Dolerite ventifact from Antarctica. (United States Polar Rock Repository collection, Byrd Polar Research Center, Ohio State University, Columbus, Ohio, USA)
Ventifacts are lustrous, polished rocks with subplanar to curved facets. They form by natural abrasion and polish by winds that carry sand grains. Ventifacts are naturally sand-blasted rocks.
The rock itself is dolerite, also known as diabase. It is the shallow intrusive equivalent of basalt. Specimen-wise, it is essentially identical to basalt - a mafic, aphanitic igneous rock composed of plagioclase feldspar and pyroxene.
Geologic unit: unrecorded/undisclosed, but almost certainly derived from the Ferrar Dolerite, Jurassic
Locality: unrecorded/undisclosed site in Antarctica (but very likely from the Dry Valleys west of McMurdo Sound, possibly Taylor Valley)
Tags: dolerite basalt ventifact ventifacts Antarctica
Fluvially-polished quartzite in the Precambrian of South Dakota, USA.
Extensive outcrops of pinkish, Paleoproterozoic-aged quartzites are present at Falls Park along the Big Sioux River in the city of Sioux Falls, South Dakota. The quartzites here have nicely water-worn, sculpted surfaces. These rocks are part of the Sioux Quartzite, which consists of 1.65 to 1.70 billion year old metamorphosed sandstones. Despite the metamorphism, original sedimentary features such as horizontal stratification, cross-bedding, and ripple marks are still preserved.
The Sioux Quartzite is an erosion-resistant unit in America’s midcontinent. It has formed a long-lived paleotopographic high since Precambrian times - the Sioux tectonic core. This high is part of a northeast-to-southwest trending series of paleotopographic highs & depressions known as the Transcontinental Arch, which extends from Arizona to Minnesota (see Carlson, 1999).
Quarries of Sioux Quartzite occur in southeastern South Dakota and southwestern Minnesota. The rocks are used as building stone, road gravel, sidewalk and paving gravel, and erosion control material.
The shiny areas on this Sioux Quartzite surface are fluvial polish. During times of energetic flow by the nearby Big Sioux River, abrasion from sediments have eroded, scoured, smoothed, and polished the rocks. Naturally polished rocks can also form by glacial action (glacial polish), faulting (structural polish or fault polish), and wind action, which results in ventifacts.
Stratigraphy: Sioux Quartzite, upper Paleoproterozoic, 1.65-1.70 Ga
Locality: Falls Park, near Sioux Falls along the Big Sioux River in the town of Sioux Falls, southeastern South Dakota, USA
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Reference cited:
Carlson (1999) - Transcontinental Arch - a pattern formed by rejuvenation of local features across central North America. Tectonophysics 305: 225-233.
Tags: South Dakota Sioux Falls Quartzite Precambrian Proterozoic Paleoproterozoic sandstone Transcontinental Arch fluvial abrasion polish
Ironstone is an iron oxide-dominated rock. Some of the examples seen here are lustrous and highly polished from wind abrasion. Wind-polished rocks are called ventifacts.
Stratigraphy: Ericson Sandstone, Mesaverde Group, upper Upper Cretaceous
Locality: desert floor south of old pumice prospect in western Zirkel Mesa, northwest of the town of Superior, Sweetwater County, southwestern Wyoming, USA (vicinity of 41° 47' 32.69" North latitude, 108° 56' 22.27" West longitude)
Tags: ironstone ventifact ventifacts Ericson Sandstone Cretaceous Zirkel Mesa Leucite Hills Wyoming Sweetwater County
Glacially striated & polished lava in California, USA.
Glaciers are rivers of ice. Ice is a mineral (H2O). Glacial ice is a rock (technically, a metamorphic rock). Despite being solid, ice does flow under certain conditions at the Earth’s surface. Occasionally, Earth experiences Ice Ages, during which extensive ice sheets cover and move over significant portions of the Earth’s surface. As ice moves over landmasses, it erodes underlying rocks and picks up small to large pieces of debris. This debris accumulates at the base of the ice sheet and scrapes bedrock as the glacier moves, resulting in glacial scratches (glacial striations) (= thin scratch lines on rock) and glacial grooves (= large channels incised in rock). Fine-grained sediments grinding between ice and rock surfaces can result in glacial polish (see above).
Locality: Bumpass Hell Trailhead, south of Mt. Lassen, Lassen Volcano National Park, northeastern California, USA
Tags: Pleistocene Ice Age glacial striations scratches lava Lassen Volcano Volcanic National Park California Cascade Range polish
Dolerite ventifact from Antarctica. (United States Polar Rock Repository collection, Byrd Polar Research Center, Ohio State University, Columbus, Ohio, USA)
Ventifacts are lustrous, polished rocks with subplanar to curved facets. They form by natural abrasion and polish by winds that carry sand grains. Ventifacts are naturally sand-blasted rocks.
The rock itself is dolerite, also known as diabase. It is the shallow intrusive equivalent of basalt. Specimen-wise, it is essentially identical to basalt - a mafic, aphanitic igneous rock composed of plagioclase feldspar and pyroxene.
Geologic unit: unrecorded/undisclosed, but almost certainly derived from the Ferrar Dolerite, Jurassic
Locality: unrecorded/undisclosed site in Antarctica (but very likely from the Dry Valleys west of McMurdo Sound, possibly Taylor Valley)
Tags: dolerite basalt ventifact ventifacts Antarctica