Geologic units in Bucks county, Pennsylvania

Brunswick Formation (Triassic) at surface, covers 30 % of this area

Reddish-brown mudstone, siltstone, and shale, containing a few green and brown shale interbeds; red and dark-gray, interbedded argillites near base. Youngest beds in Brunswick may be Jurassic in age.

Stockton Formation (Triassic) at surface, covers 19 % of this area

Light-gray to buff, coarse-grained, arkosic sandstone; includes reddish-brown to grayish-purple sandstone, siltstone, and mudstone.

Lockatong Formation (Triassic) at surface, covers 17 % of this area

Dark-gray to black, thick-bedded argillite containing a few zones of thin-bedded black shale; locally has thin layers of impure limestone and calcareous shale.

Diabase (Jurassic) at surface, covers 11 % of this area

Medium- to coarse-grained, quartz-normative tholeiite; composed of labradorite and various pyroxenes; occurs as dikes, sheets, and a few small flows. Includes the dark-gray York Haven Diabase (high titanium oxide) and the slightly younger Rossville Diabase (low titanium oxide). In chilled margins, the Rossville is distinguished from the York Haven by its lighter gray color and distinctive, sparse, centimeter-sized calcic-plagioclase phenocrysts.

Trenton Gravel (Quaternary) at surface, covers 8 % of this area

Gray or pale-reddish-brown, very gravelly sand interstratified with crossbedded sand and clay-silt beds; includes areas of Holocene alluvium and swamp deposits.

Pensauken and Bridgeton Formations, undifferentiated (Tertiary) at surface, covers 4 % of this area

Dark-reddish-brown, cross-stratified, feldspathic quartz sand and some thin beds of fine gravel and rare layers of clay or silt.

Wissahickon Formation (Probably lower Paleozoic) at surface, covers 2 % of this area

Includes oligoclase-mica schist, some hornblende gneiss, some augen gneiss, and some quartz-rich and feldspar-rich members due to various degrees of granitization.

Felsic gneiss (Precambrian) at surface, covers 2 % of this area

Light, medium grained; includes rocks of probable sedimentary origin.

Quartz fanglomerate (Triassic) at surface, covers 1 % of this area

Well-rounded quartzite pebbles, cobbles, and rare boulders set in a reddish-brown, sandy matrix.

Leithsville Formation (Cambrian) at surface, covers 1 % of this area

Medium- to dark-gray, crystalline dolomite, light-olive-gray in places, weathering to light gray and yellowish brown; massive bedded; oolitic; pink to gray, mottled chert and dark-gray chert; thin shale and dolomitic shale interbeds; scattered sand grains; upper part is very shaly.

Allentown Formation (Cambrian) at surface, covers 1.0 % of this area

Medium- to medium-dark-gray, thick-bedded dolomite and impure limestone; dark-gray chert stringers and nodules; laminated; oolitic and stromatolitic; some orange-brown-weathering calcareous siltstone at base.

Stockton conglomerate (Triassic) at surface, covers 0.7 % of this area

Quartz cobbles set in a poorly sorted, sandy matrix; includes conglomeratic sandstone.

Hardyston Formation (Cambrian) at surface, covers 0.6 % of this area

Typically light-gray, fine- to medium-grained quartzite, and feldspathic sandstone; color ranges from nearly white to dark gray; massive bedded; Scolithus present in upper part; quartz-pebble conglomerate occurs at base.

Mafic gneiss (Precambrian) at surface, covers 0.5 % of this area

Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to "PZmgh" in places.

Felsic to mafic gneiss (Precambrian) at surface, covers 0.5 % of this area

Light, medium grained; predominantly quartz and feldspar of igneous origin.

Chickies Formation (Cambrian) at surface, covers 0.5 % of this area

Light-gray, hard, massive, Scolithus-bearing quartzite and quartz schist; thin, interbedded dark slate at top; conglomerate (Hellam Member) at base.

Hornblende gneiss (Precambrian) at surface, covers 0.5 % of this area

Dark, medium grained; includes some rocks of probable sedimentary origin.

Bryn Mawr Formation (Tertiary) at surface, covers 0.2 % of this area

High-level terrace deposits; reddish-brown gravelly sand and some silt. Age uncertain.

Mafic gneiss (Probably lower Paleozoic) at surface, covers 0.2 % of this area

Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to pCAmgh in places.

Beekmantown Group (Ordovician) at surface, covers 0.2 % of this area

Includes, in descending order, the Ontelaunee (Oo), Epler (Oe), Rickenbach (Ori), and Stonehenge (Os) Formations.

Stockton Formation (Upper Triassic) at surface, covers 0.1 % of this area

Predominantly medium- to coarse-grained, light-gray, light-grayish-brown, or yellowish- to pinkish-gray arkosic sandstone and medium- to fine-grained, violet-gray to reddish-brown arkosic sandstone; with lesser, reddish to purplish-brown, silty mudstone, argillaceous siltstone, and shale. Some coarse-grained sandstone in lower part contains thick beds of conglomerate (Trsc) which have been mapped in the vicinity of Stockton. Sandstone, deposited in high-gradient stream channels, is mostly planar bedded with scoured bases containing pebble lags and mudstone rip-up clasts. Upper part of channel beds are burrowed. Large-scale trough crossbeds occur in some very coarse grained sandstone beds; smaller scale trough and climbing-ripple cross lamination occur in the upper part of channel sequences and in finer grained sandstone beds. Typical floodplain mudstones are irregularly thin bedded and extensively burrowed. Floodplain beds are thicker and more numerous in the central Newark basin, near the Delaware River. Thickness of the unit (including Trsc) near Stockton is about 1,240 m (4,068 ft).

Cocalico Formation (Ordovician) at surface, covers < 0.1 % of this area

Gray phyllitic shale, maroon shale, siltstone, and silty, siliceous shale; some interbedded argillaceous and quartzose sandstone; predominantly allochthonous, and probably closely related to Hamburg sequence, but includes some autochthonous elements.

Passaic Formation (Lower Jurassic and Upper Triassic) at surface, covers < 0.1 % of this area

(Olsen, 1980) - Reddish-brown to brownish-purple and grayish-red siltstone and shale (JTrp) maximum thickness 3,600 m (11,810 ft). At places contains mapped sandy mudstone (JTrpms), sandstone (JTrps), conglomeratic sandstone (JTrpsc) and conglomerate containing clasts of quartzite (JTrpcq), or limestone (JTrpcl). Formation coarsens up section and to the southwest. Quartzite conglomerate unit (JTrpcq) is reddish-brown pebble conglomerate, pebbly sandstone, and sandstone, in upward-fining sequences 1 to 2 m (3-6 ft) thick. Clasts are subangular to subrounded, quartz and quartzite in sandstone matrix. Sandstone is medium to coarse grained, feldspathic (up to 20 percent feldspar), and locally contains pebble and cobble layers. Conglomerate thickness exceeds 850 m (2,790 ft). Limestone conglomerate unit (JTrpcl) is medium-bedded to massive, pebble to boulder conglomerate. Clasts are subangular dolomitic limestone in matrix of brownish- to purplish-red sandstone to mudstone; matrix weathers light-gray to white near faults. Maximum thickness unknown. Conglomeratic sandstone (JTrpsc) is brownish-red pebble conglomerate, medium- to coarse-grained, feldspathic sandstone and micaceous siltstone; unit is planar to low-angle trough cross laminated, burrowed, and contains local pebble layers. Unit forms upward-fining sequences 0.5 to 2.5 m (1.6-8 ft) thick. Conglomeratic sandstone thickness exceeds 800 m (2,625 ft). Sandstone (JTrps) is interbedded grayish-red to brownish-red, medium- to fine-grained, medium- to thick-bedded sandstone and brownish-to purplish-red coarse-grained siltstone; unit is planar to ripple cross-laminated, fissile, locally calcareous, containing desiccation cracks and root casts. Upward-fining cycles are 1.8 to 4.6 m (6-15 ft) thick. Sandstone beds are coarser and thicker near conglomerate units (JTrpcq, JTrpcl). Maximum thickness about 1,100 m (3,610 ft). Sandy mudstone (JTrpms) is reddish-brown to brownish-red, massive, silty to sandy mudstone and siltstone, which are bioturbated, ripple cross-laminated and interbedded with lenticular sandstone. To southwest where similar lithologic units also occur, they have not been mapped separately, but have been included in undivided unit JTrp. Rhythmic cycles 2 to 7 m (7-23 ft) of thick gray-bed sequences (Trpg), termed Van Houten cycles by Olsen (1985), contain basal thin-bedded to finely laminated shale to siltstone, which grade upward through laminated to microlaminated, locally calcareous mudstone to siltstone and finally into massive silty mudstone. Lowest part of cycle has some desiccation features and local fossils; middle part has highest organic content and the most fossils; highest part contains mudcracks, burrows, and root casts. Gray-bed cycles are abundant in lower half of Passaic Formation and less common in upper half. Rocks of the Passaic Formation have been locally thermally metamorphosed to hornfels where in contact with the Orange Mountain Basalt, diabase dikes, and sheetlike intrusions. Total thickness of formation ranges from 3500 to 3600 m (11480-11810 ft).

Limestone fanglomerate (Triassic) at surface, covers < 0.1 % of this area

Yellowish-gray to medium-gray, angular limestone and dolomite pebbles, cobbles, and fragments set in a red, very fine grained quartz matrix; a few shale-clast interbeds.

Passaic Formation (Lower Jurassic and Upper Triassic) at surface, covers < 0.1 % of this area

Predominantly red beds consisting of argillaceous siltstone; silty mudstone; argillaceous, very fine grained sandstone; and shale; mostly reddish-brown to brownish-purple, and grayish-red. Red beds occur typically in 3- to 7-m (10- to 23-ft-)-thick, cyclic playa-lake-mudflat sequences and fining-upward fluvial sequences. Lamination is commonly indistinct due to burrowing, desiccation, and paleosol formation. Where layering is preserved, most bedforms are wavy parallel lamination and trough and climbing-ripple cross lamination. Calcite- or dolomite-filled vugs and flattened cavities, mostly 0.5 to 0.2 mm (0.02-0.08 in) across, occur mostly in the lower half. Sand-filled burrows, 2 to 5 mm (0.08-0.2 in) in diameter, are prevalent in the upper two-thirds of the unit. Desiccation cracks, intraformational breccias, and curled silt laminae are abundant in the lower half. Lake cycles, mostly 2 to 5 m (7-16 ft) thick, have a basal, greenish-gray, argillaceous siltstone; a medial, dark-gray to black, pyritic, carbonaceous, fossiliferous, and, in places, calcareous lake-bottom fissile mudstone or siltstone; and an upper thick-bedded, gray to reddish and purplish-gray argillaceous siltstone with desiccation cracks, intraformational breccias, burrows, and mineralized vugs. Thickness of the formation between Sourland Mountain and Sand Brook syncline is about 3,500 m (11,483 ft).

Metadiabase (Precambrian) at surface, covers < 0.1 % of this area

Dark-gray, fine-grained intrusives; locally, mineralogy is altered and unit has greenish color.

Passaic Formation Quatzite-clast Conglomerate facies (Lower Jurassic and Upper Triassic) at surface, covers < 0.1 % of this area

Quartzite conglomerate unit (JTrpcq) is reddish-brown pebble conglomerate, pebbly sandstone, and sandstone, in upward-fining sequences 1 to 2 m (3-6 ft) thick. Clasts are subangular to subrounded, quartz and quartzite in sandstone matrix. Sandstone is medium to coarse grained, feldspathic (up to 20 percent feldspar), and locally contains pebble and cobble layers. Conglomerate thickness exceeds 850 m (2,790 ft).

Potomac Formation, unit 3 (Upper Cretaceous, lower Cenomanian) at surface, covers < 0.1 % of this area

Sand, fine- to coarse-grained, locally gravelly, crossbedded, light-colored, interbedded with white or variegated red and yellow, massive clay, and rarely dark-gray, woody clay. The Potomac Formation crops out only in the Delaware River valley where the river and its tributaries have eroded away the overlying formations. The Potomac has been mapped in a broad belt parallel to the inner edge of the Coastal Plain. Although mapped in a broad belt, the Potomac is very poorly exposed because of the widespread cover of surficial sediments. The best exposures occur where surficial material is mined away in the Camden area. Unit is about 45 m (148 ft) thick. Contact with the overlying Magothy Formation is difficult to pick where the basal Magothy also contains variegated clays. Most of the basal Magothy has more dark-colored clay, and the contact was drawn by using this criterion. The basal contact of the Potomac with the underlying crystalline rock is not exposed in New Jersey. Biostratigraphically, the Potomac has been separated into pollen zones I, II, and III (Doyle, 1969; Doyle and Robbins, 1977). Samples from the Potomac Formation in the Camden area and along the Delaware River nearby contain pollen assemblages of early Cenomanian age (Zone III) (Les Sirkin, written commun., 1988).

Lockatong Formation (Upper Triassic) at surface, covers < 0.1 % of this area

Predominantly cyclic lacustrine sequences of silty, dolomitic or analcime-bearing argillite; laminated mudstone; silty to calcareous, argillaceous very fine grained sandstone and pyritic siltstone; and minor silty limestone, mostly light- to dark-gray, greenishgray, and black. Grayish-red, grayish-purple, and dark-brownish-red sequences (Trlr) occur in some places, especially in upper half. Two types of cycles are recognized: freshwater-lake (detrital) and alkaline-lake (chemical) cycles. Freshwater-lake cycles average 5.2 m (17 ft) thick. They consist of basal, transgressive, fluvial to lake-margin deposits that are argillaceous, very fine grained sandstone to coarse siltstone with indistinct lamination, planar or cross lamination, or are disrupted by convolute bedding, desiccation cracks, root casts, soil-ped casts, and tubes. Medial lake-bottom deposits are laminated siltstones, silty mudstones, or silty limestones that are dark gray to black with calcite laminae and grains and lenses, or streaks of pyrite; fossils are common, including fish scales and articulated fish, conchostracans, plants, spores, and pollen. Upper regressive lake margin, playa lake, and mudflat deposits are light- to dark-gray silty mudstone to argillitic siltstone or very fine grained sandstone, mostly thick bedded to massive, with desiccation cracks, intraformational breccias, faint wavy laminations, burrows, euhedral pyrite grains, and dolomite or calcite specks. Alkaline-lake cycles are similar to freshwater-lake cycles, but are thinner, averaging 3 m (10 ft), have fewer fossils (mainly conchostracans), and commonly have red beds, extensive desiccation features, and abundant analcime and dolomite specks in the upper parts of cycles. Thickness near Byram is about 1,070 m (3,510 ft). The formation thins to the southeast and northeast; thickness near Princeton is less than 700 m (2,297 ft).

Diabase (Jurassic) at surface, covers < 0.1 % of this area

Concordant to discordant, predominantly sheet-like intrusions of medium- to fine-grained diabase and dikes of fine-grained diabase; dark-greenish-gray to black; subophitic texture. Dense, hard, sparsely fractured rock composed mostly of plagioclase (An50-70), clinopyroxene (mostly augite), and magnetite-ilmenite. Orthopyroxene (En75-80) is locally abundant in the lower part of the sheets. Accessory minerals include apatite, quartz, alkali feldspar, hornblende, sphene, zircon, and rare olivine. Diabase in the map area was derived primarily from high-titanium, quartz-tholeiite magma. Sedimentary rocks within about 300 m (984 ft) above and 200 m (656 ft) below major diabase sheets are thermally metamorphosed. Red mudstone is typically altered to indurated, bluish-gray hornfels with clots or crystals of tourmaline or cordierite. Gray argillitic siltstone is typically altered to brittle, black, very fine grained hornfels. Sills are 365 to 400 m (1,197-1,312 ft) thick. Dikes range in thickness from 3 to 10 m (10-33 ft) and are many kilometers long.

Stockton Formation Cobble Conglomerate and Sandstone facies (Upper Triassic) at surface, covers < 0.1 % of this area

Predominantly medium- to coarse-grained, light-gray, light-grayish-brown, or yellowish- to pinkish-gray arkosic sandstone and medium- to fine-grained, violet-gray to reddish-brown arkosic sandstone; with lesser, reddish to purplish-brown, silty mudstone, argillaceous siltstone, and shale. Some coarse-grained sandstone in lower part contains thick beds of conglomerate (Trsc) which have been mapped in the vicinity of Stockton. Sandstone, deposited in high-gradient stream channels, is mostly planar bedded with scoured bases containing pebble lags and mudstone rip-up clasts. Upper part of channel beds are burrowed. Large-scale trough crossbeds occur in some very coarse grained sandstone beds; smaller scale trough and climbing-ripple cross lamination occur in the upper part of channel sequences and in finer grained sandstone beds. Typical floodplain mudstones are irregularly thin bedded and extensively burrowed. Floodplain beds are thicker and more numerous in the central Newark basin, near the Delaware River. Thickness of the unit (including Trsc) near Stockton is about 1,240 m (4,068 ft).

Passaic Formation Conglomerate and Sandstone facies (Lower Jurassic and Upper Triassic) at surface, covers < 0.1 % of this area

Conglomeratic sandstone (JTrpsc) is brownish-red pebble conglomerate, medium- to coarse-grained, feldspathic sandstone and micaceous siltstone; unit is planar to low-angle trough cross laminated, burrowed, and contains local pebble layers. Unit forms upward-fining sequences 0.5 to 2.5 m (1.6-8 ft) thick. Conglomeratic sandstone thickness exceeds 800 m (2,625 ft).

Leithsville Formation (Middle and Lower Cambrian) at surface, covers < 0.1 % of this area

(Wherry, 1909) - Thin- to thick-bedded dolomite containing subordinate siliciclastic rocks. Upper part is medium- to medium-dark-gray, fine- to medium-grained, pitted, friable, mottled and massive dolomite. Middle part is medium-gray, stylolitic, fine-grained, thin- to medium-bedded dolomite that is interbedded with shaly dolomite and, less commonly, vari-colored quartz sandstone, siltstone, and shale. Lower part is medium-gray, medium-grained, medium-bedded dolomite containing quartz-sand grains in stringers and lenses near the contact with the Hardyston Quartzite. Archaeocyathids of Early Cambrian age suggest an intraformational disconformity separating rocks of Middle and Early Cambrian age (Palmer and Rozanov, 1976). Thickness approximately 305 m (1,000 ft).

Granitic gneiss and granite (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Includes Springfield Granodiorite (granitized Wissahickon) in Philadelphia area.

Passaic Formation gray bed (Lower Jurassic and Upper Triassic) at surface, covers < 0.1 % of this area

Upper Triassic gray lake deposits (Trpg) consist of gray to black silty mudstone, gray and greenish- to purplish-gray argillaceous siltstone, black shale, and medium- to dark-gray, argillaceous, fine-grained sandstone and are abundant in the lower half of the Passaic Formation. Gray lakebeds occur in groups of two to five cycles although they also occur as single cycles in some parts of the formation. Several lakebed sequences consisting of one or two thick groups of drab-colored beds as much as 30 m (98 ft) thick or more can be traced over tens of kilometers. Many gray-bed sequences are locally correlated within fault blocks; some can be correlated across major faults or intrusive rock units. Thickness of the (entire Passaic) formation between Sourland Mountain and Sand Brook syncline is about 3,500 m (11,483 ft).

Ultramafic rocks (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Includes serpentine, steatite, and other products of alteration of peridotites and pyroxenites.

Hypersthene-Quartz-Oligoclase Gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

Gray- to tan-weathering, greenish-gray to greenish-brown, medium-grained, moderately well layered and foliated, greasy-lustered gneiss of charnockitic affinity composed of andesine or oligoclase, quartz, clinopyroxene, hornblende, hypersthene, and sparse amounts of biotite. Commonly interlayered with amphibolite and mafic-rich quartz-plagioclase gneiss.

Passaic Formation gray bed (Lower Jurassic and Upper Triassic) at surface, covers < 0.1 % of this area

Rhythmic cycles 2 to 7 m (7-23 ft) of thick gray-bed sequences (Trpg), termed Van Houten cycles by Olsen (1985), contain basal thin-bedded to finely laminated shale to siltstone, which grade upward through laminated to micro-laminated, locally calcareous mudstone to siltstone and finally into massive silty mudstone. Lowest part of cycle has some desiccation features and local fossils; middle part has highest organic content and the most fossils; highest part contains mudcracks, burrows, and root casts. Gray-bed cycles are abundant in lower half of Passaic Formation and less common in upper half.

Franklin Marble (Precambrian) at surface, covers < 0.1 % of this area

White, coarsely crystalline; disseminated graphite flakes.

Quartz-Oligoclase Gneiss (Middle Proterozoic) at surface, covers < 0.1 % of this area

White-weathering, light-greenish-gray, medium- to coarse-grained, moderately layered to indistinctly foliated gneiss and lesser amounts of granofels composed of quartz, oligoclase or andesine, and, locally, biotite, hornblende and (or) clinopyroxene. Contains thin amphibolite layers.

Gneiss granofels and Migmatite (Middle Proterozoic) at surface, covers < 0.1 % of this area

Gneiss and granofels range in composition from felsic to intermediate to mafic; intermediate compositions predominate. Contains a wide variety of rock types including graphitic schist and marble. Many rocks were injected by a granitoid that has blue quartz and augen of potassic feldspar and are arteritic migmatites. One body of gneiss contains a 1 m by 0.5 m (3 by 2 ft) phacoid of gabbro that is interpreted to be an olistolith. Unit probably represents a sequence of meta-sedimentary and metavolcanic rocks that have been heavily injected and migmatized by felsic magma.

Hornblende Granite (Middle Proterozoic) at surface, covers < 0.1 % of this area

Pinkish-gray- to medium-buff-weathering, pinkish-white or light-pinkish-gray, medium- to coarse-grained, gneissoid to indistinctly foliated granite and sparse granite gneiss composed principally of microcline microperthite, quartz, oligoclase, and hornblende. Some phases are quartz syenite or quartz monzonite. Includes small bodies of pegmatite and amphibolite not shown on map. U-Pb age approximately 1,090 Ma (Drake and others, 1991b).

Passaic Formation Limestone-clast Conglomerate facies (Lower Jurassic and Upper Triassic) at surface, covers < 0.1 % of this area

Limestone conglomerate unit (JTrpcl) is medium-bedded to massive, pebble to boulder conglomerate. Clasts are subangular dolomitic limestone in matrix of brownish- to purplish-red sandstone to mudstone; matrix weathers light-gray to white near faults. Maximum thickness unknown.

Hardyston Quartzite (Lower Cambrian) at surface, covers < 0.1 % of this area

(Wolff and Brooks, 1898) - Medium- to light-gray, fine- to coarse-grained, medium- to thick-bedded quartzite, arkosic sandstone and dolomitic sandstone. Basal pebble to cobble conglomerate typically contains clasts of local basement affinities. Contains fragments of the trilobite Olenellus thompsoni of Early Cambrian age. Thickness approximately 0.5 to 62 m (1.6-200 ft).

Manhattan Schist (Lower Cambrian and (or) Late Proterozoic) at surface, covers < 0.1 % of this area

(Hall, in press) - Medium-dark gray, medium- to coarse-grained schist and gneiss composed of biotite, muscovite, quartz, and plagioclase, and local accessory minerals sillimanite, kyanite, tourmaline, and garnet. Contains some interlayered amphibolite. Unit is not exposed in the map area, but is present in boring logs.

Microperthite Alaskite (Middle Proterozoic) at surface, covers < 0.1 % of this area

Pink- to buff-weathering, light-pinkish-gray or pinkish-white, medium- to coarse-grained, gneissoid to indistinctly foliated granite composed principally of microcline microperthite, quartz and oligoclase. Includes small bodies of amphibolite not shown on map.

Allentown Dolomite (Lower Ordovician and Upper Cambrian) at surface, covers < 0.1 % of this area

(Wherry, 1909) - Very thin to very thick bedded dolomite containing minor orthoquartzite and shale. Upper part is medium-light- to medium-dark-gray, fine- to medium-grained, locally coarse-grained, medium- to very thick bedded dolomite. Floating quartz sand grains and two sequences of medium-light- to very light gray, thin-bedded quartzite and discontinuous, dark-gray chert lenses occur directly below upper contact. Rhythmically bedded lower dolomite beds alternate between light and dark gray weathering, medium and very light gray, fine and medium grained, and thin and medium bedded, which are interbedded with shaly dolomite. Ripple marks, crossbeds, edgewise conglomerate, mud cracks, oolites, and algal stromatolites occur throughout unit, but more typically in lower part. Shaly dolomite increases downward toward lower conformable contact with the Leithsville Formation. Oldest beds contain trilobite fauna of early Late Cambrian age; younger beds contain latest Cambrian fauna (Howell, 1945; Howell and others, 1950). Thickness about 580 m (1,900 ft).

Amphibolite (Middle Proterozoic) at surface, covers < 0.1 % of this area

Gray- to grayish-black, medium-grained amphibolite composed of hornblende and andesine. Some phases contain biotite and (or) clinopyroxene. Ubiquitous and associated with almost all other Middle Proterozoic units. Some amphibolite is clearly metavolcanic in origin, some is metasedimentary, and some appears to be metagabbro.