U.S. GEOLOGICALSURVEY U.S. DEPARTMENT OFTHEINTERIOR Chilhowee 39 Group ° Figure 1. area ofandsurrounding LoudounCounty,Va. Zsm Zsm 17(9) Zcb a i n i g r i V BLUE RIDGEPROVINCE 11 t s e W Zcp Unconformity | 6 Zsp Zss | | Stratified | — wm Loudoun County, Va., 1992. Topographic basescannedfromtheUSGSCountyMapSeries, wu wl lc | Qal Qc | | Yp Yq | | 19 Map showinggeneralgeologicprovinces andprominent geologicfeatures ofthe cp Location of sample providing radiometric age determination age radiometric providing sample of Location Thrust fault Normal fault h a t f Zfa | | Zc s ee t smls itd n al 3 f h pmhe. oe ht those those samplenumberswithoutparentheses represent theageofintrusion that Note and metamorphism pamphlet. of age the the represent of parentheses in 3 numbers sample table in listed samples to keyed is l Zfc w Zfs Blue Ridgeanticlinorium 5 Middleburg 8(6) 600 Ma Zcr* Zcm Zcs Figure 2. Zc diagram ofMiddleProterozoic gneisses(O’Connor, 1965);An,anorthite; Ab,albite;Or, orthoclase. (Streckeisen, 1976);Q,quartz;A,alkalifeldspar;P, plagioclase. 13(7) 734 15(10) 7 —Sawteeth onupperplate SEA LEVEL 200 Ma —Bar andballondownthrown side 12 B 16 U (3) Jd* LL 14 Qc RU N 18 10(8) 10000 F 671 —

A (4) Purcellville 9 FEET U 4000 2000 2000 6000 8000 8000 6000 4000 L T (4) Zmd Loudoun A Yc County a i n i g r i V 0 0 Supergroup ANTICLINORIUM , Quartz-alkali feldspar-plagioclase ternary diagramofMesoproterozoic gneisses , Quartz-alkalifeldspar-plagioclase ternary Intrusive Ab EXPLANATION 2 BLUE RIDGE Newark Yhm*

Ymc* Yml* Ybg* Ypg* Yqp* B

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HILLSBORO SYNCLINE Qc w 2 Jdh 35 52 15 75 A Black Oak Ridge 60 4 Zc 3 5 0 2 1 50 0 Qal 1 Zmd Yml Ym 62 1 77 52 57 15 51 25 15 30 25 ° 18 Ypg 50' Late Triassic Early Jurassic Early Cambrian Late Cambrian Holocene Holocene and Zc Pleistocene and Pleistocene(?) Ypg 34 40 SCALE 1 SCALE Pliocene(?)

STATE ROUTE 690 50 Ygt 22 46 43 77 tholeiitic Zmd 15 ° calc-alkaline 50' Zss Ymc 2 5 Zmd Zcr 20 :50 000 :50 Ypg Zcp 42 F Yp 42 70 13 | | Yq Zmd 47 Qc wl 30 wm | Quaternary and Quaternary Mesoproterozoic Late Proterozoic Cambrian Triassic Jurassic Quaternary 65 Zcp 44 Ypg wl 30 24 50 84 2 20 25 Tertiary(?) Zmd 34 Zcp Jd Zmd 75 | 54 50 20 3 Ypg 35 wm 65 0.5 | 80 45 Yp 34 l 39 Zmd Ylg P 50 Qc 2 ° O Zmd Ym | 10' 88 45 29 5 25 wl Qc Yp Ymc SH 40 20 O Yq M R T Ylg

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STATE ROUTE 643 Jtr 62 Zcm ^ 55 50 35 10 Qc 75 bs J^ Js 50 Qal 43 Jss 20 47 Zcp 55 70 74 Jhg Qal Ym c Scott Southworth, WilliamC.Burton,J.Stephen Schindler, andAlbertJ.Froelich J^ 50 51 34 28 80 34 15 Qal Yp Catoctin Formationmetabasalt,n=26 TiO Low Porphyritic metadiabasedike,n=10 Fine-grained metadiabasedike,n=30 Coarse-grained metadiabase 31 cg 5 Ybg Js Qal | or flowbreccia,n=9 J^ 20 Zmd 44 w Jm 68 Zmd Zmd Zmd Qal J^ Jtr c Jtr Zsp 48 50 50 EXPLANATION 25 43 32 60 cg Jdh GEOLOGIC MAPOF LOUDOUNCOUNT 35 Jd 38 45 Jdg 44 2 J^ Jhg Jtrc Jd 44 /high MgO metavolcanic MgO /high Qc Yn ° Yp 21 J^ 55' Ybg c Yg 41 Yp 24 22 Yg Jhg Zrd Jhgs cg Jhg Yg Zcs Jmz J Qal

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GOOSE CREEK | ^ Ybg c 37 tm tm Jmz 48 ^ ^ 40 cg w 35 LOUDOUN COUNTYOFFICEOFMAPPINGANDGEOGRAPHICINFORMATION 42 Yp J cg c 38 60 48 ^ 45 J Ybg Ybg c 15 47 ^ n=6 15 40 58 Yg 78 55 tm flow Jd Qc 50 37 45 88 Zcs 50 45 J Qal 37 Zsm Zc ^ Jd 37 25 Zmd 36 Yn Yp Yg c J Q 62 ^ J al 37 Qlg Yg 50 ^ cg Qlg Zmd Yp Jdh 82 Yqp Zcs J Qal Qal Zmd cg 32 77 ^ 48 25 Ybg

Belmont 80 Yg 30 cg ^ ° J 23 51 J 35 Zmd | 35' bl Qal Ybg ^ Zsp 34 ^ 23 40 w 44 tm Yp 50 Yqp c 50 38 Qal ^ J Jdh Qal 40 J ^ bs 40 Yp 42 40 ^ 53 29 tm Ybg 27 45 tm ^ ^ ^ Zcp bl bs 41 47 37 mp Yp Qc J J Zcp Yn ^ ^ Yqp 48 25 c J 45 36 tm Yqp 43 Qlg 43 45 Qal ^ 52 53 Zmd 78 33 Qc Yqp cg Yqp 48 Jdg 59 Zrd 77 Qlg Prepared incooperationwiththe 34 32 18 Jd ^ 4 43 Jdg ° ^ Zsp Qc 23 bl Qal 35' Qc | 28 42 bs Zcp Zcm w 50 Yn 25 Jdl 25 28 30 50 Zcm 39 J Ybg Qc J 4 Yqp 35 ^ Zcp ^ 46 Zmd c ^ 35 62 tm 30 42 Zc 9 25 Zcp ^ ^ bl 39 (2) ^ Jdg 20 17 Yg bs 42 bsh 29 bl 30 28 J J 2006 B ^ Yqp Jd 35 U ^ ^ STATE ROUTE 641 ^ | By Jdh L 43 Zsp

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Broad Run 17 62 | tm bs Jd ^ Qal f 18 24 bs 23 Qal 31 23 Qc ^ Qal Jdh 15 ^ ^ QTt ^ bs 1 30 bl 30 21 bs J Jdh ^ Jdc bs ^ ^ Jd ^ bsh Jdg ^ 12 bl bs tm J ^ 17 bs ^ 34 J bs 16 Jd Qal 21 ^ tm ^ 20 tm ^ 18 bsh mp 39 25 77 ^ ° 23 13 ^ 10' ^ bs ° 8 22 30' bl bs 20 28 ^ J Jdg 10 10 Jdh 24 ^ bs Jdl Jdh tm 11 16 Jdl Qal QTt 23 QTt J^ 21 QTt 20 Jdh ^ 77 Qal Jdc c bsh ° 77 ^ 30' Jdl 22 bl ° J 30' Qal ^ 77 10 J 17 ^ 10 tm ^ Jdg mr ° 10 tm J 30' Jdl 23 ^ 23 20 22 tm Qt 12 Jdl Jdl 11 ^ ^ Jdl bsh 23 6 bsh Qt Jdl 7 25 ^ Jdl Qal QTt 10 mp QTt Jdl 18 Jd 8 Jdl 14 Qal 12 J 13 10 ^ Jdl Qal tm 21 ^ Qt bs 4 Jdh 6 Jdh 20 QTt ^ 7 17 10 21 mp Jd 11 ? 6 12 Jdc 39 12 J ° 15' ^ 38 21 tm Jdl Jd ° 55' 15 ^ 18 13 25 Jdl GROVE FAULT PLEASANT bs 12 14 11 Qal QTt 15 | J Zmg ^ 12 ^ 16 tm 39 mp 18

ALGONKIAN REGIONAL PARK ° 07'30" 10 17 Ylg 77 Qt 39 ^

SURFACE Jdh bs ° 17 ° ^ 22 52'30" bsh ^ Qal Yc Yn Ypg bs Zss Qal Yp 20 39 25 25 6 Igneous rocks Metabasalt-metadiabase Basalt-diabase Quartzite Limestone Siltstone Sandstone andsiltstone Sandstone Conglomerate Coarse cobbles,boulders,andblocks ° 05' Yq Jdc Ygt J 26 BLUE RIDGE MOUNTAINS J ^ EXPLANATION 19 ^ Qal Yp tm tm ^ Ylg ^ 23 bs 10 mr 38 iue 5. Figure Yn Zc Culpeper Group, undivided. W.F.Daniels, D.L. by interpreted and Bracken. processed R.E. U.S. were and Data Hanna, Survey. Geological the by 1989 in flown survey aeromagnetic on based Image contacts. geologic general with quadrangles 14 LOWES ISLAND Ygt Yp ^ 20 20 mp B UL EM O N T —Color-shaded-relief aeromagnetic image of the Lincoln and most of the Bluemont 7.5-min 7.5-min Bluemont the of most and Lincoln the of image aeromagnetic —Color-shaded-relief | Yq Zmg Ypg

NORTHERN VIRGINIA Zss 10 ^ 20

REGIONAL PARK J Yp bsh Yp ^ Jdh 77 tm Ylg Ylg Qal | ° Zms Jdh Qal 25' Yq 77 Yp QTt 24 Yc ° 25' Yp Yp A Ymc Jdh 23 J ' SH Yp

^ O 2000 2000 SEA LEVEL 4000 6000 8000 10000 6000 4000 8000 FEET R Yml tm T 24 Qt 33

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Garnetiferous leucocraticmetagranite(Ygt) Chilhowee Group 39 Jd Hornblende monzonitegneiss(Yhm) Formation Colluvium, laggravel,andterraces Weverton Coarse-grained metagranite(Ymc) Ymc Pink leucocraticmetagranite(Yml) Porphyroblastic metagranite(Ypg) Quartzite andquartztectonite(Yq) ^ ° 33 2 MILES Metanorite andmetadiorite(Yn) Quartz-plagioclase gneiss(Yqp) 00' Robertson RiverIntrusiveSuite Ygt Ygt bs 10 Leucocratic metagranite(Yg) Layered graniticgneiss(Ylg) Zfc Biotite granitegneiss(Ybg) Catharpin CreekFormation Marshall Metagranite(Ym) Mount ZionChurchBasalt Diabase dikesandsheets Jd Swift RunFormationand Carbonaceous phyllite Turkey RunFormation Tomstown Formation Manassas Sandstone LITHOLOGIC UNIT Jd Hickory GroveBasalt Frederick Limestone Loudoun Formation Fauquier Formation 19 18 17 16 15 14 13 12 11 10 Balls BluffSiltstone Catoctin Formation Midland Formation Ygt Harpers Formation Antietam Quartzite C W W W B A 9 8 7 6 5 4 3 2 1 Charnockite (Yc) Paragneiss (Yp) Sander Basalt Zfa SOURCES OFDATA FOR7.5-MINUTEQUADRANGLESINLOUDOUNCOUNTY Alluvium Gainesville: Lee(1979);A.J.Froelich,J.P. Smoot,andR.E.Weems (unpub. Herndon: Eggleton(1975);Lee(1979);A.J.Froelich,J.P. Smoot,andR.E. Arcola: Lee(1978);A.J.Froelich,J.P. Smoot,andR.E. Weems (unpub.data) Middleburg: Lee(1979);Klineandothers(1990) Rectortown: Espenshade(1983);Leo(1990);W.C. BurtonandScottSouthworth Upperville: W.C. Burton,A.E.Nelson,andScottSouthworth(unpub.data) Seneca: Lee(1979);A.A.Drake,Jr., A.J.Froelich,J.P. Smoot,ScottSouthworth, Sterling: Lee(1979);A.J.Froelich,J.P. Smoot,Scott Southworth, andR.E. Leesburg: Toewe (1966);Lee(1979);A.J.Froelich,J.S.Schindler, J.P. Smoot, Lincoln: Lee(1979);W.C. Burton,A.J.Froelich,J.S.Schindler, andJ.P. Smoot Bluemont: Southworth(1994);W.C. Burton(unpub.data) Ashby Gap:GathrightandNystrom(1974);Tollo andLowe(1994);W.C. Burton Poolesville: Lee(1979);Southworth(1998);A.J.FroelichandJ.P. Smoot Waterford: Lee(1979);Burtonandothers(1995);J.P. Smoot(unpub.data) Purcellville: Southworth(1995);W.C. Burton(unpub.data) Round Hill:McDowellandMilton(1992);W.C. BurtonandScottSouthworth Point ofRocks:Lee(1979);Burtonandothers(1995);J.P. Smoot(unpub.data) Harpers Ferry:Southworth(1991);andBrezinski(1996) Charles Town: Nickelson(1956);ScottSouthworth(unpub.data) n=76 n=20 Terraces Ygt Structural featuresdeveloped during metamorphismofthe data) Weems (unpub.data) (unpub. data) and R.E.Weems (unpub.data) Weems (unpub.data) and R.E.Weems (unpub.data) (unpub. data) and ScottSouthworth(unpub.data) (unpub. data) (unpub. data) Qt middle member Contoured polestoD Contoured polestoD upper member lower member Mesoproterozoic rocks ^ D D D mp 3 2 2

Yg fold hinge,n=2 fold hinge,n=5 mineral lineation,n=15 N COI L NL Ym 14 8 Jd Zfa Ylg ^ Jd QTt N N N S S S bs Yg Potomac terrane(A.A.Drake,Jr. andScott Culpeper basin(A.J.FroelichandScott Blue Ridgeanticlinorium(W.C. Burton,J.S. Sources ofGeologicMapping Yg Jd Southworth, compilers) Southworth, compilers) Schindler, andScottSouthworth,compilers) 15 9 4 1 2 1 Yg Ypg Jd foliation foliation Zcp QTt Yc ^ ^ Jmc Ylg Jdh Yg Zsm J Zcp bs Ypg | mp | ^ Zrr Jtrc cp lc Zcm cg J Zsm Yq Zss ^ Yg Ju Qlg Zsp Jhg tm LITHOLOGY AND Yg Yn J QTt Ypg MAP SYMBOL , Yhm ^ ^ 16 10 Yp Zcb bl tm 5 2 | ^ | Jmz | | | | Js wm mp wu wl Qt | h a E E E f t Zmd Ypg | Ygt Zfc Jss Yg Zsp Jdh ^ Zmg | Jdh 38 Zcs mr Qal l 30 Jhgs Jtr Zc Jm 19 17 11 30 Zc Yml Yp Zcr Ybg 6 3 J ^ Ygt Yp Zfa Zfs Zrd ^ 40 ^ bsh | mr | c 25 Jd w | Zms Qc | w 25 Figure 3. E D W W Jdh Zmg A 77 Orientation ofmetadiabasedikes Ju Ju emplaced intoMesoproterozoic ' Yml ° Jd Ybg n=34 37'30" 18 12 Ygt Yg rocks intheLateProterozoic 7 Ymc | 300–1,000 (Zss) THICKNESS, >5,000 ( >5,000 ( 2,500 ( >3,280 (J Ju 3,000 ( Zmg 0–400 (Zsp) <700–1,200 0–200 (Zfc) 0–160 (Zfs) 3,510 ( 1,500 (Zfa) 500–1,000 IN FEET 10 (Zsm) 70 ( 100–200 200–240 105–380 0– Strike directionsofcontacts <1,000 0–200 1,250 <200 <200 —Equal-area projections (Schmidtnet)ofstructuraldata.Contour intervals are 2percent per1percent area. Ym 100 330 715 5,000 ^ Yqp ? J ^ mr) ^ ^ ^ ^ Yg Ym mp) bsh) Poles tocontacts bl) Ymc ^ Ym cg) bs) 13 c) NANOTESLAS -597 -515 -454 -411 -369 -336 -301 -272 -241 -213 -183 -156 -124 -95 -61 -27 15 58 119 201 N N S E E | F H W W W | J J | ^ J Jhgs ^ | ^ Jmc Jmz | | Jdh ^ Jdg Jtrc Jdo QTt Jhg J^ Qlg Jdc Qal | ^ Jss | Jm ^ Jdl Zmg | | Jtr Qc Zms ^ Jd | Qt | Js | wm bsh mp wu mr bs wl cp bl w lc tm cg h a t f l c n=52 n=52 n=605 Chilhowee Group Culpeper Group Two generations(S luim (Holocene) Alluvium Metagraywacke—Light- to medium-gray, yellowish- to reddish-brown reddish-brown to yellowish- medium-gray, to Metagraywacke—Light- very reddish-brown-weathering, gray, to greenish-gray Schist—Lustrous, Mather GorgeFormation(LowerCambrianand(or)LateProtero Cambrian) (Lower Formation Loudoun Cambrian) (Lower undivided Formation, Weverton medium-grained, to fine- medium-gray, to member—Light- Lower to medium-grained massive, light-gray, to member—Greenish-gray Middle coarse very dark-purplish-gray, to dark-gray member—Dusky-blue, Upper Weverton Formation(LowerCambrian) Cambrian) (Lower Formation Harpers Cambrian) (Lower Quartzite Antietam Cambrian) (Lower phyllite Carbonaceous Cambrian) (Lower Formation Tomstown Cambrian) (Upper Limestone Frederick of pebbles and cobbles with conglomerate Member—Basal Reston reddish-brown, and pinkish-gray, gray, Member—Predominantly Poolesville Manassas Sandstone(UpperTriassic) conspicuous bedded crudely Member—Light-gray-weathering, Leesburg thin- member—Reddish-brown, siltstone and sandstone deltaic and Fluvial dark-gray, to member—Light- siltstone and shale Lacustrine Balls BluffSiltstone(UpperTriassic) pebbly interbedded and conglomerate Member—Lenticular Creek Goose Upper and Jurassic (Lower Formation Creek Catharpin Jurassic) (Lower Basalt Church Zion Mount Jurassic) (Lower Formation Midland Jurassic) (Lower Basalt Grove Hickory Jurassic) (Lower Formation Run Turkey Jurassic) (Lower Basalt Sander Upper and Jurassic (Lower rocks metamorphosed Thermally Jurassic) (Early sheets and dikes Diabase and (Pleistocene level high and intermediate deposits, Terrace Pleistocene?) and (Holocene level Terracelow deposits, Pleistocene?) and (Holocene gravel Lag Pleistocene?) and (Holocene Colluvium o lvl luil ercs s uh s 0 t bv sra canl and channels stream above ft 10 as much as terraces alluvial level low Includes tributaries. its and River Potomac the of plains flood underlying and slump features are abundant. Contains interbedded quartzose schist quartzose interbedded Contains abundant. are features slump and as much as to in 1.2 about from metagray- range Beds schist. well-bedded semi-pelitic and wacke generally medium-grained, to fine- weathering, of lenses County. Thicknessisunknown Loudoun few of corner a eastern extreme the in and only Found rock. calc-silicate metagraywacke interbedded Contains early cleavage. into transposed largely are beds thin very to Thin schist. hematite quartz-muscovite-chlorite-plagioclase-epidote-magnetite- grained, fine with where transitional thick ft or is 200 to present conglomerate 0 from conglomerate with Ranges Formation. contact Weverton Formation; overlying sharp Weverton is phyllite overlying of top Formation; Catoctin underlying the with transitional is formation of Base interbeds. and phyllite thin with metasandstone pebbly gray-weathering graded to white- lesser and and phyllite to dark-gray- chlorite-quartz-graphite-muscovite conglomerate of greenish-gray-weathering consists local Mountain Catoctin quartz-pebble matrix; On quartzite. silty crossbedded coarse iron-rich of grayish-black, in interbeds phyllite variegated and jasper,red gneiss(?), quartz, red and ( blue, white, subrounded conglomerate to containing rounded pebble coarse phyllite very dusky-blue, to dark-gray by dark-purple-gray overlain locally amygdules; elongated to and clasts tuffaceous light-olive-gray gray-black, asmuch120ftthick Formation; Loudoun underlying is and Formation it Harpers where overlying with Mountain conformable Catoctin to restricted is phyllite dark of interbeds thick) ft 1 than (less thin with quartzite vitreous thick-bedded to massive Thickness Ridge. Blue on ranges from 100to200ft locally gneiss basement metasiltstone; to and unconformable arkosic; metagraywacke light-gray with interbedded with exposed. quartzite vitreous thick-bedded interbedded massive, crossbedded, graded, well-sorted, well beds not is 32-ft-thick but ledges to Thickness rangesfrom 200to240ft topographic 16- Forms in metasiltstone. quartzite Hill granular Short exposed on ft poorly 105 from with ranges Thickness Mountain to380ftonBlueRidge local overlying Interbedded with Formation. contains Gradational Harpers phyllite. metasiltstone. crossbeds; phyllitic and dark-greenish-gray and quartz, beds sorted, purple Poorly and jasper,red red graded of pebbles and ilmenite, magnetite, of conglomerate. accumulations with quartz-pebble and thick-bedded quartzite grained is fabric dominant 500 ftoneastlimb to limb west and on ft 1,000 from varies Thickness quartzose laminae. micaceous transposed, 0.04-in-thick and alternating of totally consisting foliation is metamorphic bedding Catoctin on Mountain; Hill Mountain Short and Ridge bedding Blue with on grained preserved fine locally are Rocks Mountain. Hill Short on base at the locally found conglomerate fine-pebble Magnetite- and metasiltstone sandy metasiltstone. rich, and phyllite biotite-chlorite-muscovite-quartz bedded massively to laminated grained, fine very to fine- brownish-gray, or Formation Catoctin Tomstown abundant of side with east Mountain. Thicknessabout100ft on only horizons overlying Found limonite. ferruginous and hematite with local botryoidal are phyllite contact carbonaceous Near ridges. on exposed Poorly meta-arkose. massive to bedded poorly fine-grained, Maximumthicknessprobably lessthan200ft Formation. Tomstown the of dolomite with laterally Interfingers Mountain. Furnace near Quartzite slumped Antietam small above lenses or discontinuous as float occurs outcrops; as to only Found soil. 0.04- “ashy” Produces light-gray laminae. contains distinctive bedding dark-gray Locally and light- alternating phyllite. 0.4-in-thick muscovite-graphite fine-grained partings. 0.5-in-thick with well-bedded, Locally sandywithdolomiticcement.Thicknesslessthan200ft locally to Furnace Massive near Mountain. exposed dolostone, medi Jtrc Contoured polesto Contoured polesto Contoured polesto PALEOZOIC ROCKS OFTHEBLUERIDGEANTICLINORIUM 10 ft thick, averaging about 8 in. Many beds are graded; sole marks sole graded; are beds Many in. 8 about averaging thick, ft 10 Single generation(S . yls oss o atrain f ore ad ie gand layers grained finer and coarser of alternation of consist Cycles ). S S S in bothProterozoicandCambrianrocksonopposite 2 1 1 Structural featuresdevelopedduringthePaleozoic cleavage cleavage cleavage West —Sandstone, siltstone, and conglomerate, interbedded in cyclic in interbedded conglomerate, and siltstone, —Sandstone, —Hornfels, quartzite, marble, meta-arkose, and and meta-arkose, marble, quartzite, —Hornfels, MESOZOIC ROCKSOFTHECULPEPERBASIN N N N S S S —Sand, gravel, and boulders in isolated hillocks, 200 to 235 to 200 hillocks, isolated in boulders and gravel, —Sand, Jdg ROCKS OFTHEPOTOMACTERRANE DESCRIPTION OFMAPUNITS CENOZOIC SURFICIALDEPOSITS ), ferrogabbro, diorite, syenite, and aplite, which contain which aplite, and syenite, diorite, ferrogabbro, ), IN THEPIEDMONTPROVINCE 1 andS Mxue o ca, it sn, rvl ad cobbles and gravel, sand, silt, clay, of —Mixtures sides oftheShortHillfault 1 Jss ) cleavageexhibitedatanoutcroplocality 2 28 N. 21 Mean: Jdl ) cleavageexhibitedatanoutcroplocality ° SE. ). Basalts are characterized by distinctive distinctive by characterized are Basalts ). ); (3) narrow dikes and differentiated sheets differentiated and dikes narrow (3) ); E E E ° E., —Dark-gray to bluish-gray, fine- to to fine- bluish-gray, to —Dark-gray Short Hillfault Slsoe snsoe sae and shale, sandstone, —Siltstone, G W K I W W Jhgs —Medium- to dark-gray, fine- to fine- dark-gray, to —Medium- —Cobbles and gravel of rounded of gravel and —Cobbles —Coarse cobbles, boulders, and boulders, cobbles, —Coarse —Gray-weathering, pale-gray to pale-gray —Gray-weathering, —On Blue Ridge consists of of consists Ridge Blue —On n=71 n=1,017 n=806 —Gray to buff-weathering, buff-weathering, to —Gray —Buff-weathering, white to to white —Buff-weathering, ). Locally disconformable but disconformable Locally ). —Medium- to dark-gray, dark-gray, to —Medium- —Dark-greenish-gray to to —Dark-greenish-gray —Medium- to dark-gray, dark-gray, to —Medium- —Sandstone, siltstone, siltstone, —Sandstone, —Dark-gray to black, black, to —Dark-gray Contoured polesto Contoured polesto Contoured polesto n | Jdo =number ofmeasurements. lc Jd S S S wt pbls of pebbles with ) —White to gray, gray, to —White . Dikes vary from vary Dikes . 2 1 1 ); (2) dikes of of dikes (2) ); cleavage cleavage cleavage

—Sand, gravel, —Sand, East Jdh N N N S S S Jdc ). Igneous Igneous ). lwr in lower ) zoic) Jmc ft ) This mapisavailableforsalebyU.S.Geological Survey, Information Services, Box For productandorderinginformation: World WideWeb: http://www.usgs.gov; Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names in this publication is for descriptive for is publication this in names firm or product, trade, of use Any purposes onlyanddoesnotimplyendorsement bytheU.S.Government 58 N.10 Mean: 39 N. 9 Mean: 36 N. 18 Mean: ° ° 25286, DenverFederalCenter, Denver, CO80225 ° SE. SE. ° SE. ° E., E., E E E ° E., Yhm Zmd Ymc Zcm Zsm Yml Ybg Ypg Yqp Zcp Zsp Zcb Ygt Zss Ym Zcs Zrd Ylg Zfa Zcr Zfs Zfc Zrr Yg Yn Yp Yq Yc Zc 44 47 57 43 32 32 77 60 80 12 6 16 31 W W W N M L urzt ad urz tectonite quartz and Quartzite Paragneiss metadiorite and Metanorite Charnockite gneiss granitic Layered gneiss monzonite Hornblende metagranite Porphyroblastic metagranite Coarse-grained Metagranite Marshall gneiss Quartz-plagioclase metagranite leucocratic Garnetiferous metagranite Leucocratic metagranite leucocratic Pink gneiss granite Biotite buff- to Syenite—Gray- Quartz Feldspar Alkali Mountain Cobbler Robertson RiverIgneousSuite(LateProterozoic) cobble dark-gray, to meta-arkose—Light- and Metaconglomerate light-yellowish-gray- olive-gray, and medium-gray to Meta-arkose—Light- (less laminated thinly light-brown-weathering, Metamudstone—Light-gray, Fauquier Formation(LateProterozoic) meta-arkose— and quartzite, metasandstone, schist, Metagraywacke, grayish-red-purple, slate; phyllite—Medium-dark-gray and slate, Marble, calcitic white fine-grained, to medium- buff-weathering, to Marble—Gray- Swift RunFormation(LateProterozoic) Proterozoic) (Late dikes Metarhyolite Proterozoic) (Late dikes Metadiabase Proterozoic) (Late Formation Catoctin Sinkhole Sample siteforradiometricdate Paleozoic minerallineation,rodding,and(or)axisofminorfold minor of axis (or) and rodding, lineation, mineral Mesoproterozoic Strike anddipofaxialplanefold Strike anddipofjoint Strike anddipofmyloniticphylloniticfoliation (S first-generation crenulated highly of dip and Strike Strike anddipofsecond-generation(S (S first-generation of dip and Strike including foliation Mesoproterozoic of dip and Strike bed of dip and Strike Antiformal syncline Synformal anticline Overturned syncline Syncline Overturned anticline Strike-slip fault fault Reactivated Normal fault Thrust fault Lake bed Contact (Folds show trace of axial surface, direction ofdiplimbs,and,where (Foldsshowtraceofaxialsurface, ie i te xrm esen onr f odu Cut. hcns is Thickness County. Loudoun unknown of corner eastern extreme the in River Potomac the along outcrops bold Forms rock. calc-silicate of lenses and paragneiss ( cleavage. penetrative of pods metamorphic mappable larger Paleozoic and outcrop in graphite of lenses thin early Contains strong or contains bedding commonly No as foliation; such tectonite. recognized quartz-sericite textures and and primary quartzite schist massive medium-grained, quartz-chlorite-magnetite include carbonaceous phyllonite.Probable remnant ofpregranitic countryrock varieties schistose stain rusty Distinctive Graphite Retrograded, magnetite. accessory after hematite secondary by flakes. produced muscovite. rounded 0.4-in-thick and small, and disseminated, chlorite as diameter fine-grained occurs in of in. clots 0.4 lensoid green to 0.04 to retrograded and deformed typically Garnets are layers. quartzofeldspathic that garnets schist. containing zones garnet-rich 0.4-in-thick and alternating by defined Layering gneiss graphite-biotite-garnet-plagioclase-quartz layered to pyroxene thin and paragneiss belts, commonlyinproximity togarnet-graphite lenses colorless as Occurs and amphibole. pale-green to hornblende altered commonly Brown biotite-hornblende- metadiorite. medium-fine-grained plagioclase to medium- and metanorite hornblende-orthopyroxene-plagioclase well-foliated to massive grained, lensoid discontinuous as rind. Occurs weathering biotite. bodies and orange-yellow garnet 0.8-in-thick, and contains to boulders Locally 0.4- pitted a of consists with that cobbles float distinctive and soil, orange bright a highs, quartz-hornblende- topographic forms exposed; Poorly 35 rock. the of well-foliated percent about compose locally Mafics rock. to orthopyroxene-microcline-plagioclase massive coarse-grained, swirly, and layering units unknown folded include rock. throughout outcrop granitic other to Relation melting. partial of suggestive texture migmatitic scattered in diameter in structures in. and Complex 0.4 biotite of to concentrations up by garnets defined is aplite; thick, in 0.4 to Layer- 0.04 gneiss. ing, garnet-biotite-plagioclase-quartz-microcline well-layered and Biotite rock. 50 spotted as massive, much more orthopyroxene are rare maficconstituents as a microcline as and occurs Rarely percent, percent. 30 as much as hornblende percent, 20 to 10 content quartz hornblende; prismatic and aggregates hornblende-quartz-microcline-plagioclase grain feldspar and quartz flattened strongly by defined Foliation gneiss. well-foliated fine-grained, and pinkleucocraticmetagranite( or ( metagranite leucocratic garnetiferous of dikes by cut locally is which microcline foliation, define ovoids Flattened interstitially. pink occur quartz blue of clots to distinctive and opaques, plagioclase, biotite, orange garnet, diameter; in in. 1.2 of to 0.4 ovoids rounded into megacrysts deformed are that aggregates microcline-rich by Characterized metagranite. garnet-biotite-plagioclase-quartz-microcline coarse-grained microcline pink or texture augen due tooverprintingbyPaleozoicschistosity pronounced ranges contains white content Commonly percent. biotite 10 to quartz; 0 0.8-in-long from blue of to aggregates and 0.4- porphyroblasts by Characterized foliation. metagranite. biotite-plagioclase-quartz-microcline well-foliated to to massive parallel pegmatite pink of veins Resembles biotitegranitegneiss( by Layering biotite-plagioclase-quartz-microcline perc (in pamphlet) fold Paleozoic (second-generation) Mesoproterozoic (first-generation) Vertical Inclined cleavage and(or)schistosity Vertical Inclined schistosity Vertical Inclined layering Rotated more than180 Overturned Inclined structures cross sectionasadouble-headedarrow in shown fault; thrust later of plate upper on sawteeth and fault normal Zsp Zcm Orientation ofbeddingplanes n=216 n=403 n=182 in PaleozoicandMesozoic Cambrian Weverton Formation ) andnearbottomofsection,inschist( Contoured polestobeddingin ). Margins of the marble commonly contain coarse-grained coarse-grained contain commonly marble the of Margins ). Contoured polestobeddingin Cambrian Weverton Formation —Dotted where concealed LATE PROTEROZOICROCKSOFTHEBLUERIDGE MESOPROTEROZOIC BASEMENTROCKSOFTHE Triassic andJurassicrocks sedimentary rocks —Dotted where concealed on ShortHillMountain —Rusty-weathering, medium-fine- to fine-grained, well-foliated fine-grained, to medium-fine- —Rusty-weathering, Zcr known, direction ofplunge.Dottedwhere concealed) of theCulpeperbasin —Sawteeth onupperplate Poles tobeddingin (Dashed where inferred; dottedwhere concealed) —Yellow- to brown-weathering, dark-green, medium- to to medium- dark-green, brown-weathering, to —Yellow- —Ball andbarondownthrown side EXPLANATION OFMAPSYMBOLS Yp on BlueRidge ); light-gray, fine-grained phyllite and metasiltstone and gray and metasiltstone and phyllite fine-grained light-gray, ); ). Considered tobepartofmetasedimentarysuite —Arrows showrelative movement (May becombinedwithplanarfeatures) (May becombinedwithlinearfeatures) —Open ball and bar on downthrown side of early early of side downthrown on bar and ball —Open BLUE RIDGEANTICLINORIUM N N N S S S GEOLOGIC INVESTIGATIONS SERIES —Ball indicates top of bed known from sedimentary from known bed of top indicates —Ball —Pink, medium-grained, weakly to moderately moderately to weakly medium-grained, —Pink, —Orange- to gray-weathering, pink, medium- medium- pink, gray-weathering, to —Orange- —White, gray, or pink, medium- to fine-grained, to medium- pink, or gray, —White, ° PLANAR FEATURES

LINEAR FEATURES OTHER FEATURES ß Explanatory pamphletaccompaniesmap —White, medium- to medium-fine-grained, medium-fine-grained, to medium- —White, —White, medium- to medium-fine-grained, medium-fine-grained, to medium- —White, ANTICLINORIUM —Gray to pink, medium- to coarse-grained, to medium- pink, to —Gray Yloihbonwahrn, eim to medium- —Yellowish-brown-weathering, Pn, eim t medium-fine-grained, to medium- —Pink, —Gray-weathering, medium- to coarse- coarse- to medium- —Gray-weathering, FAULTS FOLDS —Refer tofigure 1andtable3 Ybg —Gray-weathering, medium-fine- to to medium-fine- —Gray-weathering, Yml 37 N. 26 Mean: —Light-gray to white, fine- to to fine- white, to —Light-gray ) inoutcrop ° ) SE. 1 E E E 2 Yg ° Plooc laae n (or) and cleavage Paleozoic ) E., ) Paleozoiccleavage —White, medium- to medium- medium- to medium- —White, Zcp —Dark-greenish-gray, fine- to to fine- —Dark-greenish-gray, —White- to gray-weathering, gray-weathering, to —White- Yg except for the presence of of presence the for except —Dark-green —Dark-green Zss Zcb ); and 3- to 10-ft-thick lenses 10-ft-thick to 3- and ); and ) ); lustrous, silvery-white silvery-white lustrous, ); Ygt Zcs a ) pattern north of of north pattern ; iey laminated, finely ); Yg Printed on recycled paper paper recycled on Printed MAPI–2553 bt potassium but ) to bluish-gray, bluish-gray, to compositional 1 Paleozoic ) Ygt )