOY RD MORRIS MILL RD DEEP BRANCH RD AVALON RD JOHNSON RD ANDERSON CORNER RD MARTIN FARM RD PRETTYMAN RD PETTYJOHN RD HUDSON RD CARPENTER RD SANDHILL RD BRIARWOOD RD SHINGLE POINT RD SPRINGFIELD RD GRAVEL HILL RD GRAVEL HILL RD NEPTUNE RD HUFF RD WILSON RD RUDD RD SHINGLE POINT RD BENNUM SWITCH RD HOLLIS RD HOLLIS RD STEINER RD DODDTOWN RD RUST RD SHORT RD SIMPLER BRANCH RD LAWSON RD AVALON RD PETERKINS RD DEEP BRANCH RD ZOAR RD KRUGER RD BUNTING RD PINEY GROVE RD GOVERNOR STOCKLEY RD GOVERNOR STOCKLEY RD LAWSON RD Qtb Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Tbd Qsw Tbd Tbd Qsw Qsw Qd Qd Qd Qd Qd Qd Qd Qd Qd Qd Qd Qd Qcb Qcb Qcb Qcb Qcb Qd Qd Qd Qd Qd Qd Qd Qd Qd Qd Qd Qd Qd Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Qsw Tbd Qlh t Qlh Qlh Qlh Qlh Qlh Qlh Qlh Qlh Qd Qd Qd Qd Qd Qd Qd Qd Qlh t Qlh t Qlh t Qlh Qlh Qlh t Qlh t Qlh t Qlh t Qlh t Qlh t Qlh Qlh Qlh t Qlh t Qlh t Qlh t Qlh t Qlh t Qlh t Qlh Tbd Qlh t Qlh t 50 50 50 50 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 46 40 46 46 46 46 46 40 40 40 30 30 30 30 30 30 30 30 30 36 36 36 36 36 36 30 36 36 36 36 36 36 36 30 35 33 38 32 37 33 33 40 45 45 38 33 43 42 44 42 43 39 34 28 33 34 44 37 28 33 Morris Millpond Cow Bridge Branch Branch Stockley Branch Mirey Horse Pound Swamp Ditch Alms House Ditch Gills Branch Walls Ditch McGee Ditch Branch Peterkins Branch Deep Branch Simpler Branch Welsh Ditch Sockorockets White Oak Swamp Ditch Swan Creek Branch Unity Branch Bundicks Beaverdam Creek Round Pole Branch Dutton Ditch Ingram Branch Savannah Ditch Pf25-e Pg21-k Pg21-e Pg21-f Pg21-d Pg22-d Pg22-c Pg32-c Pg32-f Pg33-i Pg33-g Pg33-h B' B Of13-n Of13-q Of13-r Of14-s Of14-f Of14-m Of14-j Of14-k Of15-n Of15-m Of15-l Of15-14 Of15-v Of15-u Og11-h Og11-k Og21-d Og21-g Og21-h Og21-02 Og22-a Og22-e Og22-f Og22-d Og23-d Og23-c Og23-b Og24-c Og24-d Og24-a Og25-c A' A Colors for geologic formations on the cross sections appear lighter than shown on the map explanation and stratigraphic chart because they do not include the shading effect of the DEM used on the map. LITHOLOGIC SYMBOLS (cross sections) clean sand silt pebble fine to coarse silty to clayey sand with scattered clay laminae very fine to fine moderately silty sand Sandpit State Forest and Wildlife Areas Municipal Boundary SYMBOLS (map) Og22-a Soil auger boring Of15-14 Core/drill hole Contact Tbd Qlh t Qlh Qtb Qcb Qd Qsw Tbd Qlh Qtb Qcb Qd Qsw Holocene Pleistocene Pliocene QUATERNARY TERTIARY Qlh t Kilometers 0 1 2 1 Miles 0 1 1 Feet 10,000 0 1,000 1,000 2,000 0.5 0.5 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10-foot Index with a 6-foot Intermediate Contour Interval 2-foot contour data are available for this map from the Delaware DataMIL at http://datamil.delaware.gov NORTH AMERICAN DATUM OF 1988 (NAVD88) SCALE 1:24,000 1992 MAGNETIC NORTH DECLINATION AT CENTER OF SHEET Georgetown Harbeson Millsboro Trap Pond Frankford Fairmount Lewes Ellendale Milton Quadrangle Location 2011 Kelvin W. Ramsey and Jaime L. Tomlinson by GEOLOGIC MAP OF THE HARBESON QUADRANGLE, DELAWARE MAP CREDITS Base Map Delaware state plane coordinate system Transverse mercator projection North American Datum of 1983 (NAD 83) HARN DataMIL Layers http://datamil.delaware.gov The Delaware Department of Transportation Centerline and Railroads for Delaware, 2010 The Delaware Office of State Planning Coordination Delaware Municipal Boundaries, 2010 USGS National Hydrography Dataset, 2005 USGS Delaware LiDAR Contours, 2005 USGS Delaware Miscellaneous Features, 1993 Delaware Department of Agriculture State Forest Areas, 2009 Delaware Division of Natural Resources and Environmental Control Wildlife Areas, 2009 Other Cartography by Lillian T. Wang, Delaware Geological Survey Edited by Stefanie J. Baxter, Delaware Geological Survey SWAMP DEPOSITS Found in the upper reaches of the modern stream valleys. Consist of 1 to 3 ft of gray to brown, silty and clayey gravelly sand at the base overlain by organic-rich, fine to coarse sand. In some of the larger valleys, the unit has several ft of organic silt at the top. Deposits are up to 15 ft thick in the larger stream valleys and less than 5 ft thick in the smaller tributaries. Holocene. DUNE DEPOSITS Located in raised parabolic to linear features. Consist of white to light-yellow, well-sorted, medium to fine sand. Laminae of coarse sand are common. Thin, brown soil lamellae are commonly found at depths of 1 to 3 ft within the dunes. Deposits up to 16 ft thick. The deposits are eolian features related to cold-climate processes when arboreal vegetation was scarce and winds blew sand dunes across the landscape. Some of the dunes, which have well-developed and deep (>3 ft) soil profiles, may be older than latest Pleistocene. Middle Pleistocene to Holocene. CAROLINA BAY DEPOSITS Found in circular features in the northern half of the Harbeson Quadrangle. They consist of raised rims (dunes) of well-sorted, medium to fine sand with silty sand in the interior of the circular features. A few of the features contain either seasonal or year-round bodies of water where the water table is high. The deposits are less than 5 ft thick in their interiors and up to 10 ft thick where the sand rims are best developed. Features are related to cold-climate processes during the latest Pleistocene to earliest Holocene (Ramsey, 1997). Latest Pleistocene to Holocene. TURTLE BRANCH FORMATION Clean, well-sorted, white to pale-yellow, fine sand grading down to interlaminated fine to coarse sand with opaque heavy mineral laminae, granules, and pebbles at its base. In the Harbeson Quadrangle, it is commonly less than 5 ft in thickness. The Turtle Branch Formation is distinguished from the adjacent Beaverdam Formation by better sorting and the absence of the white, silty matrix, which is characteristic of the Beaverdam. It is distinguished from adjacent and overlying dune deposits by having a better developed soil profile and the common presence of opaque heavy mineral laminae which are rare in the dune deposits. DEM images of the map area show distinctive circular features on the surface of the Turtle Branch Formation which are not readily apparent in the field. These features are likely related to periglacial eolian and freeze-thaw activity upon the surface of the Turtle Branch Formation. In places, the Turtle Branch Formation is completely removed in the center of these features and the underlying Beaverdam Formation is exposed; however, the entire area is mapped as Turtle Branch Formation. Determination of the presence of Beaverdam Formation at the land surface requires detailed mapping on a scale larger than that of this map. The Turtle Branch Formation is interpreted to be a fluvial to tidal and shoreline deposit, which has been greatly modified by periglacial activity in the map area. Middle Pleistocene. DELAWARE GEOLOGICAL SURVEY University of Delaware, Newark John H. Talley, State Geologist DELAWARE GEOLOGICAL SURVEY GEOLOGIC MAP OF THE HARBESON QUADRANGLE, DELAWARE GEOLOGIC MAP SERIES NO. 17 The project was funded in part by the cooperative agreement between the Association of American State Geologists and U.S. Geological Survey under STATEMAP program grant G09AC00166. This project would not have been possible without the cooperation of the staff of the Delaware Department of Transportation, Redden State Forest, Sussex County Engineering Dept., and the DNREC Wildlife Areas. Paul S. McCreary coordinated the drilling for the project. DGS project personnel and students who assisted in field work and data collection included Stephanie Nebel and Curt Romanchok. Groundwater recharge and water table mapping by A.S. Andres and Andrew Klingbeil of the DGS generated much of the subsurface data for the area (Andres and Klingbeil, 2006). Acknowledgements LYNCH HEIGHTS FORMATION Loose, fine to very fine, moderately silty, pale-yellow to yellow sand that ranges from 2 to 15 ft in thickness north of US Rt. 9 and east of Gravel Hill Road (Rt. 30). To the west of Gravel Hill Road, these sands are interbedded with a body of compact, gray to greenish-gray, clayey silt to silty clay from 2 to 15 ft thick that forms a topographic high oriented roughly north-south. On the western margin of this clay body a paleochannel with a similar orientation is filled with 5 to 15 ft of gravelly sand grading upward to fine to medium sand. Interpreted to be estuarine sands, tidal flat or estuarine muds, and tidal channel sands, respectively. Adjacent to Deep Branch, Peterkins Branch, and Cow Bridge Branch, the Lynch Heights Formation consists of up to 20 ft of pale-yellow, clean, gravelly sand grading up to medium to coarse sand and then to fine to medium sand. The sands continue southward into the Millsboro Quadrangle. Interpreted to be fluvial deposits related to the cutting and filling of the upper reaches of the ancestral Indian River during the middle Pleistocene. Where mapped as Qlh t in the eastern half of the Harbeson Quadrangle, the Lynch Heights Formation consists of a thin (< 5 ft thick) layer of heterogeneous deposits ranging from reddish-brown, pale-yellow, and light-gray, silty, clayey, very coarse to fine sand, to pale-yellow to light-gray, gravelly sand to sandy gravel. These deposits are considered to be the result of local reworking of the underlying Beaverdam Formation sediments. The Lynch Heights Formation is differentiated from the Beaverdam by the lack of the characteristic white silt matrix of the Beaverdam. It is also differentiated by a layer of coarse sand to gravel that is found at the base of the unit overlying the typical Beaverdam sands. The sands are interpreted to be shallow water deposits along the margins of an estuary, and later modified by eolian activity related to several periods of periglacial climate. Middle Pleistocene. BEAVERDAM FORMATION Heterogeneous unit ranging from very coarse sand with pebbles to silty clay. The predominant lithologies at the land surface are white to mottled light-gray and reddish-brown, silty to clayey, fine to coarse sand. Laminae and beds of very coarse sand with pebbles to gravel are common as are laminae and beds of bluish-gray to light-gray silty clay. In a few places near the land surface, but more commonly in the subsurface, beds ranging from 2 to 20 ft thick of finely laminated, very fine sand and silty clay are present. The sands of the Beaverdam Formation have a white, silty matrix that gives samples a milky appearance when wet. This white, silty matrix is the most distinguishing characteristic of the unit and readily differentiates the Beaverdam Formation from the adjacent cleaner sands of the Lynch Heights and Turtle Branch Formations. The Beaverdam Formation is interpreted to be a late Pliocene fluvial to estuarine deposit (Groot et al., 1990; Ramsey, 2010a, b) and ranges from 50 to 100 ft thick in the map area. Late Pliocene. Discussion The complex geologic history of the surficial units of the Harbeson Quadrangle is one of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology is further complicated by periglacial activity that produced dune deposits and Carolina Bays scattered throughout the map area. The Beaverdam Formation consists of stacked 1- to 5-ft thick beds of very coarse sand and gravel that commonly fine upwards to fine to medium sand and rarely to very fine silty sand to silty clay. These types of deposits are typical of either fluvial or estuarine environments (Ramsey, 2010a, b). Elsewhere in Delaware, rare burrows have been observed in the Beaverdam Formation indicating at least a marginal estuarine setting (DGS unpublished data; Owens and Denny, 1979). The age of the Beaverdam Formation is uncertain due to the lack of age-definitive fossils within the unit; however, stratigraphic relationships in Delaware indicate that it is no older than late Miocene and no younger than early Pleistocene and is most likely late Pliocene (Ramsey, 2010a, b). The Lynch Heights Formation is a composite unit consisting of deposits from two sea-level high stands approximately 400,000 yrs B.P. and 320,000 yrs B.P. (Groot et al., 1990; Ramsey, 2010a) which cannot be differentiated in this map area. The Lynch Heights Formation in the map area consists of three distinct lithofacies associations. In the eastern portion of the Harbeson Quadrangle, the Lynch Heights Formation (Qlh t ) is a thin unit (< 5 ft thick) consisting of fine to medium sand that coarsens to a coarse sand. In places, a pebbly sand to pebble gravel containing abundant opaque heavy minerals is found at the base of the unit overlying the Beaverdam Formation. This portion of the Lynch Heights Formation is interpreted to be a shallow water deposit consisting of sand eroded from the Beaverdam Formation as the shoreline transgressed during one or both of the sea-level high stands. In the north-central portion of the Harbeson Quadrangle, the Lynch Heights Formation consists of paleovalley fill (cross section A-A') with tidal channel sands, tidal flat and estuarine muds, and estuarine sands. These sediments were deposited in a drainage marginal to an ancestral Delaware estuary (Ramsey, 2011). In the south-central portion of the quadrangle, fluvial deposits flank the modern drainage (cross section B-B') and are related to the initial stages of drainage formation during the middle Pleistocene. Sediments closest to the modern drainage are thicker deposits (4 to >11 ft) of clean, loose, well-sorted sand with few to abundant heavy minerals. Away from the drainage, deposits become less well-sorted and siltier, as shown in the B-B' cross section. Dune deposits are fine to medium, well-sorted sands that are found throughout the map area. The dunes have a pronounced surficial expression as linear and hook-shaped features that rise above the surrounding landscape. Some of these dunes are probably latest Pleistocene to early Holocene in age (Andres and Howard, 2000) but some could be contemporaneous with deposition of the Lynch Heights Formation or the late Pleistocene Scotts Corners Formation (Ramsey, 2010a). Dune features are also associated with the rims of Carolina Bays that are found in the northern part of the map area. Some of the features mapped as Carolina Bays may be parabolic dunes with blown-out low areas on the windward side. Because the features are generally circular in shape and are consistent in size with Carolina Bays mapped elsewhere (Ramsey, 2001, 2003, 2010b), they are mapped as Carolina Bays. Both the dunes and the Carolina Bays are considered to be cold-climate related features located where winds moved sand across a landscape barren of forests (Ramsey, 1997). The exact process by which the distinctive circular shape of the Carolina Bays was formed is unknown. EXPLANATION References Cited Andres, A.S., and Howard, C.S., 2000, The Cypress Swamp Formation, Delaware: Delaware Geological Survey Report of Investigations No. 62, 13 p. Andres, A.S., and Klingbeil, A.D., 2006, Thickness and transmissivity of the unconfined aquifer of eastern Sussex County, Delaware: Delaware Geological Survey Report of Investigations No. 70, 19 p. Groot, J.J., Ramsey, K.W., and Wehmiller, J.F., 1990, Ages of the Bethany, Beaverdam, and Omar formations of Southern Delaware: Delaware Geological Survey Report of Investigations No. 47, 19 p. Owens, J.P., and Denny, C.S., 1979, Upper Cenozoic deposits of the central Delmarva Peninsula, Maryland and Delaware: U.S. Geological Survey Professional Paper 1067-A, 28 p. Ramsey, K.W., 1997, Geology of the Milford and Mispillion River Quadrangles, Delaware: Delaware Geological Survey Report of Investigations No. 55, 40 p. _____2001, Geologic Map of the Ellendale and Milton Quadrangles, Delaware: Delaware Geological Survey Geologic Map Series No. 11, Scale 1:24,000. _____2003, Geologic Map of the Lewes and Cape Henlopen Quadrangles, Delaware: Delaware Geological Survey Geologic Map Series No. 12, Scale 1:24,000. _____2010a, Stratigraphy, correlation, and depositional environments of the middle to late Pleistocene interglacial deposits of southern Delaware: Delaware Geological Survey Report of Investigations No. 76, 43 p. _____2010b, Geologic Map of the Georgetown Quadrangle, Delaware: Delaware Geological Survey Geologic Map Series No. 15, Scale 1:24,000. _____2011, Geologic Map of the Fairmount and Rehoboth Beach Quadrangles, Delaware: Delaware Geological Survey Geologic Map Series No. 16, Scale 1:24,000. 75° 15' W 38° 37' 30" N 75° 22' 30" W 38° 37' 30" N 75° 22' 30" W 38° 45' 00" N 75° 15' W 38° 45' 00" N 17' 75° 19' 21' 17' 75° 19' 21' 40' 44' 38' 38° 42' 40' 44' 38' 38° 42' Pf25-e Pg21-k Pg21-e Pg21-f Pg21-d Pg22-d Pg22-c Pg32-c Pg32-f Pg33-i Pg33-g Pg33-h 45 40 35 30 25 20 15 10 Elevation (ft) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Distance (x 1,000 ft) 0 1 Miles Qlh Qlh t Tbd Qsw Qlh Tbd Qlh t Qlh B B' Of13-n Of13-q Of13-r Of14-s Of14-f Of14-m Of14-j Of14-k Of15-n Of15-m Of15-l Of15-14 Of15-v Of15-u Og11-h Og11-k Og21-d Og21-g Og21-h Og21-02 Og22-a Og22-e Og22-f Og22-d Og23-d Og23-c Og23-b Og24-c Og24-d Og24-a Og25-c Qsw Qcb Qd Qd Qtb Qlh Qlh Qlh Qlh t Tbd Tbd Tbd Qlh Qsw Qlh A' A Qlh t Qlh t Qlh t 20 25 30 35 Distance (x 1,000 ft) 0 5 10 15 1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29 31 32 33 34 0 1 Miles 55 50 45 40 35 30 25 20 Elevation (ft)
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Colors for geologic formations on the cross sections appear lighter than shown on the map explanation and stratigraphic chart because they do not include the shading effect of the DEM used on the map.
LITHOLOGIC SYMBOLS(cross sections)
clean sand
silt
pebble
fine to coarse silty to clayey sand with scattered clay laminae
very fine to fine moderately silty sand
Sandpit
State Forest and Wildlife Areas
Municipal Boundary
SYMBOLS (map)
Og22-a Soil auger boring
Of15-14 Core/drill hole
Contact
Tbd
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10-foot Index with a 6-foot Intermediate Contour Interval2-foot contour data are available for this map from the Delaware DataMIL at http://datamil.delaware.gov
NORTH AMERICAN DATUM OF 1988 (NAVD88)
SCALE 1:24,000
1992 MAGNETIC NORTHDECLINATION AT CENTER OF SHEET
George
town
Harbeso
n
Mill
sboro
Trap
Pond
Fran
kford
Fairm
ount
Lewes
Ellend
ale
Milt
on
Quadrangle Location
2011
Kelvin W. Ramsey and Jaime L. Tomlinsonby
GEOLOGIC MAP OF THE HARBESON QUADRANGLE, DELAWARE
MAP CREDITSBase MapDelaware state plane coordinate systemTransverse mercator projectionNorth American Datum of 1983 (NAD 83) HARNDataMIL Layers http://datamil.delaware.gov The Delaware Department of Transportation Centerline and Railroads for Delaware, 2010 The Delaware Office of State Planning Coordination Delaware Municipal Boundaries, 2010 USGS National Hydrography Dataset, 2005 USGS Delaware LiDAR Contours, 2005 USGS Delaware Miscellaneous Features, 1993Delaware Department of Agriculture State Forest Areas, 2009Delaware Division of Natural Resources and Environmental Control Wildlife Areas, 2009
OtherCartography by Lillian T. Wang, Delaware Geological Survey Edited by Stefanie J. Baxter, Delaware Geological Survey
SWAMP DEPOSITS
Found in the upper reaches of the modern stream valleys. Consist of 1 to 3 ft of gray to brown, silty and clayey gravelly sand at the base overlain by organic-rich, fine to coarse sand. In some of the larger valleys, the unit has several ft of organic silt at the top. Deposits are up to 15 ft thick in the larger stream valleys and less than 5 ft thick in the smaller tributaries. Holocene.
DUNE DEPOSITS
Located in raised parabolic to linear features. Consist of white to light-yellow, well-sorted, medium to fine sand. Laminae of coarse sand are common. Thin, brown soil lamellae are commonly found at depths of 1 to 3 ft within the dunes. Deposits up to 16 ft thick. The deposits are eolian features related to cold-climate processes when arboreal vegetation was scarce and winds blew sand dunes across the landscape. Some of the dunes, which have well-developed and deep (>3 ft) soil profiles, may be older than latest Pleistocene. Middle Pleistocene to Holocene.
CAROLINA BAY DEPOSITS
Found in circular features in the northern half of the Harbeson Quadrangle. They consist of raised rims (dunes) of well-sorted, medium to fine sand with silty sand in the interior of the circular features. A few of the features contain either seasonal or year-round bodies of water where the water table is high. The deposits are less than 5 ft thick in their interiors and up to 10 ft thick where the sand rims are best developed. Features are related to cold-climate processes during the latest Pleistocene to earliest Holocene (Ramsey, 1997). Latest Pleistocene to Holocene.
TURTLE BRANCH FORMATION
Clean, well-sorted, white to pale-yellow, fine sand grading down to interlaminated fine to coarse sand with opaque heavy mineral laminae, granules, and pebbles at its base. In the Harbeson Quadrangle, it is commonly less than 5 ft in thickness. The Turtle Branch Formation is distinguished from the adjacent Beaverdam Formation by better sorting and the absence of the white, silty matrix, which is characteristic of the Beaverdam. It is distinguished from adjacent and overlying dune deposits by having a better developed soil profile and the common presence of opaque heavy mineral laminae which are rare in the dune deposits. DEM images of the map area show distinctive circular features on the surface of the Turtle Branch Formation which are not readily apparent in the field. These features are likely related to periglacial eolian and freeze-thaw activity upon the surface of the Turtle Branch Formation. In places, the Turtle Branch Formation is completely removed in the center of these features and the underlying Beaverdam Formation is exposed; however, the entire area is mapped as Turtle Branch Formation. Determination of the presence of Beaverdam Formation at the land surface requires detailed mapping on a scale larger than that of this map. The Turtle Branch Formation is interpreted to be a fluvial to tidal and shoreline deposit, which has been greatly modified by periglacial activity in the map area. Middle Pleistocene.
DELAWARE GEOLOGICAL SURVEYUniversity of Delaware, NewarkJohn H. Talley, State Geologist
DELAWARE GEOLOGICAL SURVEYGEOLOGIC MAP OF THE HARBESON QUADRANGLE, DELAWARE
GEOLOGIC MAP SERIES NO. 17
The project was funded in part by the cooperative agreement between the Association of American State Geologists and U.S. Geological Survey under STATEMAP program grant G09AC00166. This project would not have been possible without the cooperation of the staff of the Delaware Department of Transportation, Redden State Forest, Sussex County Engineering Dept., and the DNREC Wildlife Areas. Paul S. McCreary coordinated the drilling for the project. DGS project personnel and students who assisted in field work and data collection included Stephanie Nebel and Curt Romanchok. Groundwater recharge and water table mapping by A.S. Andres and Andrew Klingbeil of the DGS generated much of the subsurface data for the area (Andres and Klingbeil, 2006).
Acknowledgements
LYNCH HEIGHTS FORMATION
Loose, fine to very fine, moderately silty, pale-yellow to yellow sand that ranges from 2 to 15 ft in thickness north of US Rt. 9 and east of Gravel Hill Road (Rt. 30). To the west of Gravel Hill Road, these sands are interbedded with a body of compact, gray to greenish-gray, clayey silt to silty clay from 2 to 15 ft thick that forms a topographic high oriented roughly north-south. On the western margin of this clay body a paleochannel with a similar orientation is filled with 5 to 15 ft of gravelly sand grading upward to fine to medium sand. Interpreted to be estuarine sands, tidal flat or estuarine muds, and tidal channel sands, respectively.
Adjacent to Deep Branch, Peterkins Branch, and Cow Bridge Branch, the Lynch Heights Formation consists of up to 20 ft of pale-yellow, clean, gravelly sand grading up to medium to coarse sand and then to fine to medium sand. The sands continue southward into the Millsboro Quadrangle. Interpreted to be fluvial deposits related to the cutting and filling of the upper reaches of the ancestral Indian River during the middle Pleistocene.
Where mapped as Qlht in the eastern half of the Harbeson Quadrangle, the Lynch Heights Formation consists of a thin (< 5 ft thick) layer of heterogeneous deposits ranging from reddish-brown, pale-yellow, and light-gray, silty, clayey, very coarse to fine sand, to pale-yellow to light-gray, gravelly sand to sandy gravel. These deposits are considered to be the result of local reworking of the underlying Beaverdam Formation sediments. The Lynch Heights Formation is differentiated from the Beaverdam by the lack of the characteristic white silt matrix of the Beaverdam. It is also differentiated by a layer of coarse sand to gravel that is found at the base of the unit overlying the typical Beaverdam sands. The sands are interpreted to be shallow water deposits along the margins of an estuary, and later modified by eolian activity related to several periods of periglacial climate. Middle Pleistocene.
BEAVERDAM FORMATION
Heterogeneous unit ranging from very coarse sand with pebbles to silty clay. The predominant lithologies at the land surface are white to mottled light-gray and reddish-brown, silty to clayey, fine to coarse sand. Laminae and beds of very coarse sand with pebbles to gravel are common as are laminae and beds of bluish-gray to light-gray silty clay. In a few places near the land surface, but more commonly in the subsurface, beds ranging from 2 to 20 ft thick of finely laminated, very fine sand and silty clay are present. The sands of the Beaverdam Formation have a white, silty matrix that gives samples a milky appearance when wet. This white, silty matrix is the most distinguishing characteristic of the unit and readily differentiates the Beaverdam Formation from the adjacent cleaner sands of the Lynch Heights and Turtle Branch Formations. The Beaverdam Formation is interpreted to be a late Pliocene fluvial to estuarine deposit (Groot et al., 1990; Ramsey, 2010a, b) and ranges from 50 to 100 ft thick in the map area. Late Pliocene.
Discussion
The complex geologic history of the surficial units of the Harbeson Quadrangle is one of deposition of the Beaverdam Formation and its subsequent modification by erosion and deposition related to sea-level fluctuations during the Pleistocene. The geology is further complicated by periglacial activity that produced dune deposits and Carolina Bays scattered throughout the map area. The Beaverdam Formation consists of stacked 1- to 5-ft thick beds of very coarse sand and gravel that commonly fine upwards to fine to medium sand and rarely to very fine silty sand to silty clay. These types of deposits are typical of either fluvial or estuarine environments (Ramsey, 2010a, b). Elsewhere in Delaware, rare burrows have been observed in the Beaverdam Formation indicating at least a marginal estuarine setting (DGS unpublished data; Owens and Denny, 1979). The age of the Beaverdam Formation is uncertain due to the lack of age-definitive fossils within the unit; however, stratigraphic relationships in Delaware indicate that it is no older than late Miocene and no younger than early Pleistocene and is most likely late Pliocene (Ramsey, 2010a, b). The Lynch Heights Formation is a composite unit consisting of deposits from two sea-level high stands approximately 400,000 yrs B.P. and 320,000 yrs B.P. (Groot et al., 1990; Ramsey, 2010a) which cannot be differentiated in this map area. The Lynch Heights Formation in the map area consists of three distinct lithofacies associations. In the eastern portion of the Harbeson Quadrangle, the Lynch Heights Formation (Qlht) is a thin unit (< 5 ft thick) consisting of fine to medium sand that coarsens to a coarse sand. In places, a pebbly sand to pebble gravel containing abundant opaque heavy minerals is found at the base of the unit overlying the Beaverdam Formation. This portion of the Lynch Heights Formation is interpreted to be a shallow water deposit consisting of sand eroded from the Beaverdam Formation as the shoreline transgressed during one or both of the sea-level high stands. In the north-central portion of the Harbeson Quadrangle, the Lynch Heights Formation consists of paleovalley fill (cross section A-A') with tidal channel sands, tidal flat and estuarine muds, and estuarine sands. These sediments were deposited in a drainage marginal to an ancestral Delaware estuary (Ramsey, 2011). In the south-central portion of the quadrangle, fluvial deposits flank the modern drainage (cross section B-B') and are related to the initial stages of drainage formation during the middle Pleistocene. Sediments closest to the modern drainage are thicker deposits (4 to >11 ft) of clean, loose, well-sorted sand with few to abundant heavy minerals. Away from the drainage, deposits become less well-sorted and siltier, as shown in the B-B' cross section. Dune deposits are fine to medium, well-sorted sands that are found throughout the map area. The dunes have a pronounced surficial expression as linear and hook-shaped features that rise above the surrounding landscape. Some of these dunes are probably latest Pleistocene to early Holocene in age (Andres and Howard, 2000) but some could be contemporaneous with deposition of the Lynch Heights Formation or the late Pleistocene Scotts Corners Formation (Ramsey, 2010a). Dune features are also associated with the rims of Carolina Bays that are found in the northern part of the map area. Some of the features mapped as Carolina Bays may be parabolic dunes with blown-out low areas on the windward side. Because the features are generally circular in shape and are consistent in size with Carolina Bays mapped elsewhere (Ramsey, 2001, 2003, 2010b), they are mapped as Carolina Bays. Both the dunes and the Carolina Bays are considered to be cold-climate related features located where winds moved sand across a landscape barren of forests (Ramsey, 1997). The exact process by which the distinctive circular shape of the Carolina Bays was formed is unknown.
EXPLANATION
References Cited
Andres, A.S., and Howard, C.S., 2000, The Cypress Swamp Formation, Delaware: Delaware Geological Survey Report of Investigations No. 62, 13 p.Andres, A.S., and Klingbeil, A.D., 2006, Thickness and transmissivity of the unconfined aquifer of eastern Sussex County, Delaware: Delaware Geological Survey Report of Investigations No. 70, 19 p.Groot, J.J., Ramsey, K.W., and Wehmiller, J.F., 1990, Ages of the Bethany, Beaverdam, and Omar formations of Southern Delaware: Delaware Geological Survey Report of Investigations No. 47, 19 p.Owens, J.P., and Denny, C.S., 1979, Upper Cenozoic deposits of the central Delmarva Peninsula, Maryland and Delaware: U.S. Geological Survey Professional Paper 1067-A, 28 p.Ramsey, K.W., 1997, Geology of the Milford and Mispillion River Quadrangles, Delaware: Delaware Geological Survey Report of Investigations No. 55, 40 p._____2001, Geologic Map of the Ellendale and Milton Quadrangles, Delaware: Delaware Geological Survey Geologic Map Series No. 11, Scale 1:24,000._____2003, Geologic Map of the Lewes and Cape Henlopen Quadrangles, Delaware: Delaware Geological Survey Geologic Map Series No. 12, Scale 1:24,000._____2010a, Stratigraphy, correlation, and depositional environments of the middle to late Pleistocene interglacial deposits of southern Delaware: Delaware Geological Survey Report of Investigations No. 76, 43 p._____2010b, Geologic Map of the Georgetown Quadrangle, Delaware: Delaware Geological Survey Geologic Map Series No. 15, Scale 1:24,000._____2011, Geologic Map of the Fairmount and Rehoboth Beach Quadrangles, Delaware: Delaware Geological Survey Geologic Map Series No. 16, Scale 1:24,000.
75° 15' W38° 37' 30" N
75° 22' 30" W38° 37' 30" N
75° 22' 30" W38° 45' 00" N
75° 15' W38° 45' 00" N
17'75° 19'21'
17'75° 19'21'
40'
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38° 42'
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0 5 10 151 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 21 22 23 24 26 27 28 29 31 32 33 34
0 1
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