Geology Hydrostratigraphy 

Period

Epoch

Group

Formation

Member

(formal and informal)

Lithology and ichnology 

Map abbreviations

and color

Thickness2 

(outcrop in the 

study area,

 in feet) 

Hydrologic 

function 

Porosity 

type Field identification 

Cretaceous 

Late Cretaceous

Washita 

Del Rio

Clay

* 

Fossiliferous blue-green to yellow-brown 

clay, packstone, iron nodules; Ilymatogyra 

arietina 

Kdr Confining 

unit *

40–50 Confining None 

Clay, holds water, 

fossiliferous; Ilymatogyra 

arietina 

Georgetown 

* 

Reddish-brown, gray to light tan shaley 

mudstone and wackestone, black dendrites, 

iron nodules, iron staining; Plesioturrilites 

brazoensis, Waconella wacoensis 

Kg

Edwards aquifer 

I 20–30 Confining MO 

Black dendrites, iron 

nodules and staining, 

Plesioturrilites brazoensis, 

Waconella wacoensis 

Early Cretaceous 

Edwards 

Person 

Cyclic and

marine

(undivided)

Pelletal limestone, mudstone, miliolid 

grainstone, packstone, chert (bedded and 

large nodules); caprinid, cross-bedded 

Kpcm II 80–90 Aquifer 

MO, BU, 

VUG, BP, 

FR, CV 

Thin graded cycles; 

massive beds to relatively 

thin beds; cross-beds, 

caprinids 

Leached and

collapsed

(undivided)

Recrystallized limestone, mudstone, 

wackestone, packstone, grainstone; chert 

(bedded and large nodules); iron-stained, 

stromatolitic, Toucasia sp., Montastrea 

roemeriana, oysters 

Kplc III 70–90 Aquifer 

BU, VUG, 

FR, BP, 

BR, CV 

Bioturbated iron-stained 

beds separated by 

massive limestone beds; 

stromatolitic limestone, 

Montastrea roemeriana 

Regional dense

Dense, shaley, mudstone, wackestone, 

oyster-shell mudstone and wackestone, 

iron staining, chert 

Kprd IV 20 Confining FR, CV 

Wispy iron-oxide stains, 

thin bedded, often white 

in aerial photographs 

Walnut Clay Kainer 

Grainstone

Miliolid, skeletal fragmented grainstone, 

mudstone, wackestone; chert (beds and 

nodules); cross-bedded and ripple marked 

Kkg V 40 Aquifer IP, BU, 

FR, BP, 

CV 

Cross-bedded, ripple 

marks, miliolid 

grainstone 

Kirschberg

evaporite 

Highly altered crystalline limestone, chalky 

mudstone, occasional grainstone associated 

with tidal channels; chert (beds and nodules); 

coarse-grained spar, breccia and travertine, 

dissolution has removed all evaporites in 

the study area 

Kkke VI 40–50 Aquifer 

IP, MO, 

VUG, FR, 

BR, CV 

Boxwork porosity with 

neospar and travertine 

frame 

Dolomitc 

Chert (absent in lower 20 ft), dolomitic 

mudstone, wackestone, packstone, 

grainstone 

Kkd VII 90–120 Aquifer 

IP, IC, 

MO, BU, 

VUG, FR, 

BP, CV 

Massively bedded light 

gray, Toucasia sp., 

abundant 

Basal

nodular 

* 

Shaley, nodular limestone, burrowed 

mudstone, wackestone, packstone, miliolid 

grainstone, dolomite, contains dark, spherical 

textural features locally known as black 

rotund bodies (BRBs); caprinid, Ceratostreon 

[Exogyra] texana, miliolid, gastropods; area 

transitions to basal nodular containing oyster 

beds and shale 

Kkbn

Kwc

VIII 40 

Aquifer, 

confining unit in 

areas without 

caves 

IP, MO, 

BU, BP, 

FR, CV 

Massive, nodular and 

mottled limestone, BRBs 

and orange wisps, 

Ceratostreon 

[Exogyra] texana, seeps 

and springs, ferns 

growing near contact of 

underlying unit 

Trinity 

Glen Rose Limestone 

Upper 

Alternating beds of burrowed wackestone, 

packstone, miliolid grainstone, argillaceous 

limestone 

Kgrcb

Upper zone of the Trinity aquifer 

Camp Bullis 230 Confining 

BU, BP, 

FR, 

occasional 

CV 

Alternating beds of 

limestone and argillaceous 

limestone, fossils rare, 

stair-step topography 

Dissolved evaporites, highly altered 

crystalline limestone and chalky mudstone, 

breccia, boxwork voids 

Kgrue Upper 

evaporite 

10 Aquifer IP, MO, 

BU, BR Spring and seeps 

Fossiliferous 120–130 Semi-confining MO, BU, 

FR, CV 

Limestone and 

argillaceous limestone, 

Orbitolina minuta 

(Douglas, 1960) 

Alternating wackestone, packstone, 

miliolid grainstone, argillaceous limestone, 

mudstone, silty mudstone at base; 

Hemiaster sp., Neithea sp., Orbitolina minuta 

(Douglas, 1960), Porocystis golobularis, 

Protocardia texana, Tapes decepta, 

Turritella sp., gastropods, mollusks; 

(section not subdivided into an upper and 

lower unit as in Bexar and Comal Counties) 

Kgrf

Dissolved evaporites, highly altered 

crystalline limestone and chalky mudstone, 

breccia, boxwork voids; Corbula beds Kgrle Lower 

evaporite 10 Aquifer IP, MO, 

BU, BR 

Weathers to an orangish 

red with a pebbly texture, 

often has less cedar 

growth and thicker grasses, 

boxwork porosity, 

Corbula sp., spring and 

seeps 

Lower 

Wackestone, grainstone, argillaceous 

wackestone, shale, evaporites; 

monopleurid, Toucasia sp., 

Macraster sp., Nerinia sp., Orbitolina 

texana (Roemer, 1852), Porocystis 

golobularis, Salenia texana, gastropods, 

pecten, and pelecypods 

Kgrb

Middle zone of the Trinity aquifer 

Bulverde 30–40 

(typically 30) Semi-confining MO, BR, 

BP, FR 

Salinia texana bed 

immediately below 

Corbula bed, abundant 

fossils including 

Porocystis golobularis, 

Orbitolina texana 

(Roemer, 1852), 

Macraster sp., Nerinia sp., 

pecten, gastropods, 

pelecypods 

Mudstone, wackestone, argillaceous 

wackestone, boundstone; caprinid, 

monopleurid, Toucasia sp., 

Orbitolina texana (Roemer, 1852), 

gastropods, pectens, pelecypods 

Kgrlb Little Blanco 30–40 

(typically 30) Aquifer MO, BU, 

BP, FR 

Limestone beds thicker 

and more resistive to 

erosion than overlying and 

underlying units, Orbitolina 

texana (Roemer, 1852), 

patch reefs 

Argillaceous wackestone, shale; Orbitolina 

texana (Roemer, 1852), gastropods, 

pelecypods 

Kgrts Twin Sisters 

30–40 

(typically 30) 

Semi-confining; 

Confining 

shale beds 

IP 

Thick argillaceous beds, 

thin shale beds, Orbitolina 

texana (Roemer, 1852), 

contains ponds and 

seeps, often little 

vegetation, steeper slopes 

often with “badlands” type 

weathering, thinner in areas 

where patch reefs are 

present in the underlying 

Doeppenschmidt HSU 

Mudstone, wackestone, packstone, 

grainstone, boundstone, argillaceous 

wackestone and packstone, miliolid 

grainstone; caprinid, Toucasia sp. 

Kgrd Doeppenschmidt 

40–80 

(typically 40) Aquifer 

IP, MO, 

BU, BP, 

FR, CV 

Orbitolina texana 

(Roemer, 1852), 

limestone beds thicker 

and more resistive 

than overlying and 

underlying patch reefs 

formed on rudist, reefal 

talus 

Alternating beds of argillaceous wackestone, 

packstone; mudstone, wackestone, 

packstone, grainstone, milliolid grainstone; 

monopleurid, Nerinia sp., Orbitolina 

texana (Roemer, 1852), Tylostoma sp., 

and oysters, pectens, and pelecypods 

Kgrr Rust 

40–70 

(typically 40) Semi-confining IP, FR, CV 

Forms stair-step 

topography with soils, 

Orbitolina texana 

(Roemer, 1852) 

Wackestone, packstone, grainstone, 

boundstone, burrows; caprinid, miliolid, 

Orbitolina texana (Roemer, 1852), 

Toucasia sp., Trigonia sp., Turritella sp., 

various corals, pectens, shell fragments 

Kgrhc Honey Creek 

45–60 

(typically 55) Aquifer 

IP, MO, 

BU, BP, 

FR, CH, 

CV 

Thick beds of 

wackestone, packstone, 

grainstone; corals, 

caprinid, Trigonia sp., 

cliff forming; outcrop 

often contains large 

limestone float with large 

channel and moldic 

porosity, caves and 

springs 

Table 1. Summary of bedrock geology and hydrostratigraphy of the Edwards and Trinity aquifers within the Driftwood and Wimberley 7.5-minute quadrangles, Hays and Comal Counties, Tex. 

[Period, Epoch, Group, Formation, members, and lithology modified from Imlay (1945), Whitney (1952), Lozo and Stricklin (1956), Stricklin and others (1971), Rose (1972), Stricklin and Smith (1973), Amsbury 

(1974), Inden (1974), Perkins (1974), Clark and others (2009, 2013, 2014), Weirman and others (2010), Blome and Clark (2014), and the U.S. Geological Survey National Geologic Map Database, GEOLEX 

(http://ngmdb.usgs.gov/Geoloex.html); Orbitolina minuta (Douglas, 1960), Orbitolina texana (Roemer, 1852); aquifers from Maclay and Small (1976), and Ashworth (1983); thickness from outcrop, Clark and 

others (2009, 2014), Weirman and others (2010); hydrologic function modified from outcrop (Clark and others, 2009, 2013, 2014; Weirman and others, 2010; Clark and Morris, 2015); porosity types modified from 

Choquette and Pray (1970). 

Fabric selective: IP, Interparticle porosity; IC, Intercrystalline porosity; SH, Shelter porosity; MO, Moldic porosity; BU, Burrowed porosity; FE, Fenestral; BP, Bedding plane porosity. 

Not-fabric selective: FR, Fracture porosity; CH, Channel porosity; BR, Breccia; VUG, vug porosity; CV, Cave porosity; *no further subdivision; **not present in the study area] Trinity aquifer Hydrologic unit

1 Hydrostratigraphic unit

(HSU)

EXPLANATION OF HYDROSTRATIGRAPHIC UNITS 

Member Informal 

hydrostratigraphic Formation (Formal and informal) unit 

Del Rio Clay 

Georgetown 

* *

Kdr Upper Confining 

Kg

Unit (UCU) 

I 

Person 

Cyclic and marine, undivided 

Leached and collapsed, undivided 

Regional dense member 

Kpcm

Kplc

Kprd

II 

III 

IV 

Grainstone Kkg V 

Kainer Kirschberg evaporite Kkke VI 

Dolomitic Kkd VII 

*

Kwc

Kkbn

Clay 

Walnut Basal nodular VIII 

Kgrcb Camp Bullis

Kgrue Upper evaporite Upper Glen Rose 

Limestone Kgrf Fossiliferous

Kgrle Lower evaporite

Kgrb Bulverde Glen Rose 

Limestone 

Lower Glen Rose 

Limestone 

Kgrlb

Kgrts

Little Blanco 

Twin Sisters 

Kgrd Doeppenschmidt 

Kgrr Rust 

Kgrhc Honey Creek 

1

Informal 

2

Thickness range based on field mapping in the study area