Geomorphology, Neotectonics and Process Studies in the Rappahannock River Basin, Virginia: Marshall to Oak Grove, Virginia, July 15-16, 1989

31 Pages·3.259 MB·English

by  Milan Pavich, Robb Jacobson, Wayne Newell(auth.)

Geomorphology, Neotectonics
and Process Studies in the
Rappahannock River Basin, Virginia
Marshall to Oak Grove, Virginia
July 15-16, 1989
Field Trip Guidebook T218
Leaders:
Milan Pavich RobbJacobson
wayne Newell
American Geophysical Union, Washington, D.C.
COVER Terrace sediments flanking the
Rappahannock River record episodes of fluvial
and estuarine deposition, and provide evidence
for the interpretation of the tectonic and
climatic. history of this basin.
Leaders:
Milan Pavich Robb Jacobson and Wayne Newell
U.S. Geological Survey U.S. Geological Survey
928 National Center 926 National Center
Reston, VA 22092 Reston, VA 22092
Published 1989 by American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C. 20009
ISBN: 0-87590-594-3
Printed in the United States of America
IGCFIELDTRIPTI1S:
GEOMORPHOLOGY,NEOTECTONICS,ANDPROCESSSTUDIES
INTHERAPPAHANNOCKRIVERBASIN,VIRGINIA
MilanJ.Pavich,RobertB.Jacobson,andWayneL.Newell
U.S.GeologicalSurvey,Reston,Virginia
CONTENTS: and ofveryvaried character, that nearly everywhereforms the
INTRODUCTION surfaceofthelandandoverliesorcoversmorecoherentbedrock.
BLUERIDGEANDPIEDMONT (AGI,1972)]"
Geology Apoorlyunderstoodrecordofepisodicdeposition,limitedto
Geomorphology afewrecognizedterracesequences, existsontheerodingPied
RegolithandSurficialStratigraphy mont(CostaandCleaves,1979;Pavich,1985;Sevonandothers,
RAPPAHANNOCKESTUARY 1983). Atpresentwe cannotindependentlydate erosionaland
Geology&Stratigraphy depositionalperiodsonthePiedmont,thuswecannotcorrelate
Geomorphology erosionalanddepositionaleventsindifferentpartsofthesame
Regolith& SurficialStratigraphy drainagesystem. Developmentofdatingmethodsandaprecise
GEOMORPHICEVOLUTIONOFTHERAPPAHANNOCK chronostratigraphyarethelong-termgoalsofourresearch. To
RIVERBASIN makeaccuratecorrelationsofeventsbetweenthepatchysurficial
RegolithandEquilibriumonthePiedmont recordofthePiedmontanddepositionalepisodesontheCoastal
EpisodicDepositionintheRappahannockEstuary Plain,we needto establishnewisotopicdatingmethodsand to
Summary quantifythelonger-term rates ofweatheringand erosionproc
TABLES essesoperatingonavarietyofrocktypes.
REFERENCES The Rappahannock River Basin (fig. 1) is a good area for
ROADLOG·DAY1:D.C.,InnerPiedmont, studyoftherelationsofsedimenttosourceareaandcomparison
Culpeperbasin,OuterPiedmont,FallZone oferosionalanddepositionalepisodesbecause:
ROADLOG•DAY2:CoastalPlain a)itssizeislimitedtoasingleclimaticzonesoclimaticepisodes
probablyhavebeensynchronousoverallpartsofthebasin.
b)itcontainsavarietyofrocktypesthatweatheranderodedif
INTRODUCTION ferently.
c) the estuarinepart oftheriverbasin servedas a depositional
Geomorphologyandsurficialgeologyincreasinglyarebeing sinkforsedimenterodedfromtheupperbasin,thusallowingfor
usedtoaddressquestionsaboutclimaticandtectonichistoriesof time-stratigraphiccomparisonsofsedimentsandsource.
specificlandscapes. IntheAppalachianPiedmont, an eroding Weview the river basinas a unit ofintegratedgeomorphic
landscapeindynamicequilibrium(Hack,1960),theadjustment evolution. Weassumethaterosionalresponseoftheupperbasin
offormtoprocessmayoccurrapidlyenoughtoprecludeaccurate tomajorclimaticchange,base-levelchange,ortectonismwillbe
identificationofclimaticortectoniceventsinlandformsorsurfi recordedinthesedimentsinthedownstreamdepositionalpartof
cialstratigraphy.Incontrast,depositionalareasliketheAtlantic thebasin. Eventsalsomayoccurthataresosmallinmagnitude
CoastalPlainaremorelikelytoretainastratigraphicandmor orextentthattheyarenotrecordedinthestratigraphicrecordof
phologicrecordofeventsaffectingthecontributingdrainagebasins. thelowerbasin. Sucheventsmayberecordedinsteadinephem
Thepurposesofthisfieldtriparetoexhibitthetypesofweathered eralsurficialdepositsorlandformsintheupperbasin. Wecan
surficialmaterialsoccurringinthePiedmontand CoastalPlain not,atthistime,differentiatebetweenclimatic,tectonic,orbase
partsoftheRappahannockRiverBasin,todiscussthesynthesis levelcausesofacceleratederosion. Duringthetripwewillfocus
ofaCenozoicclimaticandtectonichistoryfromsurficialdeposits on the evidence for episodic acceleration and deceleration of
andlandforms,andtodiscussproblemsofage-datingandmeas erosiononadeeplyweatheredterrane,andweleavetheanswers
urementsofgeomorphicprocessesoveravarietyoftimescales. aboutcausalrelationstofutureinvestigations.
TheAtlanticCoastalPlainoftheU.S.containsevidencefor Wewillstresstheprimarycontroloflandformsbydifferential
multiple episodes of deposition and erosion during the Creta weatheringanderosion. Thiscanbeobservedonmanydifferent
ceousandCenozoic(Owens,1970;Newell,1985). Thestratigra scales. Atthebroadestscale,thelithologicdifferencesbetween
phicandmorphologicrecord oftheCoastalPlain couldreflect Precambrian,.Paleozoic, Mesozoic, and Cenozoicstratigraphic
climaticallyortectonicallytriggerederosional/depositionalevents, unitsprovidefourdistinctphysiographicprovinceswithinthebasin.
orsomecombinationofthe two. Interpretationisalso compli Withineachprovince,rockstructureandlithologystronglyinflu
catedbytheadditionofstratarelatedtoeustaticsea-levelchanges; encelocalregolithdevelopmentandrelief. Wewillcontrastthese
thesestratamaybeunrelated,oronlyindirectlyrelated,toproc primarycontrolswiththeeffectsofclimatechangeandneotec
esses operating in upland drainage basins. Accurately under tonics.
standing the underlying mechanisms causing depositional epi ThefirstdayofthetripwillbedevotedtoPiedmontgeology,
sodesrequires refining techniques of mapping, analyzing, and geomorphologyandregolith. CoastalPlainstratigraphy,struc
interpretingboththedepositsandthesource-arearegolith. [Regolith tureandsoils,willbeinvestigatedonthesecondday. Theguide
is"...the mantle offragmental andloose,incoherent, oruncon bookcontainsseparatediscussionsofthePiedmontandCoastal
solidatedrockmaterialofwhateverorigin(residualortransported) Plain, followed byroadlogsfor eachday. Dueto the ongoing
T218: 1
Maryland BlueRidge
InnerPiedmont
CentralLowland
Washington, D.C
EasternUpland
FallZone
~1-- 1---t0Pkm
Virginia
North Carolina
FIGURE1 LocationoftheRappahannockRiverbasinandmajorPhysiographicprovinces
researchandthelimitedspaceinthisvolume,theguidebookwill Geomorphology
belargelydescriptive. More detailed arguments anddataper
tainingtofieldtripstopswillbepresentedinhandouts. Manyof Physiographyofthe upperpartofthe RappahannockRiver
thehandoutswillbefrom publishedjournalarticles,andothers basinisstronglyrelatedtotheNWtoSEchangesinmajorSWto
areintheform ofU.S.G.S.BulletinsorOpen-filereports. NEstrikingrockunits. ThefIrstorderdistinctionisbetweenthe
BlueRidgephysiographicprovinceandPiedmontphysiographic
province. Thewesternhalfoftheupperbasinisdevelopedonthe
BLUERIDGEANDPIEDMONT rocksoftheBlueRidgeAnticlinorium. Theridgesformingthe
westerndrainagedivideofthebasin, andtheBlueRidge physi
Geology ographicprovince,areunderlainbyerosionallyresistant,younger
Precambrian, quartzite and metabasalt. The core ofthe Blue
TheupstreamtributariesoftheRappahannockRiver(fig.1) Ridge Anticlinorium is designated the inner Piedmont and is
drainanareaunderlainbyavarietyofPrecambrian-toMesozoic underlainbylessresistant,olderPrecambrian,metamorphicgranites
agemetamorphic,igneousandsedimentaryrocks. Forsimplicity andgranodioritegneisses. TheouterPiedmonttotheeastcom
thelithologiescanbesubdividedintogenerallyNE-SWstriking prisestwodistinctareas: acentrallowlanddevelopedonunmeta
mapunitsasinfigure2. Lithologicunitsandphysiographicsub morphosedMesozoicsedimentsandbasalts,andaneasternup
divisionsrelatedtothoseunitsaresummarizedonTable1andthe landdevelopedonavarietyofPaleozoicmetasedimentary,metaigne
elevation contour map shown on figure 3. Rocks ofthe Blue ous,andmetavolcanicrocks. ThedivisionbetweentheMesozoic
Ridge Anticlinorium underlie the Blue Ridge and Inner Pied rocksandthePaleozoicrockscoincideswiththeMarticLine(fig.
montPhysiographicProWtccs,Mesozoicsedimentaryrocksunderlie 2;Stose,1939).
thecentrallowlandoftheCulpeperBasin, andPaleozoicmeta ThephysiographicdistinctionbetweentheBlueRidgeprov
morphic rocks underlie the eastern upland and theFall Zone. inceandtheinnerPiedmontprovincecanbeseenonfigure3.The
Thefieldstopswillshowexamplesofthesephysiographic/lithol BlueRidgephysiographicprovinceistopographicallyhigherand
ogicrelationsinthedifferentphysiographicprovinces. hasgreaterreliefthanthePiedmont.Itshigherorderstreamsare
Table2describes theregolithandsoilcharacteristicsofthe orientedNW-SE,andthesestreamscommonlyflowbetweenNW
physiographicunitsdefinedinTable1andonfigure3. Regolith SEorientedhighreliefridgesorspurs. ThegradientoftheRap
canbesubdividedintotransportedsediment(alluviumandcollu pahannock River across the Piedmont is steeper than that of
vium) and in situ weathering profiles on bedrock (residuum). otherrivers in Virginia, asshown byfigure 4 takenfrom Hack
Solumcomprises thepedogenicA andBhorizonsdescribedby (1982).
pedologists. Solaaredistinctfrom underlying,weatheredrego TheinnerPiedmonthasarollingtopographyofroundedhills
lith by their structure, texture, mineralogy and chemistry and withmoderate relief. Numerous SW-NEelongated inselbergs
contrastindensityandfabricwithisovolumetricweatheringproducts stand in higher relief (Kesel, 1974). Unlike NW-SE oriented
suchassaprolitedevelopedfrom crystallinerocks. spursoftheBlueRidge,theSW-NEtrendinginselber~areoriented
T218: 2
y
ar
Granodiorite Phyllite MetasedimentRocks,undiff.
y
CentralLowland ~Triassicintrusive~rocksIIfTr:lTriassicsedimentarLBJrocks Upland ......~.:~,,~~:~~ChopawamsicFmGrn"-;roo.r.., ~Plagiogranite lliillTrondjemite B~sinhannockRiver
n I pa
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Tertiarysediment FallsRunGneiss QuanticoFormati 30·kmI divisionsofthe
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IN07830'+045'38 ueRidge CatoctinGreenstone PedlarFormation OldRagGranite rPiedmont CatoctinGreenstone MechumsRiverFm. LynchburgFm. LovingstonFm. RobertsonRiverFm.anMarshallMetagranite. FIGU
BlIlepCcEa ~ Inne ICpeel ~ IIpC1yli I~I I~I
~f\.)~ex> w
TABLE1 LithologicandPhysiographicSubdivisionsoftheRappahannockBasinshownon
figures2and3.
Lithologic Geologic Physiographic
Unit Unit Lithologies Unit
A OldRagGranite, Coarsegrainedgranodiorite, A -BlueRidge: steep,longslopes
Pedlar Fm., shale,quartzite,pebble comingofftheBlueRidge
CatoctinFm. conglomerate,metabasalt drainagedivide. Streamsincised
(greenstone) drainagedivide.
alongNW-SEstructures
B RobertsonRiverFm., Granodioriteandgneiss B-InnerPiedmontrollingconvex
LovingstonGraniteGneiss profiles,commonisolated
MechumsRiverFm., inselbergsofMarshall
CatoctinFm., Metagraniteatvariouselevations
LynchburgFm., strikingSW-NE,variable-width
MarshallMetagranite valleys
C ManassasFm., Sandstone,siltstone,shale, C-CentralLowland,outer
BullRun·Fm. diabase,andconglomerate Piedmont:flat drainagedivides
andlong,low-angleslopes,
rare,isolatedinselbergs,broad
streamvalleys
D FallsRunGraniteGneiss, Schist,gneiss,granite, D -EasternUpland,outer
QuanticoFm., granodiorite,trondhjemite, Piedmont:flat drainagedivides
ChopawamsicFm. plagiogranite,phyllite steepconvexslopes,narrow
valleys
withthe regionalstrikeofmajorstructures. Thesefeatures de localphysiographyiscloselyrelatedtothehydrologicresponseof
velopfromseveralofthegranodioriteandgneissiclithologiesof therockunitstoprecipitation. Hydrologicresponseiscontrolled
the inner Piedmont and theyshowa roughly constant ratio of primarilybytheroutingofrunoffbyrockstructure. Thephysi
width/heightirrespectiveofparentrocktype. ographicunitsshownonfigure2andinTable2canbedifferen
ThecentrallowlandofthePiedmontisdevelopedonMeso tiatedwithrespecttostructuraland,therefore,hydrologicprop
zoicsandstones,shales, anddiabasesills. Topographicallyhigh erties. Theirhydrologicresponsesexercisefundamentalcontrol
areas ofthebasin, orinselbergs, areformed onconglomerates overweatheringanderosionand,therefore,thedevelopmentof
andmassivediabasestocks. Thestreamsinthebasinhavebroad regolithandthelandformsrelatedtoregolithdistribution. These
floodplains and are not asdeeplyincised asthechannelsinthe groupingscanbesummarizedas:
innerPiedmontoreasternupland. a)Structurallymassive,insolublerockssuchasorthoquartzites.
Theeasternuplandisformedoncomplexlyfoldedmetasedi b)Structurallymassiverockscontainingsolubleminerals--low
mentaryandmetaigneousunits. Streamsegmentsoftheeastern permeability,igneous andmetamorphicrockssuchas the Blue
uplandarecloselycontrolledbyrockjointsandfoliation (Cran Ridgegranodiorite, metabasalt and the Mesozoic diabase that
ford and others, 1982). Evenlow-order, perennialstreams are arevariablyjointedandfractured.
deeplyincised. c)Foliatedrocks-withpredominantlyverticalorsteeplydipping
Mapsofbedrock(fig.2),elevation(fig.3),andregoliththick structures
ness(fig.5) showthat: Relativelysolublerocksthat dissolvecongruently(e.g.with
a) Despitetheprimarycontrolofphysiographicprovincesbyli no solidresidue), suchaspure carbonates, donot occurinthis
thology,atamoredetailedscalethephysiographyofthebasinis basin.
complexandnotentirelyrelatedtoobviouslithologicvariations.
ForexampIe,inselbergsontheInnerPiedmontform onlitholo RegolithandSurficialStratigraphy
gies that also underlielower relief, lower elevationareas. The
generallyflat"peneplain"oftheeasternuplandcutsacrossmajor Regolithproducedbytheweatheringorphysicaldisintegra
lithologiccontactsbetweenrocksofvariedcompositio tionofrocksisthemostusefulmappingunitforinterpretingthe
b) OntheinnerPiedmont, theregoliththicknessdistributionis geomorphichistoryofthevariousphysiographicsubdivisionsof
not relatedto topographyas it isgenerallybeneaththeeastern theriverbasin.Regolithcharacteristicschangedramaticallybetween
upland. physiographicsubdivisionsduetothegrosslithologicdifferences.
To explainvariations ofregolith thickness and variations of Theregolithcharacteristicsarevariablebothinarealdistribution
ratesofproduction,erosiontransport,andlossofregolithfrom and in vertical sequence at different landscape positions. For
thebasin,detailedstudiesinsmallareasatamapscaleof1:24,000 comparisons,themajorregolithcharacteristicsaresummarized
or larger are required. Some ofthe areas for whichthere are intable3.
detailed data are listed ontable3. These studiesindicate that Wewillnothavetimetolookindetailatalltheregolithele-
T218: 4
•
D D
• 0-200ft • 400-500ft > 1000ft
..........
• 200-300ft 500-600ft
0 30km
300-400ft 600-1000ft
approx. scale
FIGURE3 Mapshowingcontoursofgeneralelevationintheupperbasin.
mentsofthevariousphysiographicprovinces. Inallcases,how hasdeveloped.Saproliteexceeding15mthickilessalsodevelops
ever,wecanuseasimplemodelofinsituweatheredregolithfor inplacesoflowreliefattheheadsofdrainagesontopoftheBlue
comparisontomostoftheareastobevisited(fig.6;Pavich,1986). Ridge(DeKay,1972)
Themodelhasaverticalsequenceofathinsoilatthesurface,a InnerPiedmont. OntheOldRagGranite,PedlarFormation,
transitionalmassivezone,athicksaprolite,andweatheredrockat andMarshallMetagratiite,andsomeotherrockunitsintheinner
thebase. Piedmont,thehilltopsarecommonlybarerockbalds. Hillsides
Onasingleparentmaterial,thethicknessanddegreeofgeo typicallyarevegetatedandcoveredwithcolluviumand/orresid
chemicalalterationofregolithvariesasafunctionofslopeposi uum. Figure7showstwoexamplesfrom Kesel(1974) from the
tion. Thevariationofregoliththicknessonslopesis,alongwith Orleans7.5'quadrangleshowingthedistributionofregolithatthe
reliefandvalleymorphology,akeyelementindifferentiatingthe footofarockbald. ObservationsintheOrleansandBrightwood
landscapesoftheRappahannockRiverValley. quadrangles(Pavich,unpublisheddata)alongstreamvalleysin
dicatethatsoilandsaprolitecanreach10+metersonhillsides,
BlueRidge. Theoldestrocksinthe coreoftheBlueRidge andKesel(1974) foundareasexceeding30m ofweathering.
Anticlinorium mappedbyEspenshade and Clarke (1976) typi The inselbergs on the inner Piedmont provide an excellent
callydonotexhibitthickweatheringprofilesondrainagedivides opportunitytostudycontrolsonregolithdistribution. BigCob
(Kesel, 1974). Exceptinsome cases ofintenselyfractured and bler Mountain and the adjacent surfaces are underlainby the
jointedrock,uplandsunderlainbytheOldRagGraniteandthe Marshall Metagranite, a gray, coarse-grained to fine-grained
MarshallMetagraniteareexposed,barerock.Inthecaseofthe hornblendegneiss (Kesel, 1974).Kesel (1974) related thebare
OldRaggranite,theresistancetoweatheringisduetoitsmassive rocktocrystalsize: afmer-grainedgneissweathersmoreslowly
structure as well as its high content ofpotassium feldspar and thana coarse-grainedgneissandthusisnotlowered asrapidly.
. quartz. For the Pedlar Formation and MarshallMetagranite, Kesel (1974, p. 16)stated that: "...the slopes ofBigCobbler...
massive, unfoliatedrockstructureisthe more likelyreasonfor composed ofmassive rocks, have numerous bedrock outcrops
resistancetoweathering. ItissignificantthatwheretheOldRag andarecoveredbybouldersmanyfeetindiameter.Theregolith
Graniteisintenselyfractured, a thicksaproliteexceeding15m ontheseslopesislimitedtoisolatedpockets..."and"theregolith
T218: 5
TABLE2 PhysiographicUnits,RegolithCharacteristicsandSoilCharacteristics
Physiographic
andLithologicUnit Regolith Soil
(fromtable1)
A Thicktothin,discontinuoussaprolite, Thintopatchy,somefragicproperties
overlainbygravellycolluviumordiamicton
onsteepslopes(> 15degrees)
B Thinsaproliteandrockbaldsonuplands. Thinonuplands,thickeronsideslopes
Thicksaproliteonsideslopesandadjacent andoversaproliteinsomeareas
toinselbergs
c
Thinsaproliteonflatlyingshalesandsandstones Thin,generallycolorofparentmaterial
D Thicksaprolitebeneathuplanddivides, Thin,brownonsideslopes,thickerand
thinnersaprolitebeneathsideslopes,colluvium reddertowarddrainagedivide
onsteepersideslopes,angularvein-quartz
laggravelcommononupland,gradesinto
stonelinesonsteepsideslopes,several
generationsofcolluviumexposedbeneathsome
sideslopes
Altitude,
m
~~
100 /FZRappahannockRiver
0
~ JamesRiver
100 FZ
0
200 ~
AppomattoxRiver
100
~FZ
0
200 RoanokeRiver
100
/FZ
0
0 200 400 600 800
Distancefromsource, km
FIGURE4 Longitudinalproftlesofsomeriversdrainingthemid-AtlanticPiedmont(modifiedfromHack,1980).
T218: 6
TABLE3 SITESTUDIES
Site PhysiographicProvince InformationaboutRegolith
(1) OldRagGranite BlueRidge mappinganddrilling
(2) BigCobblerMtn. InnerPiedmont mapping,drilling,andseismicsurvey(Kesel,1974)
Orleans7.5'quad
(3) MountainRunFaultZone InnerPiedmont mapping,drilling,andmineralogy
(4) Brightwoodquad InnerPiedmont mappinganddrilling
(5)Triassicconglom~rate CentralLowland mapping,drilling,andmineralogy
(6)SalemChurchquad EasternUpland mapping,drilling,geochemicalanalysisofregolith
coveronPiedmontslopesdevelopedonfoliatedrocksisgenerally granitoidandgneissicrocksoftheinnerPiedmontaresimilarin
deeperthanthatontheadjacentinselbergslopes."Thethickness thattheyaredominantlyquartzandfeldspar,andthattheresis
ofPiedmontregolithadjacenttoinselbergsvariesasmuchas15 tanceoftheOldRagGranitecanbeattributedtoitspotassium
mverticallyover35mhorizontaldistance,whereastheregolith feldspar content. Boththe resistant Pedlar Formationandthe
onthe inner Piedmontgenerally averagesless than 5m inthe deeplyweathered, lowerelevationRobertsonRiverFormation
samedistance. areplagioclase rich; their differenceinresistanceis due to the
Kesel(1974,p.17)notedthatadjacenttoBigCobbler: "...the muchfmercrystalsizeoftheRobertsonRiverFormation. The
bouldersarenotconfinedtotheside-slopes,butalsocanbefound rocksoftheinnerPiedmontarea1110winmuscoviteandgenerally
asboulderfansontheadjacentpiedmontsurface." Weathering lowinbiotite. Without resistant micas forming afoliatedrock
aswellaserosionisevidencedbythelackofplagioclasefeldspar structure,aporoussaproliteapparentlydoesnotdevelopasread
inthecoarsesandfractionontheslopeadjacenttoBigCobbler, ilyintherocksoftheinnerPiedmontasitdoesinthemicaceous
comparedwith33percentintheunweatheredrock(Kesel,1974). easternuplandrocks(Pavich,1986). Thequartzandmuscovitic
Kesel's(1974)observationsonthesignificanceofrockstruc micainthefoliatedeasternuplandrocksformaresistant"skele
tureindeterminingsusceptibilitytoweatheringcorroboratesother ton" allowing recharge ofpercolatingwater to the base ofthe
evidencethatrockresistancetoweatheringisnotonlyduetorock saproliteafterplagioclasehasdissolved.
mineralogy. Modal analyses from Allen (1963) show that the Thus, ingeneral, theinnerPiedmontregolithisthinnest on
Reg 01ith thiekness
~ < 3mwithnarrowzonesofthick
~ reaolith (>15m) infracturezpnes,
gellerall)lcoveredWithcollUVium
variable-generally < 3mon
massiverockswithzones15-30m
thickonfoliated rocks
< 3mthick,withoutgravel
coverexceptforallUVium
3-20m,thickestbeneath
uplanddrainagedivides,
thinningdownvalleys
o
30km
----------
appro x. sea Ie
FIGURE5 MapshowingregoliththicknessinthephysiographicsubdivisionsoftheRappahannockRiverBasin.
T218: 7
flects adynamicbalancebetweenerosionalprocesses androck
c
resistanceratherthanrandompreservationofremnantsofolder
I
erosion surfaces or of strictly tectonic control. An argument
Soil x againsttheinfluenceofperiglacialprocessesoninselbergforma
tionis that such features also exist much farther south onthe
Piedmont(e.g. inner Piedmont ofGeorgia) where Pleistocene
glacial-pluvialprocessesweremuchlesseffectiveinalteringthe
landscape.
CentralLowland. Theregolithofthecentrallowlandisgen
Saprolite erallythin,commonlylessthan3monavarietyofrocktypes. On
thewidelyexposedMesozoicsandstones,siltstonesandshales,a
thin residual soil has developed with little or no recognizable
saprolite or weathered rock zone. The igneous rocks that in
trudedthesedimentsshowmorevariabilityinregolith. Themost
YI massive and fmest grained diabase generallystands above the
Rock surroundinglandscapeasrockbaldsorascobble-mantledslopes.
Where the diabase is intenselyjointed, the weathered regolith
exceeds 10m in depth. These areas are not common, and the
FIGURE 6 Regolith development model for the Piedmont.
Thegroundsurface,X,lowersassaproliteiscondensedintosoil, generaldepthofweatheringofdiabaseaveragesabout3m. Hornfels
zonesaregenerallyquitemassiveandstandupasresistantbare
c,andsoiliserodedfromthesurface. Thicknessofsaproliteisa
functionoftherateofloweringofthemassivezoneatb,andrate rockareas.
Ofparticularinterestindeterminingthephysiographicevolu
ofloweringoftherock-saprolitecontact,aty.
tion ofthe lowland basin are a few isolatedinselbergs formed
from conglomeratic rocks. These rocks were originally high
hilltopbaldsor inselbergs andthicker towardthevalleys. This energysedimentsdepositedneartheborderfaultsactiveduring
distributionisprobablydeterminedbytheorientationandspac theMesozoic.Thereisnodoubtfromtheirdegreeofinduration
ingofsheetjoints, andtherouting ofgroundwaterflowbythe thattheywereburiedbeneathyoungerMesozoicsediments,and
joints. Itislikelythatwaterfallingonthebaldsdoesnotinfiltrate theyhavesincebeenexhumed. Onesuchinselbergnowsitswell
intothemassiverockstructure,andthatitmoveslaterallyalong abovethesurroundinglandscapeprovidingevidenceoftherela
therocksurfaceuntilitencountersanopenjoint. Solution-mineral tiveerodibilityofthefmergrainedsandstoneandshale.
reactions are thenpossible asthewatermoves throughjointed OtherevidenceoftherelativelyhigherosionrateoftheMeso
rockadjacenttomoremassivebalds. zoicsedimentsincludesthehighsuspendedsedimentcontentof
Kesel(1974)hypothesizedthatformationofinnerPiedmont high magnitude flash floods that are common during summer
inselbergswascontrolledbyactivetectonicsorrelatedtoperiods storms.Apparentlythelowinfiltrationcapacityofthethinsoils
ofstreamdowncutting. Weputemphasisontherockstructureas createsconditions conduciveto overlandflowand higher peak
adeterminant ofthe locus ofweatheringand,therefore, ofthe dischargesfromsmall drainagebasinsrelative tothe samesize
resultinglandforms. Inselbergsdisplayasimplerelationbetween basinsdrainingtheeasternupland.
averagewidthandheight. Thissuggeststhatinselbergform re-
Eastern Upland. The regolith distribution ofthe eastern
uplandofthe outer Piedmont is not as easily seen. In drilling
transects,weobserve a correspondence ofthe thickestregolith
BigCobblerMt.,Virginia
withflatteneddrainagedivides,withthinnerregolithbeneathsteep
slopesandinvalleybottoms(fig.8). Pavich(1986)explainedthe
flattened divides as the result ofweathering ofsteeplydipping
foliatedrockstosaprolite,alterationofsaprolitetosoilwithvol
umeloss,andcontinualremovaloffme particlesfrom theclay
richsoilsurface.Structuralcontrolbynearlyverticalskeletalrock
BuckMt.,Virginia
fabriconthedirectionofwatermovementfacilitatesdeepweath
eringbeneaththeflat divides.
TerraceDeposits andDebris Fans on thePiedmont. Al
though regolith relations suggest that the dominant landforms
T Piedmontangle resultfromadjustmenttorocktype,surficialdepositsandland
0, 100 200 m formsofmuchsmallerextentexistasevidencethatepisodicproc
, ,
esses have operated on the Piedmont. In the Rappahannock
--Regolith-bedrockcontact vertical exaggeration2x basin,alluvialterraceshavebeendescribedbyDunford-Jackson
(1978) and colluvialdepositshavebeendescribedbyWhittecar
FIGURE 7 Cross sections from the Orleans 7.5' quadrangle (1985)andKesel(1974).Extensiveslopedebrisandalluvialter
showing the distribution of regolith beneath uplands and racedepositshavebeenmappedbytheUSGSintheMountain
hillslopes of Big Cobbler and Buck Mountains (from Kesel, Runvalleyinanongoingstudyofneotectonicsandgeomorphol
1974). ogy. In the Mountain Run valley, a sequence offive distinct
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