OBJECTID |
MapUnit |
Name |
FullName |
Age |
Description |
HierarchyKey |
ParagraphStyle |
Label |
Symbol |
AreaFillRGB |
AreaFillPatternDescription |
DescriptionSourceID |
GeoMaterial |
GeoMaterialConfidence |
_ID |
1 |
Qal |
Alluvium |
Alluvium |
Quaternary |
Poorly sorted, unconsolidated, tan, reddish brown, to dark-gray mud, silt, sand, and pebbles. These deposits form in stream channels, and on the flood plain adjacent to the streams. Thickness estimated a 3 to 10 feet. |
1 |
header1 |
Qal |
--- |
--- |
--- |
DS1 |
Alluvial sediment |
High |
DMU_1 |
2 |
Qc |
Colluvium |
Colluvium |
Quaternary |
Unconsolidated and unsorted cobbles and boulders that accumulate on steep slopes overlying and downslope sandstone and quartzite units. These deposit may move slowly downslope under the influence of frost action and gravity. Two types were recognized in the Blue Ridge Summit Quadrangle. The first type is composed predominately of angular boulders that overlie outcroppings of quartzites of the Weverton Formation on the flanks of Catoctin Mountain. This type of colluvium appears to be the direct result of the mechanical breakdown of the quartzite intervals. The thickness was not determined, but mat be highly variable. The second type of colluvium is composed of highly weathered, reddish brown, rounded pebbles and cobbles of quartzite and vein quartz within a sandy matrix. This type of colluvium is present along the eastern base of Catoctin Mountain and covers outcrops of the Harpers, Frederick, and Gettysburg formations near Thurmont. The thickness of this type of colluvium ranges from a thin veneer less 3 feet to more than 50 feet. |
2 |
header1 |
Qc |
--- |
--- |
--- |
DS1 |
Colluvium and other widespread mass-movement sediment |
High |
DMU_2 |
3 |
Trg |
Gettysburg Formation |
Gettysburg Formation |
Triassic |
Cyclically interbedded reddish gray, laminated, very fine-grained, argillaceous sandstone, sandy siltstone and red to reddish brown shale to rooted mudstone. Sandstone and siltstone intervals are commonly laminated to cross-laminated and siltstone intervals are mudcracked. Mudstone intervals are pervasively rooted and show signs of incipient soil development. Along the western margin of the outcrop belt the Gettysburg Formation is composed of a narrow belt of interbedded reddish brown, thin-bedded, fine-grained, argillaceous sandstone and red-brown limestone conglomerate [Trgc]. The thickness of the Gettysburg Formation has been estimated at 8,000 to 22,000 feet (Brezinski, 2004; Root, 1988; Brezinski and Adams,2021). |
3 |
header1 |
^rg |
--- |
--- |
--- |
DS1 |
Sedimentary rock |
High |
DMU_3 |
18 |
Trgc |
Gettysburg conglomerate |
Gettysburg Formation, conglomerate |
Triassic |
Along the western margin of the outcrop belt the Gettysburg Formation is composed of a narrow belt of interbedded reddish brown, thin-bedded, fine-grained, argillaceous sandstone and red-brown limestone conglomerate [Trgc]. |
3-1 |
header2 |
^rgc |
--- |
--- |
--- |
DS1 |
Sedimentary rock |
High |
DMU_18 |
4 |
Cfu |
Frederick Formation |
Frederick Formation, undivided |
Cambrian |
Medium to dark gray, laminated to thin-bedded, pyritic, argillaceous limestone and highly sheared and deformed, laminated limestone. Because of limited exposure no member assignment could be made for the Frederick Formation in the Blue Ridge Summit Quadrangle (see Brezinski, 2004). In this area, the Frederick Formation is interpreted to occur as the foot-wall of the Owens Creek overthrust. Thickness in the Blue Ridge Summit Quadrangle is estimated at 500 to 1000 feet. |
4 |
header1 |
_fu |
--- |
--- |
--- |
DS1 |
Carbonate rock |
High |
DMU_4 |
5 |
--- |
Chilhowee Group |
--- |
--- |
--- |
5 |
header3 |
--- |
--- |
--- |
--- |
DS1 |
Metasedimentary rock |
High |
DMU_5 |
6 |
Ch |
Harpers Formation |
Harpers Formation |
Cambrian |
Predominately tan-weathering, dark grayish green to dark gray, sheared phyllitic shale and siltstone with lesser amounts of metasandstones that may be quartzitic. Shale and siltstone are gray to greenish gray, dark brown-gray to medium gray in color and commonly containing thin 2- 6 inch, fine-grained sandstone layers. Local areas of intense shearing create phyllitic areas where cleavage obscures bedding. Because of intraformational folding and intense cleavage, determining the thickness of the formation was not possible. Estimates range from 1,500 to 3,000 feet. |
6 |
header1 |
_h |
--- |
--- |
--- |
DS1 |
Metasedimentary rock |
High |
DMU_6 |
7 |
Chm |
Mont Alto Member |
Harpers Formation, Mont Alto member |
Cambrian |
Within the lower Harpers Formation there is a mappable metagraywacke that can be traced along the flank of Catoctin Mountain. This interval is light- to medium gray, medium-bedded, coarse-grained to conglomeratic, quartzitic, and contains Skolithos-burrowed. This interval is considered equivalent to the Mont Alto Member of the Harpers Formation in Pennsylvania (Brezinski, 1992). The Mont Alto Member is between 30 to 50 feet thick in the Blue Ridge Summit Quadrangle, but thickens northward into southern Pennsylvania. |
6-1 |
header2 |
_hm |
--- |
--- |
--- |
DS1 |
Metasedimentary rock |
High |
DMU_7 |
8 |
--- |
Weverton Formation |
Weverton Formation |
Cambrian |
Light- to medium-gray quartzite, conglomerate, medium to dark gray metagraywacke with intervals of dark-gray to black metasiltstone and phyllite. Three members are recognized (Brezinski, 1992). |
7 |
header1 |
--- |
--- |
--- |
--- |
DS1 |
Metasedimentary rock |
High |
DMU_8 |
9 |
Cwo |
Owens Creek Member |
Weverton Formation, Owens Creek member |
Cambrian |
Medium to dark gray, very coarse grained to conglomeratic, cross-bedded, medium- to thick-bedded metagraywacke. The coarse-grained character of this member makes recognition of cross-bedding difficult. Conglomeratic intervals contain large, (1 to 3 cm) white and pink quartz pebbles. The type section of this member is along MD 550 along Owens Creek. Thickness ranges from 200 to 300 feet. |
7-1 |
header2 |
_wo |
--- |
--- |
--- |
DS1 |
Metasedimentary rock |
High |
DMU_9 |
10 |
Cwm |
Maryland Heights Member |
Weverton Formation, Maryland Heights member |
Cambrian |
Interbedded, thin (less than 30 feet), white quartzite and medium gray metagraywacke and very dark gray, highly cleaved, volcaniclastic siltstone and phyllitic shale. Shales and siltstones are very poorly exposed and individual quartzites and graywackes are only locally traceable. Near the middle of this member there is a light gray, massive, coarse-grained quartzite. Herein termed the Wolf Rock Quartzite Bed [Cwmw] (=middle quartzite of Whitaker, 1955 and ledge-maker quartzite of Fauth, 1977). This unit is up to 30 feet thick in the Blue Ridge Summit Quadrangle. Thickness of the Maryland Heights Member ranges from 200 to 400 feet. |
7-2 |
header2 |
_wm |
--- |
--- |
--- |
DS1 |
Metasedimentary rock |
High |
DMU_10 |
17 |
Cwmw |
Wolf Rock Quartzite Bed |
Weverton Formation, Maryland Heights member, Wolf Rock quartzite bed |
Cambrian |
Near the middle of the Weverton Formation, Maryland Heights member there is a light gray, massive, coarse-grained quartzite. Herein termed the Wolf Rock Quartzite Bed [Cwmw] (=middle quartzite of Whitaker, 1955 and ledge-maker quartzite of Fauth, 1977). This unit is up to 30 feet thick in the Blue Ridge Summit Quadrangle. |
7-2-1 |
header2 |
_wmw |
--- |
--- |
--- |
DS1 |
Quartzite |
High |
DMU_17 |
11 |
Cwb |
Buzzard Knob Member |
Weverton Formation, Buzzard Knob member |
Cambrian |
The lowest member of the Weverton Formation consists of two ledge-forming quartzites, which are often difficult to discern between. The lower ledge consists of light- to medium-gray to light greenish gray, medium-bedded quartzite with dark-gray argillaceous layers up to 4 cm thick, separating the quartzite beds. Cross-bedding within individual quartzite strata is pervasive and is commonly accentuated by purplish or yellow-gold bands demarcating the individual cross-bed foresets. The upper ledge-forming quartzite is composed of medium- to thick-bedded, very light gray, quartzite. Cross-bedding is abundant, but less obvious than in the lower ledge. This member is the main ridge-forming unit of the Maryland Blue Ridge. Thickness ranges from 175 to 350 feet. |
7-3 |
header2 |
_wb |
--- |
--- |
--- |
DS1 |
Quartzite |
High |
DMU_11 |
12 |
CZl |
Loudoun Formation |
Loudoun Formation |
Cambrian-Late Proterozoic |
Interbedded, medium-gray, medium-to thin-bedded, tuffaceous sandstone, granule to pebble conglomerate, and dark gray, tuffaceous phyllite. Massive, cross-bedded, quartzite, and thin, quartz-pebble conglomerate and pebbly polymictic lenses of conglomerate are interbedded within phyllite as are layers of flattened phyllite pebbles. The width of the outcrop belt widens dramatically in the Blue Ridge Summit and Emmitsburg quadrangles indicating a localized thickening of the formation. In the Blue Ridge Summit Quadrangle, the Loudoun Formation is graphically determined to be from 800 to 1,500 feet in thickness. |
8 |
header1 |
_Zl |
--- |
--- |
--- |
DS1 |
Lower-grade metamorphic rock, of unspecified origin |
High |
DMU_12 |
13 |
--- |
Catoctin Formation |
Catoctin Formation |
Late Proterozoic |
The Catoctin Formation is a suite of volcanigenic rocks that consists principally of metabasalt, metarhyolite, and tuffaceaous phyllites. |
9 |
header1 |
--- |
--- |
--- |
--- |
DS1 |
Meta-volcaniclastic rock |
High |
DMU_13 |
14 |
Zcm |
Metabasalt |
Catoctin Formation, metabasalt |
Late Proterozoic |
Medium to dark greenish gray, medium-grained, massive, metabasalt. Textures vary greatly between exposures, but commonly consists of massive to highly cleaved aphanitic, greenish gray metabasalt exhibiting a range of primary textures including amygdaloidal, porphyoblastic, banded, vesicular, and flow-top brecciation. Areas of tectonic deformation are suggested by schistose chlorite- and actinolite-bearing metabasalt that are epidote-rich. Porphyroblasts most commonly epidote, plagioclase feldspar, or chlorite that are flattened and elongated and composed of chlorite, actinolite, or epidote, ranging between 10 and 25 mm (0.4 to 1 inch) in length (Fauth, 1977). Vesicles are oval-shaped voids or may be filled amygdules that contain epidote, feldspar, and quartz. Primary layering is present in many exposures along the eastern margin of the Blue Ridge Anticlinorium. Prominent veins and nodular masses of epidote and quartz are widely distributed throughout the unit. Thickness of the entire metabasalt unit is estimated at greater than 2,000 feet. |
9-1 |
header2 |
Zcm |
--- |
--- |
--- |
DS1 |
Meta-volcaniclastic rock |
High |
DMU_14 |
15 |
Zcr |
Metarhyolite |
Catoctin Formation, metarhyolite |
Late Proterozoic |
Medium gray to medium dark bluish gray, dense, metarhyolite. A variety of variations are noted on the map, not delineated separately. The most common is aphanitic bluish gray metarhyolite that is massive and dense, and exhibits a conchoidal fracture. Rare phenocrysts, are present, but comprise less than 1 percent of the rock. Weathers light gray to light medium gray. Several textural subunits have been noted but noted mapped. Medium bluish gray, flow-banded metarhyolite containing planar to convoluted flow bands 0.12 to 0.4 inch (3 to 10 mm) thick. Banding is discontinuous and most prominent on weathered surfaces. Phenocrysts typically constitute less than 5 percent of the rock. Medium to dark bluish gray, porphyritic, aphanitic metarhyolite contain very light gray to very pale orange or pinkish gray feldspar laths. Phenocrysts are up to 0.2 inches (6mm) long, rectangular, and make up more than 5 percent of the rock. Minimum aggregate thickness is estimated at 500 to 800 feet (Fauth, 1977). |
9-2 |
header2 |
Zcr |
--- |
--- |
--- |
DS1 |
Meta-volcaniclastic rock |
High |
DMU_15 |
16 |
Zcrp |
Interbanded Metabasalt, Metarhyolite and Tuffaceous Phyllite |
Catoctin Formation, Interbanded Metabasalt, Metarhyolite and Tuffaceous Phyllite |
Late Proterozoic |
Interlayered greenish gray granular phyllite, pale purplish gray, to very light gray, tuffaceous phyllite, greenish gray metabasalt, and dark gray metarhyolite. Medium greenish gray phyllite contains small elongate felspathic laths and is commonly interbedded with thin bands (4-10 cm) of gray metarhyolite or metabasalt. Gray phyllites typically are tuffaceous and volcanoclastic. Especially well-developed in the Harbaugh Valley. Estimated thickness 1000 to 1,500 feet. |
9-3 |
header2 |
Zcrp |
--- |
--- |
--- |
DS1 |
Meta-volcaniclastic rock |
High |
DMU_16 |
19 |
Zcmp |
Porphyritic metabasalt |
Catoctin Formation, porphyritic metabasalt |
Late Proterozoic |
Medium to dark greenish gray, medium-grained, massive, metabasalt. Textures vary greatly between exposures, but commonly consists of massive to highly cleaved aphanitic, greenish gray metabasalt exhibiting a range of primary textures including amygdaloidal, porphyoblastic, banded, vesicular, and flow-top brecciation. Areas of tectonic deformation are suggested by schistose chlorite- and actinolite-bearing metabasalt that are epidote-rich. Porphyroblasts most commonly epidote, plagioclase feldspar, or chlorite that are flattened and elongated and composed of chlorite, actinolite, or epidote, ranging between 10 and 25 mm (0.4 to 1 inch) in length (Fauth, 1977). Vesicles are oval-shaped voids or may be filled amygdules that contain epidote, feldspar, and quartz. Primary layering is present in many exposures along the eastern margin of the Blue Ridge Anticlinorium. Prominent veins and nodular masses of epidote and quartz are widely distributed throughout the unit. Thickness of the entire metabasalt unit is estimated at greater than 2,000 feet. |
9-4 |
header2 |
Zcmp |
--- |
--- |
--- |
DS1 |
Meta-volcaniclastic rock |
High |
DMU_19 |