U.S. DEPARTMENT OF THE INTERIOR Prepared in cooperation with the GEOLOGIC QUADRANGLE MAP U.S. GEOLOGICAL SURVEY MARYLAND GEOLOGICAL SURVEY BUCKEYSTOWN QUADRANGLE, MARYLAND AND VIRGINIA 77°30′ 77°22′30″ MAP GQ–1800 ° ′ ″ ° ′ ″ 39 22 30 21 39 22 30 E LEVEL SEA Qt fl 80 77 85 CORRELATION OF MAP UNITS 54 22 16 4 REFERENCES CITED a 17 21 45 82 5 33 FEET 1600
2000 82 1200 1200 2000 1600 15 20 Cenozoic surficial deposits 400 400 800 800 h fa Brezinski, D.K., 1992, Lithostratigraphy of the western Blue Ridge cover rocks in 85 81 73 34 fr Qal 36 26 Holocene B 25 Qal A 16 11 Ogu 30 37 Maryland: Maryland Geological Survey Report of Investigations 55, 69 p. ufca eoisntshown not deposits Surficial meters feet=122 400 20 72 QUATERNARY 27 79 Holocene and Burton, W.C., Froelich, A.J., Pomeroy, J.S., and Lee, K.Y., 1995, Geologic map of Qt Qc Qr Qt 40 26 25 27 84 Pleistocene 82 frs 42 the Waterford and Virginia portion of the Point of Rocks quadrangles, Virginia: bs 82 11 Pleistocene and QUATERNARY 29 71 22 QTt Qc 52 late Tertiary AND TERTIARY U.S. Geological Survey Bulletin 2095, 30 p. 61 fa h 5 81 62 bl 74 Clark, F.W., 1924, The data of geochemistry: U.S. Geological Survey Bulletin 770, Ogl 54 69 42 Unconformity mt 71 22 70 Qal 79 72 20 9 10 841 p. Qal Ogl Qt Qal 86 50 65 Jd Early Jurassic JURASSIC mt 71 Cloos, Ernst, and Cooke, C.W., 1953, Geologic map of Montgomery County and the surface 10 45 bl 37 80 frs 23 60 Intrusive contacts District of Columbia: Baltimore, Maryland Department of Geology, Mines, and Wa- mp 21 15 81 76 Culpeper basin 61 51 ter Resources, scale 1:62,500. 80 77 Qr 29 81 54 12 52 11 78 43 55 51 Drake, A.A., Jr., and Lee, K.Y., 1989, Geologic map of the Vienna quadrangle, Fair- Qc 17 39 F bl 65 76 ar fax County, Virginia, and Montgomery County, Maryland: U.S. Geological Survey a 25 22 70 fr 61 32 bs tm Geologic Quadrangle Map GQ–1670, scale 1:24,000. ? Newark lower part of 25 Upper Triassic TRIASSIC 55 Qt 58 Supergroup Culpeper Group Drake, A.A., Jr., Sinha, A.K., Laird, Jo, and Guy, R.E., 1989, The Taconic orogen, 41 18 82 60 mp
bs 11 22 fr 45 in Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds., The Appalachian-Oua- 15 QTt 60 77 41 mt mr chita orogen in the United States: Boulder, Colo., Geological Society of America, 17 fl 23 40 51 80 62 61 50 The Geology of North America, v. F–2, p. 101–177. 35 85 72 75 Unconformity Route 180 40 frs frs Jd 25 41 Ogu fr western Piedmont Edwards, Jonathan, 1986, Geologic map of the Union Bridge quadrangle, Carroll and 43 57 67 71 42 Frederick Counties, Maryland: Baltimore, Maryland Geological Survey, scale 39 20 80 Blue Ridge- Frederick Valley Sugarloaf Mountain Westminster 23 F 72 14 22 Qt 40 17 South Mountain synclinorium anticlinorium terrane 1:24,000.
35 70 Qc 15 62 anticlinorium ———1988, Geologic map of the Woodsboro quadrangle, Carroll and Frederick mp 9 fl 25 81 37 67 72 Ogu Lower Ordovician ORDOVICIAN Counties, Maryland: Baltimore, Maryland Geological Survey, scale 1:24,000.
fa 21 77 mt 25 42 F 31 79 81 46 Lower Ordovician ORDOVICIAN Evans, N.H., and Milici, R.C., 1994, Stratigraphic relations and structural chaos on 19 65 75 45 60 Ogl 42 66 81 frs 82 52 and Upper Cambrian AND CAMBRIAN the southeastern limb of the Blue Ridge anticlinorium and points east, central Vir- 37 22 52 80 Qt 86 43 49 39 fl ginia Piedmont, in Schultz, Art, and Henika, Bill, eds., Fieldguides to southern Ap- Qc mt 80 85 palachian structure, stratigraphy, and engineering geology: Virginia Tech Depart- 31 70 fa Upper Cambrian Qr 39 63 82 ment of Geological Sciences Guidebook Number 10, p. 31–64. 62 28 O gl 72 bl 62 35 fr frs Fauth, J.L., 1968, Geology of the Caledonia Park quadrangle area, South Mountain, 39 81 75 ar? 11 42 ? Pennsylvania: Pennsylvania Geological Survey, 4th Series, Atlas 129a, 132 p. ar Middle and 53 79 75 Zi Lower Cambrian Fisher, G.W., 1978, Geologic map of the New Windsor quadrangle, Carroll County, 62 fr fl Maryland: U.S. Geological Survey Miscellaneous Investigations Series Map I–1037, 42 32 65 59 t 80 Qc 47 59 Jd scale 1:24,000. Tuscarora Creek bs 12 65 fl a Lower Cambrian CAMBRIAN
? Froelich, A.J., 1975, Bedrock map of Montgomery County, Maryland: U.S. Geologi- frs Chilhowee Jd 42 Qt Group cal Survey Miscellaneous Investigations Series Map I–920–D, scale 1:62,500. 9 25 h 40 79 Qal 33 57 80 Goddard, E.N., Trask, P.D., DeFord, R.K., Rove, R.N., Singewald, J.T., and Over- 60 40 15 78 ul
fr ar 37 45 85 Qal A u uq beck, R.M., 1948, Rock-color chart: Washington, D.C., National Research Coun- 39 87 56 72 67 75 33 55 ar cil, 6 p. [reprinted by Geological Society of America, 1951, 1963, 1970]. mp 7 fa 55 su 57 45 79 65 Lower Cambrian(?) Hopson, C.A., 1964, The crystalline rocks of Howard and Montgomery Counties, in 21 22 mt 42 sm The geology of Howard and Montgomery Counties: Baltimore, Maryland Geologi- mp 21 Qt cal Survey, p. 27–215. Qr 65 sl B 28 65 31 fr 84 Horton, J.W., Jr., Drake, A.A., Jr., and Rankin, D.W., 1989, Tectonostratigraphic 17 uq 27 Ogu 24 T Zi Lower Cambrian(?) 17 71 L CAMBRIAN AND terranes and their Paleozoic boundaries in the central and southern Appalachians, 11 62 31 U and fa FA LATE PROTEROZOIC in Dallmeyer, R.D., ed., Terranes in the Circum-Atlantic Paleozoic orogens: Geo- B&O Railroad 57 25 Zs Late Proterozoic(?) 59 frs 11 85 logical Society of America Special Paper 230, p. 213–245. fa 59 Jd mt 27 21 T S Hoy, R.B., and Schumacher, R.L., 1956, Fault in Paleozoic rocks near Frederick, 22 Jd U T 23 75 R L Maryland: Geological Society of America Bulletin, v. 67, no. 11, p. 1521–1528. Jd 25 70 H 37 U 11 81 T ? ? FA Jonas, A.I., 1924, Pre-Cambrian rocks of the western Piedmont of Maryland: Geo- bl 17 21 logical Society of America Bulletin, v. 35, no. 2 p. 355–363. Qr 22 70 C 30 New Design Road 10 I Jd T 1 ———1927, Geologic reconnaissance in the Piedmont of Virginia: Geological Soci- 32 R ST 21 61 A sl U ety of America Bulletin, v. 38, no. 4, p. 837–846. 62 81 69 42 58 29 10 M R Ogu 57 H Jonas, A.I., and Stose, G.W., 1938a, Geologic map of Frederick County and adjacent 80 10 T 75 32 77 73 frs Qc 65 Qt parts of Washington and Carroll Counties: Baltimore, Maryland Geological Survey, bl 39 27 62 21 uq Qr 70 72 37 C scale 1:62,500. 42 I DESCRIPTION OF MAP UNITS 80 69 T a Antietam Formation of Chilhowee Group (Lower Cambrian)—Light- 10 fr 78 R ———1938b, New formation names used on the geologic map of Frederick County, 13 7 27 17 10 A [Color designations, in parentheses, are from Goddard and others (1948)] olive-gray (5 Y 6/1) to olive-gray (5 Y 4/1), medium- to coarse-grained, 19 65 10 12 46 52 fr 71 60 M u Maryland: Washington Academy of Sciences Journal, v. 28, no. 8, p. 345–348. Qt fr 81 fa 75 medium-bedded, locally ferruginous, micaceous, silty metasandstone in- 11 35 Keith, Arthur, 1894, Harpers Ferry Folio, Va.-Md.-W. Va.: U.S. Geological Survey 70 RDRC ALYSYNCLINORIUM VALLEY FREDERICK 23 Qal 78 60 CENOZOIC SURFICIAL DEPOSITS terbedded with very fine grained, silty metasandstone to sandy metasilt- fl 20 59 11 65 25 Geologic Atlas of the United States, Folio 10, scale 1:125,000, 11 p. 13 7 34 78 ar 70 stone. Poorly exposed. Thickness estimated at 300 ft bs 21 77 Alluvium (Holocene)—Unconsolidated mixture of clay, silt, sand, gravel, O 35 Qal Keyes, C.R., 1890, Discovery of fossils in the limestones of Frederick County, Mary- 53 Zi fr 57 80 10 81 75 38 67 cobbles, and some boulders underlying flood plains of Potomac and Harpers Formation of Chilhowee Group (Lower Cambrian)—Brownish- land: Johns Hopkins University Circular 10, p. 32. gl 28 Jd h Qt Monocacy Rivers and their tributaries. Includes alluvial terraces as gray (5 YR 6/1) to dark-greenish-gray (5 G 4/1), silty, phyllitic metashale 81 ———1891, A geological section across the Piedmont Plateau in Maryland: Geologi- frs Ogu 45 69 80 much as 10 ft above stream channels, and fine colluvial debris from ad- fl mp 27 82 to highly sheared, phyllitic metasiltstone containing intervals of brownish- cal Society of America Bulletin, v. 2, p. 319–322. 17 45 73 77 jacent slopes. Sediments well to poorly stratified, commonly in fining- fa 70 65 42 gray (5 YR 4/1), medium-grained, silty metasandstone. Poorly exposed. Knopf, E.F.B., 1931, Retrogressive metamorphism and phyllonitization: American Qr fl fl 70 81 Qal 11 5 upward sequences as much as 20 ft thick Thickness estimated at greater than 900 ft
Journal of Science, 5th Series, v. 21, p. 1–27. 61
fl 81 3 Ogl 77 Knopf, E.B., and Jonas, A.I., 1929, Geology of the McCalls Ferry-Quarryville district, O gl Qc Colluvium (Holocene and Pleistocene)—Coarse cobbles, boulders, and LOWER CAMBRIAN(?) METASEDIMENTARY ROCKS OF THE 35 5 79 frs frs uq 32 83 Pennsylvania: U.S. Geological Survey Bulletin 799, 156 p. Route 85 11 blocks of quartzite that were transported by gravity and debris flow, and SUGARLOAF MOUNTAIN ANTICLINORIUM 72 Kunk, M.J., Froelich, A.J., and Gottfried, David, 1992, Timing of emplacement of fa 35 subsequently modified by freezing and thawing. Concentrated in hill- Qr 67 35 47 12 11 50 35 diabase dikes and sheets in the Culpeper basin and vicinity, Virginia and Mary- 24 9 slope depressions and hollows on Sugarloaf Mountain. Thickness rang- u Urbana Formation (Lower Cambrian?)—Predominantly medium-olive- 55 85 Qt 75 88 70 50 40 39 80 land: Ar/ Ar age spectrum results from hornblende and K-feldspar in grano- 30 es from thin veneer to greater than 10 ft. Includes subangular to sub- brown (5 Y 4/4) to light-olive-gray (5 Y 5/2), poorly sorted, graded,
frs 21 37 5 11 55 54 rounded pebbles and cobbles of quartzite and vein quartz derived from uq crossbedded, ripple-marked, calcareous metagraywacke and metasilt- phyres [abs.]: Geological Society of America Abstracts with Programs, v. 24, no. 20 65 81 34 40 fr rocks of Blue Ridge-South Mountain anticlinorium in fan-like aprons stone. Contains light-olive-gray (5 Y 6/1), and light-brownish-gray (5 2, p. 125. 42 19 65 65 ul 51 frs Jd YR 6/1), fine- to coarse-grained, thin- to medium-bedded, crossbedded, Lee, K.Y., 1977, Triassic stratigraphy in the northern part of the Culpeper basin, Vir- Qt 55 69 covering strata along western margin of Culpeper basin. Thickness
75 21 pitted, friable, lensoidal, discontinuous very calcareous metasandstone ginia and Maryland: U.S. Geological Survey Bulletin 1422–C, 17 p.
fr ranges from thin veneer to 3 ft 21 and quartzite (|uq). Interbedded with light-brown (5 YR 5/6) laminated ———1979, Triassic-Jurassic geology of the northern part of the Culpeper basin, 22 67 67 62 Qal 70 68 uq Qr Residuum (Holocene and Pleistocene)—Unconsolidated mixture of ar 10 72 70 60 metasiltstone. Also contains light-gray (N7) to greenish-gray (5 G 6/1), Virginia and Maryland: U.S. Geological Survey Open-File Report 79–1557, scale 35 22 72 moderate reddish brown (10 R 4/6) soil, pebbles, and blocks of grayish 31 88 55 40 53 | 21 thin-bedded crystalline marble ( ul)inlaminated beds of indeterminate 1:24,000. 33 13 11 76 81 69 55 pink (5 R 8/2) to white (N9) angular, locally euhedral quartz derived 23 45 38 35 thickness marked by seams of sericite and chlorite. Poorly exposed; Lee, K.Y., and Froelich, A.J., 1989, Triassic-Jurassic stratigraphy of the Culpeper 23 Jd 40 25 23 Qt 55 45 u from in-place weathering of underlying carbonate rocks. Thickness and Barboursville basins, Virginia and Maryland: U.S. Geological Survey Professio- 46 55 produces distinctive reddish-orange (10 R 6/6) soils bs 85 69 86 Zi ranges from thin veneer to 10 ft 5 42 nal Paper 1472, 52 p. Monocacy River u 5 Sugarloaf Mountain Quartzite (Lower Cambrian?)—Pinkish-gray (5 85 32 75 su 14 80 Qt Terrace deposits (lowest level) (Holocene and Pleistocene)—Sand, Lindholm, R.C., Hazlett, J.M., and Fagin, S.W., 1979, Petrology of Triassic-Jurassic 37 71 65 YR 8/1) to white (N9), fine- to medium-grained, medium-bedded to 67 55 gravel, and boulder deposits 10 to 20 ft thick underlying nearly flat conglomerates in the Culpeper basin, Virginia: Journal of Sedimentary Petrology, fr fr 60 sm massive, well-sorted, graded, crossbedded, ripple-marked, granular 43 53 85 15 Jd uq 44 benches that are 33 to 80 ft above Potomac and Monocacy Rivers v. 49, no. 4, p. 1245–1261.
frs 55 32 75 80 45 quartzite. Quartzite interbedded with seldomly exposed medium-brown 65 45 25 20 sl Muller, P.D., Candela, P.A., and Wylie, A.G., 1989, Liberty Complex; polygenetic 24 69 55 35 38 72 QTt (5 YR 4/4), quartzose metasiltstone and dusky-blue (5 PB 3/2), laminat- 50 20 Terrace deposits (highest level) (Pleistocene and late Tertiary)—Gravel mp frs 65 30 55 58 melange in the central Maryland Piedmont, in Horton, J.W., Jr., and Rast, Nicho- 5 36 and boulder deposits on isolated hillocks as much as 183 ft and 140 ft ed metasiltstone (similar to that of the conformably overlying Urbana 34 34 20 35 fr 10 | | las, eds., Melanges and olistostromes of the U.S. Appalachians: Geological Society M 40 Qal above Potomac and Monocacy Rivers. Clasts of predominantly quartzite Formation) underlies topographic swales. Lower ( sl), middle ( sm), U 12 45 of America Special Paper 228, p. 113–134. I 62 ? 45 | ar 11 and upper ( su)members (informal) separately mapped based on topo- R 74 50 75 71 with Skolithos (trace fossil) have thick weathering rinds 25 67 55 18 50 Nickelsen, R.P., 1956, Geology of the Blue Ridge near Harpers Ferry, West Virginia: O ar 62 45 55 85 65 20 30 fr 80 36 50 graphic expression of ridge-forming basal units because quartzites are 57 N 67 75 82 16 Qr fr Qt 37 42 Geological Society of America Bulletin, v. 67, no. 3, p. 239–269. LI 72 75 65 80 Bennett Creek 72 38 62 62 INTRUSIVE ROCKS virtually identical. Total thickness is approximately 2,000 ft 19 C 50 84 33 60 62 55 ul 60 62 Rankin, D.W., Drake, A.A., Jr., Glover, Lynn, III, Goldsmith, Richard, Hall, L.M., N 42 Y 70 75 60 fr 29 22 36 52 56 Jd Diabase dikes (Early Jurassic)—Medium (N5)- to dark-gray (N3), moder- Murray, D.P., Ratcliffe, N.M., Read, J.F., Secor, D.T., Jr., and Stanley, R.S., 1989, 25 S 42 72 METASEDIMENTARY ROCKS OF THE WESTMINSTER TERRANE 67 54 5 Qal 45 u ately to coarsely crystalline, equigranular, massive diabase with charac- MARTIC THRUST FAULT Pre-orogenic terranes, in Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds., mt 12 82 teristic light-brown (5 YR 5/6) weathered surface. Discontinuous and Ijamsville Phyllite (Lower Cambrian? and Late Proterozoic?)—Dusky- 52 55 Zi The Appalachian-Ouachita orogen in the United States: Boulder, Colo., Geological 19 55 75 55 52 24 65 en echelon subvertical tabular bodies blue (5 PB 3/2), grayish-blue (PB 5/2), very dusky reddish-purple (5 RP 11 50 60 80 45 Society of America, The Geology of North America, v. F–2, p. 7–100. 21 55 50 60 45 5 52 71 46 66 46 2/2), and greenish-gray (5 G 6/1) to pale-olive (10 Y 6/2) phyllite, 19 mp 12 15 50 6 70 Rankin, D.W., Drake, A.A., Jr., and Ratcliffe, N.M., 1990, Geologic map of the U.S. Qal 42 10 60 77 u SEDIMENTARY ROCKS OF THE CULPEPER BASIN 75 58 phyllonite, and minor slate. Contains abundant vein quartz. Intensely Appalachians showing the Laurentian margin and the Taconic orogen, plate 2, in
Zi 27 74 85 Qt 75 47 70 75 48 42 21 15 75 65 Thermally metamorphosed rocks (Upper Triassic)—Dark-gray (N3) to folded and sheared. Finely laminated beds seen only in slate. Phyllite 65 60 19 60 25 60 tm Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds., The Appalachian-Ouachi- 29 65 55 25 consists mostly of muscovite and chlorite and local paragonite and 19 Y 85 Qt 80 Jd 24 75 olive-black (5 Y 2/1) cordierite-spotted hornfels in zoned contact aur- ta orogen in the United States: Boulder, Colo., Geological Society of America, The E 62 34 16 45 55 chloritoid. Lustrous sheen results from paragonite (determined by X-ray 21 L 82 62 80 eole adjacent to diabase dike at Monocacy Natural Resources Area Geology of North America, v. F–2, scale 1:2,000,000. 27 13 L Qr 60 55 54 40 50 A frs 30 78 diffraction) and dark color results from abundant hematite dust Reinhardt, Juergen, 1974, Stratigraphy, sedimentology, and Cambro-Ordovician pa- V 18 55 80 32 80 Balls Bluff Siltstone (Upper Triassic) 32 69 68 50 leogeography of the Frederick Valley, Maryland: Maryland Geological Survey Re- fr 38 Silver Run Limestone (Lower Cambrian? and Late Proterozoic?)—Light- 10 21 19 42 72 su bl Leesburg Member—Light-gray-weathering carbonate conglomerate with Zs 21 21 58 T 12 Jd Qc port of Investigations 23, 74 p. MARTIC THRUST FAULT 45 L 30 bluish-gray (5 B 7/1), and medium-light-gray (N6), thin-bedded, laminated,
fa 30 36 subangular to subrounded boulders, cobbles, and pebbles of grayish and 80 U 56 52 65 carbonaceous and argillaceous metalimestone and minor medium-dark- ———1977, Cambrian off-shelf sedimentation, central Appalachians: Society of Eco- 62 A sm mp 25 77 50 reddish lower Paleozoic limestone and dolomite in reddish-brown peb- frs F 60 82 nomic Paleontologists and Mineralogists Special Publication 25, p. 83–112. 20 82 65 15 gray (N4), finely laminated carbonaceous phyllite. Complexly folded and 24 60 82 50 20 27 55 68 55 60 bly sandstone and calcareous siltstone matrix
uq 40 Scotford, D.M., 1951, Structure of the Sugarloaf Mountain area, Maryland, as a key 19 80 75 85 40 exposed only along a tributary of Monocacy River west of St. Paul Church 35 31 82 60 18 fr 20 T Qc bs Fluvial sandstone and siltstone member—Dark-reddish-brown (10 R 3/4), to Piedmont stratigraphy: Geological Society of America Bulletin, v. 62, no. 1, p. 70 28 75 70 30 S uq 55 29 35 su 9 U 77 fine- to medium-grained, thin- to medium-bedded, locally crossbedded, 45–75. K 18 R 35 25 C 36 Smoot, J.P., 1989, Fluvial and lacustrine facies of the early Mesozoic Culpeper basin, 27 35 35 I 60 H 60 54 feldspathic, silty sandstone interbedded with dusky-red (5 R 3/4), thin- 27 R T 38 55 15 35 29 E Qc bedded, bioturbated, calcareous, micaceous, feldspathic, clayey and san- Virginia and Maryland, in Hanshaw, P.M., ed., Field trips for the 28th International 15 D 85 83 20 21 42 Zi Qal 68 Bear Branch 10 65 E ar 70 dy siltstone in repetitive sequences 3 to 10 ft thick. Grades down into Geological Congress; Field Trip Guidebook T213: Washington, D.C., American R 55 70 C 78 I 27 F EXPLANATION OF MAP SYMBOLS 43 ul 5 62 68 T ^ Geophysical Union, 15 p. 79 64 41 30 20 Poolesville Member of Manassas Sandstone ( mp) and intertongues lat-
u 11 14 mt R 18 32 49 A 44 50 erally with Leesburg Member (^bl). Composite thickness estimated to Southworth, Scott, 1996, The Martic fault in Maryland and its tectonic setting in the 11 21 82 81 30 Contact—Dashed where approximately located or projected in cross sec- 5 M Jd 68 45 exceed 5,000 ft central Appalachians, in Brezinski, D.K., and Reger, J.P., eds., Studies in Maryland 62 Jd sl tion; dotted where concealed u bs mp 82 67 16 36 geology: Maryland Geological Survey Special Publication 3, p. 205–221. Jd 78 55 57 ar 60 Manassas Sandstone (Upper Triassic) Faults—Dashed where approximately located or projected in cross sec- 31 78 80 28 44 40 21 Qr 72 52 sm ———1998, Geologic map of the Poolesville quadrangle, Frederick and Montgomery C 10 77 80 54 65 80 mp tion; dotted where concealed Jd 85 10 Poolesville Member—Predominantly medium-gray (N5), pinkish-gray (5 Counties, Maryland, and Loudoun County, Virginia: U.S. Geological Survey Geo- Qal 42 Qc 11 75 85 50 80 73 19 30 YR 8/1), and pale-reddish-brown (10 R 5/4), fine- to coarse-grained, Thrust fault—Sawteeth on upper plate. In cross section, half arrow
u 76 logic Quadrangle Map GQ–1761, scale 1:24,000. 10 45 65 68 86 u 84 60 60 32 thick-bedded, arkosic and micaceous sandstone; locally pebbly and shows direction of relative movement 37 32 30 sm ———1999, Geologic map of the Urbana quadrangle, Frederick and Montgomery 55 57 74 77 27 5 25 68 Qal 75 20 crossbedded where it fills channels; commonly interbedded with calcare- 77 61 72 45 18 55 Overturned thrust fault with recurrent thrust motion—Sawteeth on up- Counties, Maryland: U.S. Geological Survey Geologic Quadrangle Map GQ–1768, 67 55 38 82 58 22 72 60 ous, dark-reddish-brown (10 R 3/4) siltstone in upward-fining sequen- su frs 72 27 scale 1:24,000. 17 45 per plate, open bar on early upper plate Qt 35 55 72 18 ces in upper part of unit. Grades down into and intertongues with Re- 15 56 73 11 Southworth, Scott, Burton, W.C., Schindler, J.S., Froelich, A.J., Aleinikoff, J.N., and 20 72 40 Qc 68 80 49 ston Member of Manassas Sandstone (^mr). Estimated thickness as Normal fault—Bar and ball on downthrown block 25 Qal 20 65 Drake, A.A., Jr., in press, Geologic map of Loudoun County, Virginia: U.S. Geo- mt 82 30 47 85 46 18 46 30 70 much as 3,000 ft 67 Zi 38 Folds—Dotted where concealed logical Survey Miscellaneous Investigations Series Map I–2533, scale 1:50,000. 78 35 27 15 41 sm 78 70 62 52 Qal 25 B 60 77 30 46 40 30 Stose, A.I.J., and Stose, G.W., 1946, Geology of Carroll and Frederick Counties, UALA ONANANTICLINORIUM MOUNTAIN SUGARLOAF Jd Tuscarora Creek Member—Light (N7)- to dark-gray (N3) and light-red (5 Qt 82 52 37 11 mt Anticline—Showing axial trace and direction of plunge where known 35 fr 81 UM 38 58 54 20 Qc [Md.], in Carroll and Frederick Counties: Baltimore, Maryland Department of Geol- 45 68 80 55 su 45 R6/6) conglomerate composed of very fine to very coarse grained, an- 61 78 73 10 Overturned anticline—Showing axial trace surface and direction of dip 32 11 44 gular to subangular pebbles and cobbles of limestone and dolomite with- ogy, Mines, and Water Resources, p. 11–131. 54 49 81 67 21 62 of limbs and plunge where known 30 21 25 35 40 in matrix chiefly of limestone and dolomite granules and dusky-red (5 R ———1951, Structure of the Sugarloaf Mountain area, Maryland, as a key to Pied- 35 3 27 82 78 32 TICLINORI 75 18 65 77 77 62 3/4) to grayish-red (5 R 4/2), clayey sand and silt with calcite cement. Syncline—Showing axial trace and direction of plunge where known mont stratigraphy: Geological Society of America Bulletin, v. 62, p. 697–699. 72 75 Qr frs 75 50 34 21 31 32 83 85 su AN 26 Stose, G.W., 1906, The sedimentary rocks of South Mountain, Pennsylvania: Jour- 11 75 Limestone and dolomite clasts derived from Cambrian and Ordovician 25 Qal 46 10 Qc 40 25 Overturned syncline—Showing axial trace and direction of dip of limbs
sl 66 75 35 30 48 Qal 21 33 29 75 50 25 35 carbonate strata. Estimated thickness ranges from 0 to 233 ft nal of Geology, v. 14, p. 201–220. fa 75 80 55 54 and plunge where known 5 67 57 30 82 13 65 65 74 44 70 Stose, G.W., and Jonas, A.I., 1935, Limestones of Frederick Valley, Maryland: 7 75 ar 55 60 70 80 28 mr Reston Member—Light-gray (N7) to pinkish-gray (5 YR 8/1) variegated F 22 76 30 65 86 82 Arch in transposition foliation (F)—Found in Ijamsville Phyllite in north- 56 60 80 5 70 63 Washington Academy of Science Journal, v. 25, no. 12, p. 564–565. 10 59 60 32 40 17 72 24 fr 65 86 60 u 60 25 pebble, cobble, and boulder conglomerate containing clasts of phyllite, eastern part of quadrangle mp 87 70 Qc 62 25 Thomas, B.K., 1952, Structural geology and stratigraphy of Sugarloaf anticlinorium 18 65 58 7 10 80 25 schist, quartzite, and quartz in poorly sorted, coarse-grained, arkosic ? 32 60 35 F 65 24 Qal 43 24 25 Trough in transposition foliation (F)—Found in Ijamsville Phyllite in and adjacent Piedmont area, Maryland: Baltimore, Md., The Johns Hopkins Uni- 16 11 5 78 77 sm 45 70 51 sandstone matrix; locally interbedded with pale-reddish-brown (10 R 59 20 17 50 20 45 northeastern part of quadrangle versity, unpublished Ph.D. dissertation, 95 p. mr 54 67 40 5/4) sandstone and siltstone. Basal conglomerate unconformably over- 25 32 5 25 Walcott, C.D., 1896, The Cambrian rocks of Pennsylvania: U.S. Geological Survey 22 16 50 32 46 IN 11 74 QTt Jd 34 lies metasedimentary rocks of Westminster terrane. Estimated thickness Minor anticline—Showing bearing and plunge of axis 23 T 63 65 A 32 Bulletin 134, 43 p. 77 L 30 50 Qc T 56
sm U as much as 70 ft N 54 55 9 A 77 Minor syncline—Showing bearing and plunge of axis Whitaker, J.C., 1955, Geology of Catoctin Mountain, Maryland and Virginia: Geo- 40 U 40 u 24 F 42 Zs 20 44 62 30 46 O 25 37 20 31 Qal 25 logical Society of America Bulletin, v. 66, no. 4, p. 435–462. 73 M 53 METASEDIMENTARY ROCKS OF THE FREDERICK VALLEY SYNCLINORIUM Minor asymmetric antiform (F2)incomplex fold train—Showing bearing 10 75 77 Qal mt 37 10 65 70 59 54 uq 82 and plunge of axis 52 fr T 30 20 38 38 Grove Formation (Lower Ordovician and Upper Cambrian) 15 S Qal 14 Zi 32 35 40 25 65 50 sl 45 30 67 RU 31 76 Ogu Upper member (Lower Ordovician)—Medium-light-gray (N6), locally san- H 62 36 su 75 T 85 20 70 30 50 dy, thrombolitic and stromatolitic algal limestone thickly interbedded Zi 65 41 27 mp 82 32 42 Qc 38 72 with medium-gray (N5), laminated dolomitic limestone and olive-gray (5 C 46 I Jd 20 AF 80 PLANAR FEATURES 50 O 50 B 80 T YR 4/1) dolomite. Thickness is greater than 450 ft Qt 57 L 78 50 R 40 ul 59 su 40 A AR 50 uq (May be combined with each other or with linear features)
FEET 75 Qt 57 1600 1200 2000
E LEVEL SEA 51 400 2000 1200 400 1600 800 800 sm 81 O gl Lower member (Lower Ordovician and Upper Cambrian)—Medium-light- 56 M 37 G 65 mt Qc 48 U Qal 50 65 QTt S 52 gray (N6) to medium-gray (N5), thickly bedded and crossbedded, arena- Strike and dip of beds—Ball indicates top of bed known from sedimen- 76 Jd 50 23 35 ceous limestone and sandy dolomitic limestone containing 1-ft-thick in- 67 32 sm 88 tary structures fr fa 42 82 uq 72 mp 76 74 terbeds of medium-light-gray (N6) sandy dolomite. Thickness is 30 35 27 Qt 80 tm 72 36 54 39°15′ C 39°15′ approximately 150 to 400 ft Inclined 77°30′ 77°22′30″ Frederick Formation (Upper Cambrian) Vertical 1 ° Base from U.S. Geological Survey, 1952 10 /2 SCALE 1:24 000 Geology mapped in 1992 and 1994 fl Lime Kiln Member—Interbedded, thinly laminated to thinly bedded, dark- Horizontal Photorevised 1984 1 1/ 2 0 1MILE M
A NORTH TRUE gray (N3), fine-grained limestone; calcareous shale; and fine-grained, G MARYLAND 17 Overturned N
E
Polyconic projection T medium-bedded limestone near base. Becomes more thickly interbed-
I C 1 .5 0 1KILOMETER Strike and dip of slaty cleavage 1927 North American Datum N ded toward the top with medium-dark-gray (N4), fine-grained, wavy- O
R 10,000-foot grid based on Maryland coordinate system T MAP LOCATION 67 H bedded limestone containing local stromatolitic algal beds. Near top, 1000-meter UniversalTransverse Mercator ticks, zone 18 Inclined APPROXIMATE MEAN member becomes interbedded with medium-light-gray (N6), crossbed- DECLINATION, 2001 CONTOUR INTERVAL 20 FEET ded, sandy limestone. Thickness is 600 ft Vertical NATIONAL GEODETIC VERTICAL DATUM OF 1929 fa Adamstown Member—Medium-dark-gray (N4) to dark-gray (N3), fine- Strike and dip of crenulation cleavage grained, argillaceous limestone thinly interbedded with dusky-yellow (5 Y 57 Inclined 6/4) to medium-dark-gray (N4), silty dolomite. Limestone beds range Vertical A A from .01 to 1.55 in. in thickness. Includes several thin, dark-greenish- gray (5 G 4/1) to greenish-black (5 G 2/1), light-olive-brown (5 Y 5/6) Strike and dip of transposition foliation FEET FEET 70 FREDERICK VALLEY SYNCLINORIUM weathering, silty, calcareous shale intervals 6 to 16 ft thick throughout 2000 M 2000 Inclined ARTI member. Top of member is mapped at base of the lowest medium to C 1600 TH 1600 RUST thick bed of sandy or algal limestone. Thickness is approximately 1,600 ft Vertical
FA 1200 1200 U L fr Rocky Springs Station Member—Dark-gray (N3), nodular to lumpy-bed- Strike and dip of joint T 800 t 800 ded, argillaceous, dolomitic limestone at base containing an interval of 66 Route 180 h a Zi Route 355 Interstate 270 Inclined Route 15 and 340 u frs surface bl bs mp mt fr frs ar | fa fl O gl Ogu O gl Route 85 fl fa fr fr grayish-black (N2), platy shale ( frs)45to60ftthick mapped along the 400 Monocacy River 400 Jd eastern flank of the Frederick Valley synclinorium. Upsection grades in- Vertical SEA LEVEL SEA LEVEL to dark-gray (N3), laminated to flaggy-bedded limestone containing dus- 400 mt 400 ky-yellow (5 Y 6/4) to light-olive-gray (5 Y 6/1), silty dolomitic partings ? 800 800 and laminations and contains 1- to 32-ft-thick intervals of medium-dark- ar? frs gray (N4), polymictic breccia that grade upsection into medium-gray 1200 1200 LINEAR FEATURES ? (N5), planar-bedded, arenaceous limestone. Clast sizes in breccia range MARTIC THRUST FAULT 1600 1600 from sand size to 1 ft diameter on western flank of Frederick Valley syn- (May be combined with planar features) ? ? 2000 2000 clinorium and diminish to less than 1.2 to 2 in. in diameter on eastern 80 Bearing and plunge of mineral lineation 400 feet=122 meters flank of Frederick Valley synclinorium. Top of member is mapped at Surficial deposits not shown top of stratigraphically highest polymictic breccia or sandstone interval. 17 Bearing and plunge of intersection of bedding and slaty cleavage Thickness ranges from approximately 1,200 to 2,500 ft ar Araby Formation (Middle and Lower Cambrian)—Light-olive-gray (5 Y 5/2), pale-brown (5 YR 5/2), and moderate-brown (5 YR 4/4), mottled metasiltstone containing sandy intervals. Pervasively cleaved bedding typically obscure C C OTHER FEATURES METASEDIMENTARY ROCKS OF THE BLUE RIDGE- sl FEET FEET Quarry—sl, slate FREDERICK VALLEY SYNCLINORIUM SOUTH MOUNTAIN ANTICLINORIUM 2000 2000 1 Locality of geochemical sample in table 1 M 1600 t Tomstown Formation (Lower Cambrian)—Medium-light-gray (N6) to 1600 A R fr T IC medium-gray (N5), saccharoidal dolomite containing thin (0.04 in.) lay- Sinkhole TH 1200 1200 R bs U S ers of sericite. Found in northwest corner of quadrangle where com- T mr Metasiltstone interbed in Sugarloaf Mountain Quartzite FA 800 800 U su pletely covered by colluvium (Qc). Thickness of 150 ft measured in drill L ul uscarora Creek ar T Route 85 bs T New Design Road ar Zi Bedrock landslide—Hachures on escarpment Monocacy River uq 400 surface mp mt fr fa fr frs mp 400 core by Hoy and Schumacher (1956) fr SEA LEVEL SEA LEVEL Zs mr Jd 400 400 mt u 800 800 Jd 1200 ar? ? 1200 A A´ 1600 1600 c 1 WEST Shelf Shelf/Slope Slope/Rise EAST 2000 2000 lt Sea level fau INTERIOR—GEOLOGICAL SURVEY, RESTON, VA—2003 400 feet=122 meters ust N thr Surficial deposits not shown rtic M Ma Stonehenge Limestone PENNSYLVANIA Conococheague Limestone Grove Formation MARYLAND Elbrook Formation Frederick Formation Waynesboro Formation RG BASI SR Tomstown Formation Araby Formation detrital sphene, tourmaline, zircon, and ilmenite, and characteristic clots of hematite Lime Kiln Member Thermally Metamorphosed Rocks Antietam Formation ? DISCUSSION YSBU T Harpers Formation Urbana Formation after magnetite(?). Dark-blue to black, laminated metasiltstone and phyllite are inter- | T The Lime Kiln Member ( fl) (Reinhardt, 1974), the uppermost member of the Siltstone, shale, and minor sandstone were baked by metamorphism to hornfels Zu E Weverton Formation Sugarloaf Mountain Quartzite INTRODUCTION AND GEOLOGIC SETTING bedded with the quartzite. Erosion of the rocks forms recessive topographic swales. G Ijamsville Phyllite Frederick Formation, consists of interbedded, thinly bedded limestone and algal lime- (^tm) adjacent to the diabase dike at the southern edge of the map. These brittle Catoctin Formation ? The laminated metasiltstone is lithologically similar to metasiltstone of the Urbana For- The Buckeystown quadrangle is underlain mostly by rocks of the western Piedmont stone. The member records the aggradation from basinal deposition to shallow shelf rocks are poorly exposed along the bluff of the Monocacy River. Metasiltstone of the Swift Run Formation Silver Run Limestone Tr mation that conformably overlies the highest quartzite. Basement gneiss aj Zi ec province and a portion of the eastern Blue Ridge province. The western Piedmont deposition (Reinhardt, 1974). The Lime Kiln is best exposed along the quarry roads Urbana Formation was locally baked to hornfels by contact metamorphism to | ry The Sugarloaf Mountain Quartzite is divided into informal lower ( sl), middle Ju ar Zw of province is underlain by Late Proterozoic(?) and Lower Cambrian(?) metasedimentary within the Essroc quarry at Lime Kiln. The trilobite Olenellus, brachiopods, and echi- the Martic (|sm), and upper (|su) members that were mapped according to their ridge- and c thrust fault rocks of the Westminster terrane, Lower and Middle Cambrian metasedimentary rocks noderms are found near Buckeystown Station and cephalopods are found west of Diabase Dikes ledge-forming habit; otherwise, the rocks are virtually indistinguishable. Stereoscopic f and Upper Cambrian to Lower Ordovician carbonate rocks of the Frederick Valley Lime Kiln as fossil debris (Reinhardt, 1974). aerial photographs and an orthophotoquadrangle map were used to support field Near vertical, en echelon, north-, northeast-, and northwest-trending diabase dikes c synclinorium, Upper Triassic sedimentary rocks of the Mesozoic Culpeper basin, and 40 39 ER SC mapping of the quartzite units. The lower member (|sl)ispoorly exposed in the core (Jd) were emplaced at about 200 Ma based on Ar/ Ar data (Kunk and others, RIUM M Grove Formation O u IU Martic thrust fault Westminster terrane Early Jurassic dikes; Lower Cambrian metasedimentary rocks underlie the Blue Ridge NO NST I I 2 1992) during continental rifting that led to the opening of the Atlantic Ocean. The di- R L of the Sugarloaf Mountain anticlinorium. The rocks are well exposed along the RANE Grove Formation C province. Within the western Piedmont, Lower Cambrian(?) metasedimentary rocks R I The Grove Formation (Stose and Jonas, 1935; Jonas and Stose, 1938b; Rein- abase is dense, hard, and weathers to large spheroidal boulders aligned along ridges. NO Frederick Formation Northern Peaks trail northwest of the Mountain Loop trail. Crossbedded quartzite T I Ordovician(?) TE of the Sugarloaf Mountain anticlinorium are interpreted to be exposed in a tectonic N ESTM E Araby Formation hardt, 1974) is an interval of Upper Cambrian and Lower Ordovician carbonate rock The diabase dike swarm that transects the eastern part of this quadrangle has been A CL ? can be seen immediately east of Furnace Branch Road north of Bells Chapel. The W N ? ? ?? window (A.A. Drake, Jr., U.S. Geological Survey, oral commun., 1989; Horton and F N G A in the core of the Frederick Valley synclinorium. Two informal members were map- traced over 106 mi from Fauquier Co., Va., to Emittsburg, Md. (J.P. Smoot and oth- MARYLAND Martic thrust fault R Urbana Formation base of the lower member is not exposed; therefore, the 600 ft thickness is approxi- SY N R Silver Run others, 1989; Rankin and others, 1989) through the complexly deformed allochtho- I Sugarloaf Mountain Quartzite ped within the formation in the Buckeystown quadrangle. Rocks of the Grove Forma- VIRGI Y mate. The middle member (|sm) forms a prominent ridge that is followed by the ers, U.S. Geological Survey, unpub. data, 1994). NIA A TE ? Limestone T E nous rocks of the Westminster terrane (Muller and others, 1989). The undated rocks L Northern Peaks trail north of the summit where the quartzite is approximately 800 ft tion contain trilobites, brachiopods, cephalopods, and conodonts of Late Cambrian c N A u of the Westminster terrane are interpreted to be rise-slope deep-water deposits of the AL and early Early Ordovician age (Reinhardt, 1974, 1977). CENOZOIC SURFICIAL DEPOSITS OU thick. Quartzite of the middle member defines the anticline-syncline-anticline triplet A V I Iapetus Ocean that were transported westward onto the Laurentian margin (ancestral M K C of folds on the northern nose of the anticlinorium. These rocks are well exposed be- High-level terrace deposits (QTt)ofthe ancestral Potomac River are preserved 183 H I North America) along the Martic thrust fault during the Ordovician Taconic orogeny T R U Lower member R neath the road and trail north of the west view parking lot. Ripple-marked quartzite ft above the Potomac River north of Maryland Route 28 near Tuscarora. Cobbles of E (Horton and others, 1989). Continental margin strata, which underlie the Sugarloaf DE T Blue Ridge – South Mountain anticlinorium SOU SMA Frederick Valley Sugarloaf Mountain Westminster terrane can be seen south of the stone building at the west view. The upper member (|su)of The basal strata of the Grove Formation consist of crossbedded calcareous sand- E S rounded quartzite have armoured the underlying limestone of the Frederick Forma- R N synclinorium anticlinorium Mountain anticlinorium and continental margin-slope strata which underlie the Freder- F I 3 stone here informally called the lower member (O|gl). Because of its marked compo- M the Sugarloaf Mountain Quartzite is best seen at the summit of Sugarloaf Mountain tion. The original deposit may have been preserved in a sinkhole. Later preferential — To ick Valley synclinorium, are here correlated with the Lower Cambrian Chilhowee T msto sq S e Harpers Formation wn Frederick Formation where approximately 300 ft of quartzite has been folded into a series of anticlines and sitional difference from underlying and overlying strata, this member makes an excel- E l Forma erosion formed a hillock by topographic inversion. In the Poolesville quadrangle to n k il A tion Grove Formation Ijams Group and overlying carbonate rocks on the limbs of the Blue Ridge-South Mountain W w v Weverton Formation ntie v GE tam ille synclines. Gently folded rocks of the upper member can be seen west of Sugarloaf lent marker for mapping purposes. The lower member is well exposed within the the south, similar deposits are as much as 288 ft above the Potomac River (South- SMQ o ric rs Fo Phy t e rma U llite anticlinorium to the west. The relation of the Sugarloaf Mountain Quartzite (fig. 1) le d e Catoctin Formation tion rbana F Road where they plunge beneath rocks of the Urbana Formation. The upper mem- abandoned quarry east of the main Essroc quarry at Lime Kiln. RID d lk or worth, 1998). High-level terrace deposits of the ancestral Monocacy River are pre- re a mation (Jonas and Stose, 1938b) to surrounding rocks is controversial (Scotford, 1951; Stose id F Zs A W ber can also be seen where Bear Branch breaches “west ridge,” especially north of served 140 ft above the Monocacy River at the Monocacy National Battlefield and UE ´ M Swift Run Formation Araby Formation and Stose, 1951; Thomas, 1952). The Martic thrust fault (fig. 1) (Jonas, 1924, L ? ? the water gap at “White Rocks” (quotation marks refer to local names of features Upper member west of Monocacy Natural Resources Area. These terraces are undated but may be as B n 1927; Knopf and Jonas, 1929) and the interpretation that the Sugarloaf Mountain f w Basement gneiss MARY o o a shown on accompanying trail map). old as 5 Ma and as young as 1 Ma. LAN t t n Sugarloaf Mountain Quartzite anticlinorium (Scotford, 1951; Thomas, 1952) is a tectonic window through the Mart- Above the basal sandstone of the Grove Formation are medium- to thick-bedded Zi D s s a VIRG in k y The Sugarloaf Mountain Quartzite is correlated with the Lower Cambrian Weverton Low-level terrace deposits (Qt)ofthe Potomac and Monocacy Rivers and Tuscarora INIA o c e rb ic thrust sheet (A.A. Drake, Jr., U.S. Geological Survey, oral commun., 1989) further limestone and dolomite of the informal upper member (Ogu). The strata are arranged P k Ro c U Formation of the Chilhowee Group which crops out on the limbs of the Blue Ridge- in cycles consisting of thrombolitic and stromatolitic limestone and laminated dolo- Creek are as much as 80 ft above the present channels. Terraces of the Monocacy Late Paleozoic(?) Ma complicates the stratigraphic correlation of these rocks. Upper Triassic sedimentary Ju Bu rtic Yu thru South Mountain anticlinorium (fig. 1). The Sugarloaf Mountain Quartzite is about River are well preserved east of Buckeystown adjacent to broad meanders. n st f rocks of the Culpeper basin consist of westward-dipping conglomerate, sandstone, mite. This member is well exposed in pastures north of Adamstown Road and west of e ault Zu rd ll w 1,800 ft thick. i o Maryland Route 85 and within several abandoned quarries near the large lime kilns Residuum (Qr) superficially resembles the terrace deposits but instead is character- CULPEPER BASIN o v t and siltstone. These rocks, as well as intrusive Early Jurassic diabase dikes, accumulat- rf s n e ized by pebble- to boulder-size, angular to euhedral, white quartz. The residuum re- te l a ed during an early Mesozoic rifting event that resulted in the opening of the Atlantic along the railroad tracks east of the Essroc quarry. a o Urbana Formation o rm sulted from in-place weathering of the underlying carbonate rocks. The original Yu W P e Ocean. Contractional faults of Paleozoic orogenesis and extensional faults related to G | LOWER CAMBRIAN METASEDIMENTARY ROCKS OF THE BLUE RIDGE- quartz filled veins and cavities in the limestone. Some of the cobbles are geodes con- Mesozoic rifting indicate a complex tectonic history for this region. The Urbana Formation ( u) (Edwards, 1986; Urbana Phyllite of Jonas and Stose, Area of Buckeystown quadrangle SOUTH MOUNTAIN ANTICLINORIUM taining quartz crystals. g Cenozoic deposits, which overlie the bedrock, include high- and low-level alluvial 1938b) conformably overlies the Sugarloaf Mountain Quartzite (|su). The contact g Qc bur lin may be seen along a tributary to Bennett Creek at the north end of the Sugarloaf Colluvium ( )isfound on Sugarloaf Mountain and along the western margin of s terraces, residual gravel, colluvium, and alluvium. Terrace deposits of the ancestral e er eneca Culpeper Harpers Formation the Culpeper basin in the northwest part of the map area. Coarse colluvium derived e St S Potomac River and the Monocacy River are as much as 183 ft and 140 ft, respec- Mountain anticlinorium and on the west side of “west ridge” south of Bear Branch. L basin The Urbana Formation contains a wide variety of metasedimentary rocks that include, |h from quartzite is concentrated in hillslope depressions on Sugarloaf Mountain. Large Approximate level of tively, above the present river levels. Isolated residual gravel deposits that form in The Lower Cambrian Harpers Formation ( )(Keith, 1894) of the Chilhowee M artic t present erosion | colluvial blocks that spalled from the face of the summit can be seen at the parking lot hrust place from the weathering of the Upper Cambrian Frederick Formation superficially in decreasing abundance, metasiltstone and metagraywacke ( u), calcareous meta- Group consists of phyllitic metashale, metasiltstone, and silty metasandstone. The QUADRANGLE INDEX fault resemble terrace deposits. Colluvium of quartzite boulders is concentrated in hill- sandstone and quartzite (|uq), and marble (|ul). Locally, these rocks are poorly ex- Harpers is poorly exposed in the extreme northwest corner of the map area. The and trails at the “west view.” Colluvial deposits 10 ft thick choke Bear Branch and Triassic and Jurassic slope depressions on Sugarloaf Mountain, and fanlike aprons of colluvial quartz peb- posed because of a deep regolith of the decomposed calcareous and sandy strata. type locality is approximately 13 mi to the west at Harpers Ferry, W. Va. On the west can be seen at the water gap of “west ridge.” Fine colluvium derived from weathering bles cover the Triassic rocks on the west side of the Culpeper basin. Alluvium was The metasiltstone and metagraywacke (|u)are poorly sorted sediments that con- limb of the Blue Ridge-South Mountain anticlinorium, rocks of the Harpers Formation of the rocks of the Blue Ridge-South Mountain anticlinorium forms dissected aprons mapped along the Potomac and Monocacy Rivers and all their tributaries. Altitude tain detrital saussurite, calcite, orthoclase, tourmaline, and olivine(?). Bedding is de- contain Skolithos burrows and Rusophycus traces (Nickelsen, 1956; Brezinski, that mantle the Paleozoic and Mesozoic bedrock. This colluvium consists predomi- ranges from 200 ft along the Potomac River to 1,282 ft on the crest of Sugarloaf fined by concentrations of heavy minerals. These rocks are best seen north of Ben- 1992). nantly of subangular to subrounded pebbles and cobbles of vein quartz and minor A B Mountain. The map area includes the Chesapeake and Ohio Canal National Histori- nett Creek. The calcareous metasandstone and quartzite unit (|uq)ofthe Urbana quartzite and metasandstone. Locally, the finer colluvium resembles both terrace de- cal Park, and the Monocacy Natural Resources Area. Sugarloaf Mountain is a regis- Formation is lensoid, discontinuous, and difficult to trace in the field. The calcareous Antietam Formation posits and the residuum unit. Alluvium (Qal) lies along all drainages, but the best deposits are along the Potomac tered natural landmark. metasandstone is medium- to coarse-grained quartz sand in a clay-rich matrix contain- The Lower Cambrian Antietam Formation (|a) (Keith, 1894) of the Chilhowee and Monocacy Rivers. The current river channels are incised into bedrock and the al- Parts of the Buckeystown quadrangle were mapped by Jonas and Stose (1938a, ing abundant crystals and seams of calcite. The rock is characteristically friable and Group consists of metasandstone and sandy metasiltstone that underlies the main luvium crops out on the banks. scale 1:62,500), Scotford (1951, scale 1:12,500), Thomas (1952, scale 1:25,000), contains many vugs. The quartzite consists of medium- to coarse-grained quartz and ridge of the Blue Ridge-South Mountain anticlinorium in the extreme northwest corner REGIONAL CORRELATION OF MAP UNITS SHOWN IN FIGURE 1A AND B Cloos and Cook (1953, 1:62,500), Reinhardt (1974, scale 1:62,500), Froelich polycrystalline quartz lithic clasts cemented by silica; accessory detrital minerals include of the map area. The type locality of the Antietam Formation is approximately 13 mi STRUCTURAL GEOLOGY (1975, scale 1:62,500), and Lee (1979, scale 1:24,000). zircon, magnetite, orthoclase, perthite, plagioclase, and ilmenite. The quartzite is best to the west at Antietam, Md. The Antietam Formation is the lithic equivalent of the Blue Ridge-South Mountain Culpeper and Gettysburg Frederick Valley Westminster terrane The map area is subdivided into five domains (fig. 1). Allochthonous rocks of the seen along Bennett Creek south of Park Mills. Quartzite containing blue-black quartz- Araby Formation although the Antietam Formation is older. On the west limb of the In the Buckeystown quadrangle, polydeformed slope rise to slope prism deposits anticlinorium basins synclinorium Parautochthonous rocks of the Allochthonous rocks Westminster terrane are thrust onto rocks of the Frederick Valley synclinorium. With- ite clasts can be seen along the Frederick and Montgomery County line east of Strong- Blue Ridge-South Mountain anticlinorium, the Antietam contains the trilobite Olenel- were thrust over parautochthonous rocks of the Sugarloaf Mountain anticlinorium and Sugarloaf Mountain anticlinorium | in the Westminster terrane of Muller and others (1989), the Sugarloaf Mountain hold. Marble ( ul) contains sericite, chlorite, graphite, and detrital quartz, zircon, mi- lus (Walcott, 1896) and the trace fossils Skolithos, Rusophycus, and Planolites (Bre- Frederick Valley synclinorium along the Martic thrust fault during the Ordovician Ta- and Bush Creek window crocline, plagioclase, and orthoclase. The marble is well exposed on the bluffs of conic orogeny (A.A. Drake, Jr., U.S. Geological Survey, oral commun., 1989; Drake Jurassic and Triassic Quartzite and Urbana Formation underlie the parautochthonous Sugarloaf Mountain zinski, 1992). J u rocks, undivided JURASSIC AND TRIASSIC anticlinorium. The Lower and Middle Cambrian Araby Formation and Upper Cambri- Bennett Creek east of Bear Branch. This impure carbonate rock does not resemble and others, 1989; Horton and others, 1989; Rankin and others, 1989; Rankin and Ordovician and any other carbonate rock within the quadrangle. others, 1990). In this interpretation, the Martic thrust sheet was folded with rocks of Ou Grove Formation (G) ORDOVICIAN AND an Frederick Formation crop out on the east limb of the Frederick Valley synclinori- Tomstown Formation Cambrian rocks, undivided O g CAMBRIAN um. The Frederick Formation and Upper Cambrian and Lower Ordovician Grove Rocks of the Urbana Formation are correlated with lithologically similar metasilt- the Sugarloaf Mountain anticlinorium during the late Paleozoic Alleghanian orogeny; Dolomite of the Lower Cambrian Tomstown Formation (|t) (Stose, 1906) was f Frederick Formation (F) Formation lie in the trough of the Frederick Valley synclinorium. Lower Cambrian stone, metasandstone, and limy metashale of the Lower Cambrian Harpers Formation subsequent erosion exposed the Sugarloaf Mountain Quartzite and Urbana Formation encountered in drill core along the Mesozoic basin border fault in the adjacent Point of Harpers, Antietam, and Tomstown Formations crop out on the east limb of the Blue of the Chilhowee Group in the Blue Ridge-South Mountain anticlinorium, as earlier in a tectonic window (fig. 1). Like the Blue Ridge-South Mountain anticlinorium, c Chilhowee Group ar Araby Formation (A) u Urbana Formation (U) CAMBRIAN Rocks quadrangle by Hoy and Schumacher (1956). The Antietam and Tomstown Ridge-South Mountain anticlinorium in the extreme northwest corner of the map. proposed by Scotford (1951). Rocks of the Urbana Formation are crossbedded and rocks of the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium are Sugarloaf Mountain Formations are roughly time correlative to the Araby Formation exposed to the east. sq The Blue Ridge-South Mountain anticlinorium is separated from the Frederick Valley ripple-marked as first recognized by Hopson (1964) and are interpreted to be conti- mostly continental-margin deposits of Laurentia and they contain evidence of only Quartzite (SMQ) The type locality of the Tomstown Formation is approximately 30 mi to the northwest synclinorium by Upper Triassic sedimentary rocks in the Culpeper basin half graben. nental margin deposits. one phase of folding and axial-planar cleavage development (fig. 2). The overlying Zi Ijamsville Phyllite (I) in Pennsylvania. The trilobite Olenellus (Fauth, 1968) and the mollusc Salterella Figure 1 shows the type localities of rock units. polydeformed Silver Run Limestone and Ijamsville Phyllite of the Westminster terrane Early Cambrian and Late CAMBRIAN AND (OR) conulata (Brezinski, 1992) have been recognized in the Tomstown Formation on the Zu Zs Silver Run Limestone (SR) CAMBRIAN AND ORDOVICIAN ROCKS OF THE FREDERICK VALLEY are allochthonous. Contractional motion of the Martic thrust fault occurred during the Proterozoic rocks, undivided LATE PROTEROZOIC west limb of the Blue Ridge-South Mountain anticlinorium. Undivided rocks of the Westminster terrane: ROCKS OF THE WESTMINSTER TERRANE SYNCLINORIUM Ordovician with later thrusting and possible dextral strike-slip motion occurring during Zw Sams Creek Formation (SC), Ijamsville Phyllite, the late Paleozoic Alleghanian orogeny. Because the geology is complex and the in- Middle Proterozoic Marburg Formation (M), Wissahickon Formation, MESOZOIC ROCKS Yu basement rocks,undivided and Gillis Formation MIDDLE PROTEROZOIC Silver Run Limestone Araby Formation terpretations controversial, an interpretative sequence of cross sections (fig. 2) por- The Silver Run Limestone (|Zs) (Jonas and Stose, 1938b) was named for a karstic The Lower and Middle Cambrian Araby Formation (|ar) (Reinhardt, 1974, 1977) Culpeper Group of the Newark Supergroup trays this model. valley in the Littlestown quadrangle, Carroll County, Md., but the best exposure is consists predominantly of argillaceous, burrowed, mottled metasiltstone that contains Structural elements in the rocks include bedding, cleavage, transposition foliation, considered to be in the Union Bridge quadrangle (Edwards, 1986), Frederick County, sandy intervals and has a phyllitic metashale at the top. The type locality is along Manassas Sandstone crenulation cleavage, mineral lineations, intersection lineations, folds, and faults. Figure 1.—A, Generalized geologic map of parts of northern Virginia and Maryland showing location of the Buckeystown, Md., quadrangle (box), and type localities of rock units (indicated by initials). These elements are discussed below by terrane and geographic location. In summary, Md. In the Buckeystown quadrangle, thin-bedded, laminated, argillaceous metalime- Bush Creek south of Frederick Junction. Rocks of the Araby Formation are well ex- The Manassas Sandstone of Late Triassic age was divided into three members by B, Schematic cross sections (see A-A' in part A)showing the inferred environment of deposition (part 1) and structural evolution (parts 2 and 3) of rocks of the Piedmont and Blue Ridge provinces, Maryland. bedding, cleavage, and bedding-cleavage intersection of the rocks of the Sugarloaf stone crops out along a tributary north of the Monocacy River northwest of St. Paul posed along the railroad tracks east of Frederick Junction, along the Monocacy River Lee and Froelich (1989). The Reston and Tuscarora Creek Members are basal con- Mountain anticlinorium and Frederick Valley synclinorium support the concept of one Church. By inference, the metalimestone underlies the linear valley along strike as it east of Buckeystown, west of Greenfield Mills, and along the road west of Lilypons. glomerates that are overlain conformably by sandstone of the Poolesville Member. phase of folding. Cleavage in these parautochthonous rocks is mostly coplanar with is well exposed along the Monocacy River to the south in the Poolesville quadrangle The metasiltstone is highly cleaved and jointed and the bedding is commonly obscure. Reston Member—The Reston Member of the Manassas Sandstone (^mr) (Lee, transposition foliation in the allochthonous rocks. Second-phase folds of foliations (Southworth, 1998). There, the metalimestone is overlain by Ijamsville Phyllite. In The rocks underlie ridges that have a thin soil. Classic ridge and valley topography 1977) is a basal conglomerate that unconformably overlies rocks of the Piedmont. and vein quartz in the allochthonous rocks of the Westminster terrane help to define the Buckeystown quadrangle the metalimestone crops out within polydeformed phyl- formed on the east limb of the Frederick Valley synclinorium where the Araby is tight- The type locality is along the Washington and Old Dominion bike trail near Reston the Martic thrust sheet. lite of the Ijamsville Phyllite. Fisher (1978) mapped the Silver Run Limestone within ly folded with the Frederick Formation. Parkway in the Vienna 7.5-minute quadrangle in northern Virginia (Drake and Lee, Ijamsville Phyllite in the New Windsor quadrangle and also interpreted the metalime- The metasiltstone consists predominantly of quartz sand grains, polycrystalline 1989). The Reston Member is characterized by cobbles and pebbles of vein quartz Allochthonous Rocks of the Westminster Terrane stone to be beneath the phyllite. The Silver Run Limestone probably was deposited in quartz, and opaque minerals that are supported by a clay-rich matrix containing abun- and subangular phyllite and schist derived from the underlying Ijamsville Phyllite in deep water prior to the mud (protolith of the phyllite). Alternatively, the metalime- dant hematite. Sericite, chlorite, magnetite octahedra, sphene, zircon, microcline, sa- the map area. Good outcrops can be seen north of Monocacy River, east of St. Polydeformed, allochthonous rocks of the Silver Run Limestone and Ijamsville stone may constitute blocks (sedimentary olistoliths) deposited in the mud. nidine, and plagioclase are accessory grains. The Araby Formation probably repre- Paul Church. Phyllite of the Westminster terrane constitute the Martic thrust sheet. The Martic sents the deep-water slope facies of a starved clastic basin that persisted in the Tuscarora Creek Member—The Tuscarora Creek Member (^mt) (Lee and Froelich, thrust fault (Jonas, 1924; 1927; Knopf and Jonas, 1929; Southworth, 1996) has Ijamsville Phyllite | Cambrian (Reinhardt, 1977). Rocks of the Araby Formation contain fragments of the 1989) is a basal conglomerate that unconformably overlies carbonate rocks of the Up- brought the Ijamsville Phyllite onto the Urbana, Araby, and Frederick Formations. steeply eastward and quartzite of the Urbana Formation ( uq)istransected by the METAMORPHISM 77°25′ 77°22′30″ The type locality of the blue, green, and purple phyllitic slate of the Ijamsville Phyl- trilobite Olenellus of late Early Cambrian age (Reinhardt, 1974). The Araby Forma- per Cambrian Frederick Formation. It is laterally equivalent to the Reston Member The trace of the Martic thrust fault is straight from the Potomac River (Southworth, fault north of Flint Hill. The Urbana Formation was thrust onto the klippe of Ijams- 39°17′30″ lite (Jonas and Stose, 1938b) is the town of Ijamsville in the Urbana quadrangle tion is conformably overlain by carbonate rocks of the Rocky Springs Station Member but differs lithically because of different source rocks. The unit can be best seen at the 1996) north to the Mesozoic Gettysburg basin (Edwards, 1988), suggesting a moder- ville Phyllite during formation of the Sugarloaf Mountain anticlinorium. The klippe Westminster Terrane | (Southworth, 1999). Rocks composing the Ijamsville Phyllite are (in order of decreas- of the Frederick Formation ( fr). The contact can be seen in a tributary south of type locality along Route 28, east of Tuscarora Creek and west of Tuscarora. The ately steep attitude at the surface (fig. 1). Along Bennett Creek within one meter of masks the relationship between the Urbana Formation and the Araby and Frederick Rocks of the Westminster terrane had a complex metamorphic history under ing abundance) hematite-rich muscovite-chlorite-paragonite-chloritoid phyllite, phyllon- Greenfield Mills. Tuscarora Creek Member is characterized by the presence of limestone and dolomite the fault, transposition foliation in both the Ijamsville Phyllite (hanging wall) and Fred- Formations. A minor southwestward-directed intraformational thrust fault in the Ur- greenschist-facies conditions. The Ijamsville Phyllite contains varying proportions of ite, and slate (undifferentiated) (|Zi). The Ijamsville Phyllite is characterized by com- clasts derived from the underlying Frederick Formation. The conglomerate is locally erick Formation (footwall) dips 50° to the southeast. The Martic thrust fault was fold- bana Formation south of Bennett Creek is related to the formation of the northeast- muscovite-chlorite-paragonite-quartz-magnetite, and calcite, of a first metamorphic Frederick Formation posite foliations, abundant and strongly folded vein quartz, and polymetamorphic lensoidal and discontinuous and was interpreted to be a channel fill and (or) fan depos- ed and erosion exposed rocks of the Sugarloaf Mountain anticlinorium in a tectonic plunging anticlinorium. event. In fault zones, the phyllites have been retrogressively metamorphosed and features. The unit is commonly poorly exposed, but its presence is indicated by phyl- The Upper Cambrian Frederick Formation (Frederick Limestone of Keyes, 1890; its (Lee and Froelich, 1989). window. On the west side of the window, the Martic was reactivated, bringing foot- sheared into phyllonitic diaphthorites (Knopf, 1931). Such rocks are characterized by Frederick Valley Synclinorium lite chips and an abundant float of vein quartz that mantles the clay-rich soil of a dis- Stose and Stose, 1946; Reinhardt, 1974, 1977) is a thick interval of thin- to medium- Poolesville Member—The Poolesville Member (^mp) (Lee and Froelich, 1989) is wall rocks of the Urbana Formation onto the Ijamsville Phyllite during formation of “sick-looking” (Knopf, 1931) retrograde chlorite and recrystallized quartz. Abundant sected plateau. The Ijamsville Phyllite is located in a fault zone, the base of which is bedded limestone, dolomite, and thin intervals of shale and sandstone. Because of the an arkosic, muscovite-rich sandstone that contains sparse quartz pebbles and fines up- the anticlinorium. The asymmetrical Frederick Valley synclinorium contains Cambrian and Ordovician leucoxene in these rocks also suggests retrogressive metamorphism (Stose and Stose, the Martic thrust fault. numerous rock types present in this unit, it is here revised as the Frederick Formation. ward to siltstone. The unit is transitional with the basal conglomerate members. The The dominant planar structure in the Ijamsville Phyllite is a composite foliation that rocks (Reinhardt, 1974). On the east limb of the synclinorium, rocks of the Araby 1946; Scotford, 1951; Hopson, 1964). Ijamsville Phyllite locally contains static White Most of the Ijamsville in the Buckeystown quadrangle is phyllonite that contains The three members of Reinhardt (1974 and 1977) were recognized and mapped Poolesville Member grades up into the Balls Bluff Siltstone; therefore, the contact in consists mostly of a transposition foliation overprinted by a phyllonitic foliation and and Frederick Formations were folded into a series of northeast- and southwest-plung- chloritoid that grew after the deformation or is related to a later prograde metamor- Rocks abundant vein quartz. These rocks are best exposed along Bennett Creek west of within the Frederick Formation. this quadrangle is approximate. The type locality is 7 mi to the south at Poolesville, several sets of cleavage. Vein quartz was sheared and folded into steeply plunging iso- ing anticlines and synclines (fig. 3). These folds and attendant axial-plane cleavage are phic event. Park Mills and along Maryland Route 355 in the northeast part of the map. Slate is Md. In this quadrangle, the unit is best exposed between Pleasant View and Doubs clines. The transposition foliation and folded vein quartz were deformed by westward- best seen north of Bennett Creek. The Araby Formation is well cleaved and highly well exposed in a quarry west of Hope Hill. Rocky Springs Station Member along Route 28 west of Tuscarora Creek. verging inclined F2 folds that plunge steeply to gently northwest or southeast (fig. 3). jointed. South of Lilypons and northwest of Hope Hill, antiforms similar to those in Rocks of the Sugarloaf Mountain Anticlinorium, Frederick Valley Synclinorium, These folds have an axial-planar pressure-solution cleavage that strikes northwest and and Blue Ridge-South Mountain Anticlinorium The hematite-rich phyllite originally was deep-water mud deposited on the conti- The lower member of the Frederick Formation, the Rocky Springs Station Mem- the Araby Formation and topographic valleys suggest the presence of nonexposed Balls Bluff Siltstone dips northeast and can be seen in the immediate hanging wall of the Martic thrust nental slope (fig. 1). Chemical data (table 1 and fig. 2) of the phyllites shows that ber (|fr) (Reinhardt, 1974), is characterized by intervals of locally traceable polymictic folds of Frederick Formation. For example, the linear valley at Greenfield Mills is a The Sugarloaf Mountain Quartzite and Urbana Formation are at chlorite-grade re- the ratio of K Otototal alkalies differs and may reflect different source areas for the fault southeast of Lilypons and along Bennett Creek. Bedding is seen only as laminae 2 breccia. This member resulted from off-shelf deposition at the toe of a slope (Rein- The type locality of the Balls Bluff Siltstone (Lee and Froelich, 1989) is the Balls tight syncline in the Frederick Formation. sulting from a single greenschist-facies metamorphic event. The matrix of the quartz- in phyllite and slate in the quarry west of Hope Hill. In the Buckeystown quadrangle, parental muds (Fisher, 1978) or may indicate a volcanic component of the muds hardt, 1974). The Rocky Springs Station is best exposed along the Monocacy River Bluff National Cemetery in the adjacent Waterford quadrangle, Va. (Burton and The Lime Kiln Member of the Frederick Formation and the Grove Formation de- ites is rich in sericite and contains sparse chlorite porphyroblasts. Cleavage in meta- rocks of the Ijamsville Phyllite constitute a fault zone. The Silver Run Limestone is (Clark, 1924). within the Monocacy National Battlefield, southwest of Maryland Route 355. Good others, 1995). Although the unit is predominantly siltstone (^bs), it locally contains fine a series of folds in the keel of the westward overturned synclinorium. graywacke and metasiltstone of the Urbana Formation is marked by aligned minute poorly exposed in the map area. To the south in the Poolesville quadrangle (South- outcrops can also be seen along Bennett Creek, north of Lilypons. An interval of interbedded sandstone. In the map area, the Balls Bluff Siltstone is poorly exposed The Harpers, Antietam, and Tomstown (subsurface) Formations in the extreme sericite crystals. The metasiltstone contains abundant chlorite porphyroblasts that lo- LOWER CAMBRIAN(?) ROCKS OF THE SUGARLOAF MOUNTAIN worth, 1998), the Silver Run Limestone is folded into F antiforms that exhibit rod- gray to black shale (|frs) occurs on the east limb of the Frederick Valley synclinorium but can be seen west of Doubs. 2 northwest corner of the map area constitute a southeast-dipping homoclinal sequence cally give them a green hue. ANTICLINORIUM ding and contain a strong south-plunging lineation. and can be seen along Maryland Route 355. The trilobite Olenellus was found near Leesburg Member—The Leesburg Member (^bl)ofthe Balls Bluff Siltstone (Lee that forms the east limb of the Blue Ridge-South Mountain anticlinorium and the west The Harpers, Antietam, Tomstown, Araby, Frederick, and Grove Formations con- limb of the Frederick Valley synclinorium. Sugarloaf Mountain Quartzite the top of the member in thin-bedded limestone (Reinhardt, 1974). and Froelich, 1989) is a conspicuous carbonate-clast fanglomerate composed of Parautochthonous Rocks tain sparse tiny crystals of sericite and minor chlorite porphyroblasts, both of which subangular to subrounded boulders, cobbles, and pebbles of limestone and dolomite in are crudely aligned on the cleavage. The Sugarloaf Mountain Quartzite (Jonas and Stose, 1938b) was named for Sugar- Adamstown Member areddish-brown sandy siltstone matrix. Lindholm and others (1979) suggested that Rocks of the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium Culpeper Basin loaf Mountain. The Sugarloaf Mountain Quartzite consists of medium-bedded to mas- the carbonate clasts were derived from Cambrian and Ordovician rocks of the Grove, have experienced a deformational and metamorphic history similar to that of the Blue 77°30′ 77°22′30″ The Adamstown Member (|fa) (Reinhardt, 1974), the middle member of the Fred- The Culpeper basin in the Buckeystown quadrangle is a half-graben that is bound- 39°22′30″ sive, medium- to coarse-grained, saccharoidal, white, gray, tan, and maroon quartzite Ridge-South Mountain anticlinorium to the west (Southworth and others, in press). In e erick Formation, consists of thinly bedded limestone and thin intervals of shale. The Elbrook, and Conococheague Formations. Isolated outcrops are exposed along the ed on the west by an east-dipping normal fault that places the Upper Triassic Lees- g cemented by silica and sericite. Crossbedding and sparse ripple marks support the in- the Buckeystown quadrangle, these rocks are structurally and stratigraphically discord- Adamstown resulted from deposition in a basinal environment (Reinhardt, 1974). road northwest of Adamstown. The conglomerate was interpreted by Smoot (1989) burg Member of the Balls Bluff Siltstone against dolomite of the Lower Cambrian terpretation that these rocks are continental-margin deposits. The quartzite contains ant to the allochthonous Silver Run Limestone and Ijamsville Phyllite. In general, the The Adamstown is well exposed on the south and north faces of the Essroc quarry at to be debris flows on an alluvial fan. The conglomerate, locally called “Potomac Tomstown Formation (Whitaker, 1955). On the east side of the Culpeper basin, the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium constitute a sys- Lime Kiln. The trilobite Olenellus, brachiopods, and echinoderms are found as fossil marble,” was used for the columns in Statuary Hall of the Capitol building in Upper Triassic Tuscarora Creek Member of the Manassas Sandstone unconformably Al2O3 tem of parasitic folds to the Blue Ridge-South Mountain anticlinorium (figs. 1 and 2). hash southeast of Doubs (Reinhardt, 1974). Washington, D.C. overlies the Upper Cambrian Frederick Formation and the Upper Triassic Reston West Rid Member of the Manassas Sandstone unconformably overlies the Ijamsville Phyllite. Sugarloaf Mountain Anticlinorium Both of the carbonate-clast fanglomerates (Tuscarora Creek and Leesburg Members) Autochthonous rocks of the Allochthonous rocks of the Parautochthonous rocks of the Sugarloaf Mountain anticlinorium Scotford (1951) and Thomas (1952) demonstrated that the structural geology of are debris-flow deposits that formed along fault escarpments during early Mesozoic Frederick Valley synclinorium Westminsterterrane Sugarloaf Mountain and the surrounding area constitutes a doubly plunging anticlinori- rifting. Clasts of limestone and dolomite derived from the Elbrook and Conocochea- N N N N um that is overturned to the west. An anticline-syncline-anticline triplet of Sugarloaf gue Limestone, and the Grove Formation (Lindholm and others, 1979) suggest that Na M M A A Mountain Quartzite constitutes the north-plunging nose of the anticlinorium south of these rocks formed a topographically high fault scarp that has since been eroded. Af- R R O T T 2 IC IC 2 O Bennett Creek. The main anticlinorium of Sugarloaf Mountain Quartzite has been ter deposition and lithification of the fanglomerates, faulting continued, resulting in K T T H H R R U U thrust onto quartzite of the middle and upper members that underlie the “west ridge.” west-dipping strata. The greatest thickness of preserved basin fill is along the western S S T T F This is the Sugarloaf fault of Thomas (1952) which was first recognized by Keyes border fault. Smaller down-faulted basins formed by the intersection of northeast and F A A U U L (1891). The “west ridge” is also an anticlinorium. A south-plunging anticline can be northwest-striking normal faults. The Triassic rocks between Tuscarora Creek and L T 80 80 T seen along Furnace Branch in the Poolesville quadrangle to the south (Southworth, east of Tuscarora, Md., were down-dropped along such faults. The Araby Formation in the footwall of the normal fault east of Tuscarora is truncated by a northwest-strik- 1998) at the southernmost exposure of the upper member of the Sugarloaf Mountain Quartzite. The folded beds of Sugarloaf Mountain Quartzite are cut by normal faults ing normal fault concealed beneath alluvium of the Potomac River. At the Monocacy EXPLANATION of Mesozoic age. Most of the normal faults strike northwest and are characterized by Natural Resources Area, the Reston and Poolesville Members of the Manassas Sand- Ijamsville Phyllite (chemical sample 1, table 1) hematite-cemented breccia and float blocks of iron ore. The normal faults (first descri- stone were down-faulted against rocks of the Ijamsville Phyllite and Urbana Forma- Ijamsville Phyllite (A.A. Drake, Jr., U.S. Geological Survey, unpub. data, 1994)
bed by Scotford, 1951) displace the upper member of the Sugarloaf Mountain Quartz- tion. The present erosion level provides an opportunity to walk along the Triassic un- Muscovite phyllite of Ijamsville Phyllite (Fisher, 1978) ite that underlies the “west ridge.” conformity within and north of the Monocacy River. These exposures provide Chlorite phyllite of Ijamsville Phyllite (Fisher, 1978) ° ′ The Sugarloaf Mountain Quartzite and Urbana Formation have a cleavage that is evidence that Mesozoic faulting and the areal extent of Triassic rocks were more ex- 39 15 Chlorite phyllite of Wissahickon Formation (Fisher, 1978) axial planar to a single phase of folds that constitute the Sugarloaf Mountain anticlino- tensive. Northwest-trending normal faults that cut the Sugarloaf Mountain Quartzite 39°15′ 0 ½MILE Chlorite phyllite of Sams Creek Formation (Fisher, 1978) rium (fig. 3). The most distinctive features of the Urbana Formation are bedding, one and Frederick Formation also support the concept that rocks of the Piedmont, Freder- Index to Areas of Geologic Mapping Responsibility Paragonite-bearing white mica-chlorite phyllite, central Virginia (Evans and Milici, 1994) cleavage, and a conspicuous bedding and cleavage intersection lineation (fig. 3). ick Valley, and Blue Ridge were also affected by Mesozoic extensional faulting. The Frederick and Araby Formations Ijamsville Phyllite Urbana Formation Sugarloaf Mountain Quartzite EXPLANATION Map Showing Trails and Local Names for Geographic Features on Sugarloaf Mountain and Vicinity The contact of the Urbana Formation and Ijamsville Phyllite is the Martic thrust diabase dike swarm that trends northward through the eastern part of the map area Intersections of bedding and cleavage, n=44 F2 folds, n=43 Intersection of bedding EXPLANATION Figure 2.—Portion of an Al O -K O-Na O plot for phyllites in the Westminster Folds, n=16 Cleavage, n=27 and cleavage, n=32 Cleavage, n=46 fault, which makes the Ijamsville Phyllite a synformal klippe. This fault contact dips reflects east-west extension at roughly 200 Ma. 2 3 2 2 Southworth Brezinski Southworth and Brezinski Northern Peaks Trail Sunrise Trail terrane of Maryland (Fisher, 1978; A.A. Drake, Jr., U.S. Geological Survey, unpub. data, 1994) and phyllite on the southeastern limb of the Blue Ridge anticlinorium in Mountain Loop Trail Monadnock Trail central Virginia (Evans and Milici, 1994). Figure 3.—Lower hemisphere equal-area projections of structural data of rocks of the Buckeystown, Md., quadrangle, by structural domains. Saddleback Horse Trail A.M. Thomas Trail
GEOLOGIC MAP OF THE BUCKEYSTOWN QUADRANGLE, FREDERICK AND MONTGOMERY COUNTIES, MARYLAND, AND LOUDOUN COUNTY, VIRGINIA
By Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. AUTHOR AFFILIATIONS Scott Southworth1 and David K. Brezinski2 Government 1U.S. Geological Survey, Reston, Virginia 20192 For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225 2Maryland Geological Survey, Baltimore, Maryland 21218 2003 Printed on recycled paper