Glacial erratic

thumb|Glacial erratics from [[Norway on Schokland in the Netherlands]]

thumb|Glacial erratic boulder in [[Snowdonia (Eryri), Wales]]

A glacial erratic is a glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word ' ("to wander"), are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (16,500 metric tons) in Alberta.

Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics are significant because:

They can be transported by glaciers, and are thereby one of a series of indicators which mark the path of prehistoric glacier movement. Their lithographic origin can be traced to the parent bedrock, allowing for confirmation of the ice flow route.
They can be transported by ice rafting, which allows quantification of the extent of glacial flooding resulting from ice dam failures which release the waters stored in proglacial lakes such as Lake Missoula. Erratics released by ice rafts that were stranded and subsequently melted, dropping their load, allow characterization of the high-water marks for transient floods in areas like temporary Lake Lewis.
Erratics dropped by icebergs melting in the ocean can be used to track Antarctic and Arctic-region glacial movements for periods prior to record retention. Also known as dropstones, these can be correlated with ocean temperatures and levels to better understand and calibrate models of the global climate.

Formation of erratics

thumb|upright=1.2|Multiple erratics on the [[terminal moraine of the Okanogan Lobe. The Cascade Mountains are in the background.]]

The term "erratic" is commonly used to refer to erratic blocks, which geologist Archibald Geikie describes as: "large masses of rock, often as big as a house, that have been transported by glacier ice, and have been lodged in a prominent position in the glacier valleys or have been scattered over hills and plains. And examination of their mineralogical character leads the identification of their sources...". In geology, an erratic is material moved by geologic forces from one location to another, usually by a glacier.

Erratics are formed by glacial ice erosion resulting from the movement of ice. Glaciers erode by multiple processes including:

Abrasion/Scouring – debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till.
Plucking – pieces of bedrock are cracked off by glaciers, producing larger erratics.
Ice thrusting – the glacier freezes to its bed, moving large sheets of frozen sediment at its base along with it.
Glacially induced spalling – layers of rock are spalled off the rocks below the glacier during ice lens formation. This provides smaller debris, which is ground into the glacial basal material, to become till.

upright|thumb|[[Doane Rock, at Cape Cod National Seashore]]

Evidence supports another possibility for the creation of erratics as well: rock avalanches onto the upper surface of the glacier (supraglacial). Rock avalanche–supraglacial transport occurs when the glacier undercuts a rock face, which fails by avalanche onto the upper surface of the glacier. The characteristics of rock avalanche–supraglacial transport includes:

:*Monolithologic composition – a cluster of boulders of similar composition are frequently found in close proximity. Commingling of the multiple lithologies normally present throughout the glaciated basin, has not occurred.

Ice-rafted erratic

Glacial ice entrains debris of varying sizes from small particles to extremely large masses of rock. This debris is transported to the coast by glacier ice and released during the production, drift and melting of icebergs. The rate of debris release by ice depends upon the size of the ice mass in which it is carried as well as the temperature of the ocean through which the ice floe passes.

thumb|alt=This photo shows an automobile passing in front of a rock which is essentially fully exposed. The rock has a rough, dark surface indicating it is weathered basalt and is roughly circular in exposed cross-section. The rock is immediately adjacent to a roadway—the road cut removed much of the earth from one side of it exposing it—from the excavation it is evident that the rock sits on a mound of glacial till. The rock is approximately two times the length of the car (i.e., ≈9 metres) in one direction and five times the height of the car in the other direction (i.e., ≈9 metres). Since the rock has not tipped onto the road and no structural support is provided, it must be approximately as deep as it is wide and high. Since the density of basalt is 3 grams per cubic centimetre, this puts the mass of the rock at about 400 to 500 metric tons (consistent with the references). |[[Yeager Rock, a boulder on the Waterville Plateau, Washington. Although transported by a glacier, this boulder is not a true erratic because it is of the same lithology as the underlying, till-blanketed bedrock. Note the glacial till below the rock.]]

Sediments from the late Pleistocene period lying on the floor of the North Atlantic show a series of layers (referred to as Heinrich layers) which contain ice-rafted debris. They were formed between 14,000 and 70,000 years before the present. The deposited debris can be traced back to the origin by both the nature of the materials released and the continuous path of debris release. Some paths extend more than distant from the point at which the ice floes originally broke free.

Large erratics

thumb|Glacial erratic [[Ehalkivi, with overground volume of (weight approximately ), in Estonia]]

thumb|Area exposed by the retreat of [[Alaska's Steller Glacier in August 1996, the westernmost part of Bering Glacier's piedmont lobe. The ground surface is covered by glacial sediment deposited as lodgement and ablation till. The erratic is an angular, piece of gneiss. Bering Glacier, Alaska flows through Wrangell–St. Elias National Park and Preserve.]]

Large erratics consisting of slabs of bedrock that have been lifted and transported by glacier ice to subsequently be stranded above thin glacial or fluvioglacial deposits are referred to as glacial floes, rafts (schollen) or erratic megablocks. Erratic megablocks have typical length-to-thickness ratios on the order of 100 to 1. These megablocks may be found partially exposed or completely buried by till and are clearly allochthonous, since they overlay glacial till. Megablocks can be so large that they are mistaken for bedrock until underlying glacial or fluvial sediments are identified by drilling or excavation. Such erratic megablocks greater than in area and in thickness can be found on the Canadian Prairies, Poland, England, Denmark and Sweden. One erratic megablock located in Saskatchewan is (and up to thick). Their sources can be identified by locating the bedrock from which they were separated; several rafts from Poland and Alberta were determined to have been transported over from their source.

Nonglacial erratics

In geology an erratic is any material which is not native to the immediate locale but has been transported from elsewhere. The most common examples of erratics are associated with glacial transport, either by direct glacier-borne transport or by ice rafting. However, other erratics have been identified as the result of kelp holdfasts, which have been documented to transport rocks up to in diameter, rocks entangled in the roots of drifting logs, and even in transport of stones accumulated in the stomachs of pinnipeds during foraging.

History

thumb|Erratic rocks on Estonian northern coast

During the 18th century, erratics were deemed a major geological paradox. Geologists identify erratics by studying the rocks surrounding the position of the erratic and the rock of the erratic itself. Erratics were once considered evidence of a biblical flood, but in the 19th century scientists gradually came to accept that erratics pointed to an ice age in Earth's past. Among others, the Swiss politician, jurist and theologian saw glaciers as a possible solution as early as 1788. However, the idea of ice ages and glaciation as a geological force took a while to be accepted. Ignaz Venetz (1788–1859), a Swiss engineer, naturalist and glaciologist was one of the first scientists to recognize glaciers as a major force in shaping the earth.

In the 19th century, many scientists came to favor erratics as evidence for the end of the ice age 10,000 years ago, rather than a flood. Geologists have suggested that landslides or rockfalls initially dropped the rocks on top of glacial ice. The glaciers continued to move, carrying the rocks with them. When the ice melted, the erratics were left in their present locations.

Charles Lyell's Principles of Geology (v. 1, 1830) provided an early description of the erratic which is consistent with the modern understanding. Louis Agassiz was the first to scientifically propose that the Earth had been subject to a past ice age. In the same year, he was elected a foreign member of the Royal Swedish Academy of Sciences. Prior to this proposal, Goethe, de Saussure, Venetz, Jean de Charpentier, Karl Friedrich Schimper and others had made the glaciers of the Alps the subjects of special study, and Goethe, Charpentier as well as Schimper

Examples

Glacier-borne erratics

thumbnail|Example of mixed erratics. The boulder in the foreground is [[basalt. The boulder on the other side of the fence is granite.]]

Australia

Exhumed erratics eroded from unconsolidated 270 Ma Permian glacial sediments can be found on the beach and in the park at Hallett Cove Conservation Park just south of Adelaide, and in other South Australian locations, such as Inman Valley.

Canada

Big Rock near Okotoks, Alberta, Canada. It is the largest erratic in the Foothills Erratics Train.
Bleasdell Boulder, southern Ontario, was described as "glacial" in origin by Reverend William Bleasdell in 1872.
The Foothills Erratics Train is a deposit of rocks of many sizes. These deposits stretch in a narrow belt for about from Alberta's Athabasca River Valley to the southwest of the province.
White Rock, British Columbia gets its name from a coastal erratic the size of a garage found on the beach at Semiahmoo Bay, right at the border with Washington.
Boulder in Green Timbers Urban Forest Park in Surrey, British Columbia, described as a glacial erratic on the city websitealt=Large glacial erratic boulder in shady forest|thumb|Glacial erratic in [[Green Timbers Urban Forest Park, in Surrey, BC, Canada]]

Estonia

Ehalkivi (Sunset Glow Boulder) near Letipea, Estonia is the largest erratic boulder in the glaciation area of North Europe. Height 7 m, circumference 48.2 m, a volume of 930 m<sup>3</sup> and a mass of approx 2,500 tonnes

Finland

thumb|Kukkarokivi in March 2013

Kukkarokivi, located close to Turku at the Ruissalo island in Southwest Finland. It is the largest in Finland; length about 40 m, width about 30 m, height 12 m, weight about 36,000 tonnes.

Germany

Colossus of Ostermunzel, Lower Saxony
Der Alte Schwede, found during dredging of the river Elbe near Hamburg in 1999; oldest in Germany
Giebichenstein, Stöckse, Lower Saxony
Glacial erratics on and around Rügen

Republic of Ireland

The Clonfinlough Stone, located in central Ireland, is covered with Bronze Age and medieval carvings

Latvia

Nīcgale Great Stone, located in Nīcgale Parish
Brodu quarry stone, located in Jēkabpils
Lauču stone, located in Lauči, Skulte parish, Limbaži municipality. Believed to have separated from a glacier in the Vyborg area of Southern Finland and Russia.

thumb|Lauču Stone in Vidzeme coastline, [[Latvia]]

Lithuania

Puntukas, one of national symbols of Lithuania, near Anykščiai city

Poland

Trygław, Tychowo
Devil Stone, Kashubia
Glacial erratic on Wolności Square in Racibórz
Glacial erratic in Sulechów

thumb|Devil Stone, [[Kashubia, Poland]]

United Kingdom

England

The Crosby Erratic, Coronation Park, Liverpool, England; unearthed in a field nearby in 1898.
The Great Stone of Fourstones, at the county boundary between North Yorkshire and Lancashire, has fifteen steps carved in its side to enable it to be climbed.

thumb|The Drake Stone near [[Harbottle, Northumberland, is the height of a double-decker bus.]]

The Hitching Stone in North Yorkshire. It is the largest erratic block in the county.
The Merton Stone, Merton, Norfolk
The Norber erratics in the Yorkshire Dales are one of England's finest sets of glacial erratics.
Soulbury Stone, located in Soulbury, Buckinghamshire

Scotland

Jim Crow Rock, glacial erratic in Hunters Quay, situated on the foreshore of the Firth of Clyde. The rock has been the subject of controversy because of an allegedly racist face painted on it.

Northern Ireland

Cloughmore, near Rostrevor in County Down, Northern Ireland, is a glacial erratic found on the mountain high above the village. Located on the slopes of Slieve Martin, the stone overlooks Carlingford Lough and the Cooley Peninsula.

United States

thumb|Bubble Rock, [[Acadia National Park, Maine]]

thumb|The Glen Rock, in Glen Rock, New Jersey

Bubble Rock, perched on the edge of a cliff near the summit of South Bubble mountain in Acadia National Park, Maine.
Doane Rock, the largest exposed boulder in Cape Cod, Massachusetts.
Fantastic Erratic, a fern-covered erratic the size of a two-car garage, is found on Cougar Mountain near Seattle.
Glen Rock, a boulder weighing in Glen Rock, New Jersey, believed to have been carried to the site by a glacier that picked up the rock 15,000 years ago near Peekskill, New York.
Indian Rock in Montebello, New York is a large glacial erratic boulder of granite gneiss, formed in the Proterozoic (Precambrian) era, 1.2 billion to 800 million years ago. It is estimated to weigh ≈17,300 tons.
Madison Boulder, a glacial erratic the size of a large house in Madison, New Hampshire.
Olmsted Point in Yosemite National Park is noteworthy for having granite hills covered in numerous small glacial erratics.
Pleasantville Glacial Erratic, a Fordham gneiss boulder in Rockefeller State Park Preserve in Westchester County, New York; based on its dimensions (20 feet tall with a circumference of roughly 65 feet), its total weight may be some 630 tons.
Plymouth Rock, the site in Plymouth, Massachusetts on which the Mayflower Pilgrims landed in 1620. It is an important symbol in American history.
Rollstone Boulder, a 110-ton porphyritic granite boulder that was originally located at the summit of Rollstone Hill in Fitchburg, Massachusetts. It was carried by the last glaciation from central New Hampshire. Threatened by quarrying operations, it was moved to Litchfield Park in downtown Fitchburg in 1929–1930.
The northern portion of the town of Waterville, Washington has a large number of large basalt erratics, particularly along the moraine running east–west from McNeil Canyon.
Tripod Rock in Kinnelon, New Jersey is noteworthy for being perched on three smaller boulders.
Balance Rock in Princeton, Massachusetts is located at the base of Mount Wachusett, on the northwest side.

Flood-borne erratics

If glacial ice is "rafted" by a flood such as that created when the ice dam broke during the Missoula floods, then the erratics are deposited where the ice finally releases its debris load. One of the more unusual examples is found far from its origin in Idaho at Erratic Rock State Natural Site just outside McMinnville, Oregon. The park includes a specimen, the largest erratic found in the Willamette Valley.

References

Imbrie, J. & K. P. Imbrie. Ice Ages, Enslow Publishers, Hillside, New Jersey, 1979.

External links

Royal Alberta Museum