Gallery of Rocks

Map showing the locations of GeoLab samples and the ecoregions of Arkansas

Color symbology based on Woods et al. (2004) Ecoregions of Arkansas and physiographic delineations collected from the US EPA. Visit the Interactive Map page to pan, zoom, query, and explore the region in greater detail.

Woods A.J., Foti, T.L., Chapman, S.S., Omernik, J.M., Wise, J.A., Murray, E.O., Prior, W.L., Pagan, J.B., Jr., Comstock, J.A., and Radford, M., 2004, Ecoregions of Arkansas (color poster with map, descriptive text, summary tables, and photographs): Reston, Virginia, U.S. Geological Survey.

1: Travertine – Ouachita Mountains

Era: Cenozoic
Period: Quaternary
Epoch: Holocene
Age: 12,000 years ago (max)
Lithology: Travertine (Tufa)
Original location: 34.516014, -93.052590
Ecozone: Central Hills, Ridges, and Valleys

Note: Sample chosen as an unusually large and pure example of limestone deposited around mineral springs.

2: Bauxite – South Central Plains

Era: Mesozoic
Period: Tertiary
Epoch: Wilcox-Eocene
Age: 60–34 million years ago
Formation: Basal Midway Formation
Lithology: Bauxite (Aluminum-hydroxide Paleosol; weathered Nepheline Syenite)
Original location: 34.548284, -92.501283
Ecozone: Tertiary Uplands

Note: Classic bauxite texture reflecting mm to cm-scale aluminum hydroxide nodules. This is the weathering product of soil formation on an exposed surface of nepheline syenite. The nepheline syenite “parent rock” was emplaced during the Cretaceous. Its surface was exposed and altered to an aluminum rich paleosol under humid, tropical conditions during the Tertiary.

3: Limestone – South Central Plains

Era: Cenozoic
Period: Tertiary
Epoch: Paleocene
Age: 66–56 million years ago
Formation: Midway Group
Lithology: Limestone
Original location: 34.745302, -92.290832
Ecozone: Tertiary Uplands

Note: Cylindrical core specimen collected from AR state capital site. Representative specimen of fossiliferous, buggy limestone from shortly after the Cretaceous-Tertiary boundary that famously is marked by a large asteroid impact with global iridium layer and resulting mass extinction (including the dinosaur group).

4: Nepheline Syenite – Ouachita Mountains

Era: Mesozoic
Period: Cretaceous
Epoch: Cenomanian
Formation: Unnamed Igneous Intrusion
Age: 105–95 million years ago
Lithology: Nepheline Syenite
Original location: 34.581496, -92.467390
Ecozone: Tertiary Uplands

Note: Moderate to poor economic quality light gray, coarser grained. Sampled selected to exhibit the composition and grain size.

5: Nepheline Syenite – Ouachita Mountains

Era: Mesozoic
Period: Cretaceous
Epoch: Cenomanian
Formation: Unnamed igneous intrusion
Age: 105-95 million years ago
Lithology: Nepheline Syenite
Original location: 34.581496,-92.467390
Ecozone: Tertiary Uplands

Note: Next best economic quality mottled with lighter patches of coarser feldspar crystals. Sample was selected to illustrate internal variation of grain size and fracture fills. Note mottled appearance reflecting local changes in grain size and the sub parallel bands of coarser grained feldspars.

6: Limestone – South Central Plains

Era: Mesozoic
Period: Cretaceous
Epoch: Aptian / Albian
Age: 125–100 million years ago
Formation: Trinity Group / DeQueen
Lithology: Limestone
Original location: 34.071232, -93.905736
Ecozone: Cretaceous Dissected Uplands

Note: Sample selected to illustrate the gray-green, mud-rich, fossiliferous limestone. Dominantly wackestone to packstone textures with cm-scale interbedded layers of grainstones indicating periods of higher energy. The dinosaurs walked across this surface during periods of low tide and subaerial exposure.

7: Gypsum – South Central Plains

Era: Mesozoic
Period: Cretaceous
Epoch: Aptian / Albian
Age: 125–100 million years ago
Formation: Trinity Group
Lithology: Gypsum
Original location: 34.0758, -93.9005
Ecozone: Cretaceous Dissected Uplands

Note: Massive gypsum from “stray bed” quarried in the middle of the Interbedded green and gray calcareous claystone and gypsum. Interbedded gypsum beds periodic drying and evaporation events punctuated by flooding events and clay/silt deposition along a low relief coast line (e.g. present day Trucial Coast, Arabian Gulf).

8: Sandstone – Arkansas Valley

Era: Paleozoic
Period: Pennsylvanian
Epoch: Middle (Desmoinian)
Formation: McAlester
Age: 308–306 million years ago
Lithology: Sandstone
Original location: 35.473880, -94.272593
Ecozone: Arkansas Valley Plains

Note: Sample selected to illustrate a ‘typical’ sandstone from the upper Arkoma Basin fill. Crinkle laminations formed by coalification of macerated plant material. This indicates an adjacent vegetated area being reworked by high energy waves and currents to produce sand sized particles of plant debris commonly called ‘coffee grounds’ in the modern. This lithofacies is common in marginal marine sand flats.

9: Sandstone and Shale – Ouachita Mountains

Era: Paleozoic
Period: Pennsylvanian
Epoch: Morrowan
Age: 323–318 million years ago
Formation: Jackfork
Lithology: Sandstone and Shale
Original location: 34.830326, -92.371917
Ecozone: Fourche Mountains

Note: Sample selected to illustrate deepwater gravity flow event bed. The massive unit below the scale is a Bouma Ta bed. It is overlain by a sandstone matrix conglomerate as a ‘linked debrite’.

10: Volcanic Tuff – Ouachita Mountains

Era: Paleozoic
Period / Epoch: Late Mississippian / Early Pennsylvanian
Age: 331–318 million years ago
Formation: Stanley Group / Hatton Tuff
Lithology: Volcanic Tuff, Debris Flow
Original location: 34.351096, -94.353428
Ecozone: Central Mountain Ranges

Note: Hatton Tuff Lentil within the Lower Stanley Gp. Bedded volcanic tuff deposits interpreted as deepwater gravity flows triggered from thick ash fall deposits along the basin margin or from vertical settling through the water column. Interpretation is still being debated.

11: Concretion – Ouachita Mountains

Era: Paleozoic
Period: Mississippian
Epoch: Middle-Late
Age: 346–323 million years ago
Formation: Stanley Group
Lithology: Concretion
Original location: 34.351096, -94.353428
Ecozone: Central Mountain Ranges

12: Sandstone – Ozark Highlands

Era: Paleozoic
Period: Mississippian
Epoch: Late
Age: 330–323 million years ago
Formation: Batesville
Lithology: Sandstone
Original location: 35.795835, -91.856530
Ecozone: Springfield Plateau

Note: Sample originally commissioned by the U of A as a replacement capstone for wall column. It was rejected. Selected for GeoLab for its Liesegang structures and unique cut pyramid shape.

13: Limestone – Ozark Highlands

Era: Paleozoic
Period: Mississippian
Epoch: Early-Middle
Age: 358–330 million years ago
Formation: Boone
Lithology: Limestone
Original location: 36.23810, -94.18510
Ecozone: Springfield Plateau

14: Limestone – Ozark Highlands

15: Chert – Ouachita Mountains

Era: Paleozoic
Period: Mississippian
Epoch: Early
Formation: Arkansas Novaculite
Age: 358–346 million years ago
Lithology: Chert
Original location: 34.430218, -93.243680
Ecozone: Central Mountain Ranges

Note: Sample selected to illustrate bedding plane vs. fracture orientation. Note diagenetic alteration along the fracture planes. Also note ‘blobular’ areas of alteration.

16: Chert – Ouachita Mountains

Note: Sample selected to illustrate color variation within the novaculite. There are subtle variations in grain size the effect the economic value as well as the best application of the various types from standard sharpening to high polish finishing.

17: Limestone – Ozark Highlands

Era: Paleozoic
Period: Mississippian
Epoch: Early
Age: 358–346 million years ago
Formation: St. Joe
Lithology: Limestone
Original location: 36.160190, -93.005969
Ecozone: Springfield Plateau

Note: Fresh sample that preserves a dark gray, blobular chert clast. Chert clast contains mm to cm-scale crinozoan columnals. Overall sample exhibits cm to dm-scale bedding defined by wavy, clay laminations indicating periods of diminished sedimentation rates. Some laminations exhibit pressure solution and stylitozation.

18: Limestone – Ozark Highlands

Note: Weathered sample that accentuates crinozoan debris (disaggregated and transported) weathered in relief. Blobular, white chert indicating upward deepening and transition toward the Lower Boone penecontemporaneous chert.

19: Shale – Ozark Highlands

Era: Paleozoic
Period and Epoch: Late Ordovician (and Early/Middle Silurian)
Formation: Cason
Age: 458–430 million years ago
Lithology: Shale
Original location: 35.808810, -91.626766
Ecozone: Dissected Springfield Plateau-Elk River Hills

Note: Unconformity at base of Cason Shale on the underlying Fernvale Limestone. Blocks of Fernvale Limestone developed on the unconformity surface incorporated into basal Cason Shale “matrix”.

20: Shale – Ozark Highlands

Era: Paleozoic
Period: Ordovician
Epoch: Late
Age: 458–443 million years ago
Formation: Fernvale Basal Unconformity
Lithology: Shale
Original location: 35.808810, -91.626766
Ecozone: Dissected Springfield Plateau-Elk River Hills

Note: Phosphatic nodules concentrated along bedding planes with the Cason Shale.

21: Sandstone – Ozark Highlands

Era: Paleozoic
Period: Ordovician
Epoch: Middle
Formation: St. Peter
Age: 470–458 million years ago
Lithology: Sandstone
Original location: 36.348592, -93.533754
Ecozone: Central Plateau

Note: Essentially pure quartz sandstone. Cryptic to wavy lamination. Evidence of compaction and loading on bed bases and tops.

22: Dolomite, Sandy Dolostone – Ozark Highlands

Era: Paleozoic
Period: Ordovician
Epoch: Middle (?)
Age: 470–458 million years ago
Formation: Powell (?)
Lithology: Dolomite, Sandy Dolostone
Original location: 36.1883, -92.0358
Ecozone: Central Plateau

Note: ‘The Wedge’. Ripple lamination in relief on weathered surface. Centimeter-scale scour and fill packets attributed to tidal currents. Wavy to ripple lamination well defined on cut surfaces.

23: Limestone – Ozark Highlands

Era: Paleozoic
Period: Ordovician
Epoch: Middle
Age: 470–458 million years ago
Formation: Plattin
Lithology: Limestone
Original location: 35.8633, -91.3241
Ecozone: Central Plateau

Note: Plattin Limestone. Sample selected to illustrate typical intertidal carbonate deposits. Note cm-scale, interbedded micrite mud layers and mm-scale, laminated algal bands. Mud cracks are preserved on bedding planes recording low tide, exposure surfaces. Sample exhibits en echelon, calcite filled fractures.

24: Sandstone, Limestone, Shale – Ouachita Mountains

Era: Paleozoic
Period: Ordovician
Epoch: Early
Age: 485–470 million years ago
Formation: Crystal Mountain
Lithology: Sandstone, Limestone, Shale
Original location: 34.662469, -93.103819
Ecozone: Central Hills, Ridges, and Valleys

Note: An example of a submarine mass transport block or olistolith. A rare representative of debris flow from the early Ordovician Crystal Mountain Sandstone which is a source of museum-quality quartz crystal clusters.

25: Limestone – Ouachita Mountains

Era: Paleozoic
Period: Ordovician
Epoch: Early
Age: 485–470 million years ago
Formation: Collier
Lithology: Limestone
Original location: 34.5317, -93.6806
Ecozone: Central Hills, Ridges, and Valleys

Note: Sample was selected to illustrate gravity flow event beds (Tb,c) and bedding constrained fracture fills. There are indications of soft sediment deformation.

26: Dolomite – Ozark Highlands

Era: Paleozoic
Period: Ordovician
Epoch: Early
Age: 485–470 million years ago
Formation: Cotter
Lithology: Dolomite
Original location: 36.34912, -93.53057
Ecozone: Central Plateau

Note: Medium grained dolostone with common algal structures suggesting pertidal to inter tidal conditions. Vugs containing saddle dolomite crystals and or calcite fillings are common. Note aligned vugular porosity along algal bedding plane