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Coastal Plain Rocks and Sediments

The information contained on this page was adapted from Maryland Geological Survey's Geologic Map of Maryland (1968). This information reflects geologic interpretations from over 20 years ago and do not necessarily represent an accurate interpretation of currently accepted geologic theory. We present this information for historic purposes only. Do not use this information for anything other than illustrative purposes. When a corrected and updated geologic map of Maryland is available you will see a notification on our web site.

Guilford Quatrz Monzonite (Paleozoic)   Lowland Deposits (Quaternary) Lowland Deposits
Undifferentiated gray to buff sand and gravel, gray to brown lignitic silt and clay, occasional boulders, and rare shell beds.
Surficial deposits occur as intercalated fluvial sands and marsh muds (e.g. in upstream floodplains of the Wicomico and Nanticoke Rivers), well sorted, stabilized sand dunes (e.g. eastern Wicomico County), shell-bearing estuarine clays and silts (e.g. lower Dorchester County and Pocomoke River basin of Worcester County), and beach zone sands (e.g. Fenwick and Assateague Islands). Wisconsin to Holocene in age.
Subsurface deposits of pre-Wisconsin age consist of buff to reddish-brown sand and gravel locally incised into Miocene sediments (e.g. Salisbury area), estuarine to marine white to gray sands, and gray to blue, shell-bearing clays (e.g. Worcester County).
  Lowland Deposits (Quaternary) Lowland Deposits
Gravel, sand, silt and clay. Medium- to coarse-grained sand and gravel; cobbles and boulders near base; commonly contains reworked Eocene glauconite; varicolored silts and clays; brown to dark gray lignitic silty clay; contains estuarine to marine fauna in some areas (includes in part Pamlico, Talbot, Wicomico and Sunderland Formations of earlier reports); thickness 0 to 150 feet.
  Upland Deposits - Eastern Shore (Quaternary)

Upland Deposits (Eastern Shore)
Gravel, sand, silt, and clay. Mostly cross-bedded, poorly sorted, medium- to coarse-grained white to red sand and gravel, boulders near base; minor pink and yellow silts and clays; (Wicomico Formation of earlier reports); thickness 0 to90 feet, locally thicker in paleochannels.

    Upland Deposits -Western Shore (Quaternary) Upland Deposits (Western Shore)
Gravel and sand, commonly orang-brown, locally limonite-cemented; minor silt and red, white, or gray clay; (includes Brandywine, Bryn Mawr, nad Sunderland Formations of earlier reports); lower gravel member and upper loam member in Southern Maryland; thickness 0 to 50 feet.
Yorktown Formation
Yellowish-white to gray, fine- to medium-grained sand, gray to bluish gray clayey silt, and granule gravel; generally nonfossiliferous; sands locally black or green; present in subcrop of Eastern Shore; thickness 0 to 400 feet.
St. Mary Formation  (Miocene) St. Mary Formation
Greenish-blue to yellowish-gray sandy clays and fine-grained argillaceous sand; thickness 0 to80 feet.
Choptank Formation (Miocene) Choptank Formation
Interbedded brown to yellow very fine-grained to fine-grained sand and gray to dark bluish-green argillaceous silt; locally indurated to calcareous sandstone; prominent shell beds; thickness 0 to 50 feet.
Calvert Formation (Miocene) Calvert Formation
Plum Point Marls Member: Interbedded dark green to dark bluish-gray, fine-grained argillaceous sand and sandy clay; contains prominent shell beds and locally silica-cemented sandstones. Fairhaven Member: Greenish-blue diatomaceous clay, weathers to pale gray; pale brown to white, fine-grained argillaceous sand and greenish-blue sandy clay; total thickness 0 to 150 feet.
Piney Point Formation
Light gray to yellowish glauconitic medium- to coarse-grained sand and interbedded shell beds; no known surface outcrop; occurs in subsurface in St. Marys, Calvert, Queen Annes, Talbot, Caroline, Dorchester, Wicomico, and Somerset Counties; thickness 10 to possibly 280 feet.
Nanjemoy Formation  (Eocene) Nanjemoy Formation
Dark green to gray, argillaceous, glauconitic, fine- to medium-grained sand; minor gray to pale brown clay; Marlboro Clay Member at base: Pink to gray, homogeneous plastic clay with local lenses of very fine-grained white sand; thickness 0 to 30 feet; present west of Chesapeake Bay only; total thickness 0 to 125 feet.
Aquia Formation (Paleocene) Aquia Formation
Dark green to gray-green, argillaceous, highly glauconitic, well sorted fine- to medium-grained sand; locally indurated shell beds; thickness 0 to 100 feet.
    Brightseat Formation
Gray to greenish-gray, micaceous, argillaceous, sparsely glauconitic, fine- to coarse-grained sand, locally indurated calcareous beds; phosphatic pebbles; thickness 0 to 20 feet; present in southwestern Prince Georges County only. Mapped as part of Monmouth Formation.
Monmouth Formation (Cretaceous) Monmouth Formation
Dark gray to reddish-brown, micaceous, glauconitic, argillaceous, fine- to coarse-grained sand; basal gravel in Prince Georges County; thickness 0 to 100 feet.
Matawan Formation (Cretaceous) Matawan Formation
Dark gray, micaceous, glauconitic, argillaceous, fine-grained sand and silt; absent in outcrop southwest of Patuxent River; thickness 0 to 70 feet.
Magothy Formation (Cretaceous) Magothy Formation
Loose, white, cross-bedded, "sugary", lignitic sands and dark gray, laminated silty clays; white to orange-brown, iron-stained, subrounded quartzose gravels in western Anne Arundel County; absent in outcrop southwest of Patuxent River; thickness 0 to 60 feet.
Potomac Group (Cretaceous)

Potomac Group
Interbedded quartzose gravels; protoquartzitic to orthoquartzitic argillaceous sands; and white, dark gray and multicolored silts and clays; thickness 0 to 800 feet.

Raritan and Patapsco Formations
Gray, brown, and red variegated silts and clays; lenticular, cross-bedded, argillaceous, subrounded sands; minor gravels; thickness 0 to 400 feet.
Arundel Clay
Dark gray and maroon lignitic clays; abundant siderite concretions; present only in Baltimore-Washington area; thickness 0 to 100 feet.
Patuxent Formation
White or light gray to orange-brown, moderately sorted, cross-bedded, argillaceous, angular sands and subrounded quartz gravels; silts and clays subordinate, predominately pale gray; thickness 0 to 250 feet.
Undifferentiated Mesozoic Rocks
Coarse-grained conglomerate with pebbles of quartzite, pegmatite, serpentine, and vein quartz at base; reddish-brown, gray and green, mottled, fine- to coarse-grained sandstone, silts tone, and shale; present in subsurface; maximum thickness penetrated 600 feet.
Undifferentiated Crystalline Rock
Weathered schist and mica gneiss with pegmatite dikes, serpentine, and metagabbro encountered in Eastern Shore deep test wells; hornblende gneiss and biotite-quartz gneiss encountered in southern Prince Georges County test wells.

Updated 7/7/00

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