Glacial erratic

Two small icebergs at right clearly retain fragments of the moraine (rock debris) that forms a dark line along the upper surface of the glacier. The inclusion of the moraine illustrates how land-based rocks and sediment are carried by ice.

Erratics provide an important tool in characterizing the directions of glacier flows, which are routinely reconstructed used on a combination of moraines, eskers, drumlins, meltwater channels, and similar data. Erratic distributions and glacial till properties allow for identification of the source rock from which they derive, which confirms the flow direction, particularly when the erratic source outcrop is unique to a limited locality. Erratic materials may be transported by multiple glacier flows prior to their deposition, which can complicate the reconstruction of the glacial flow.[6]

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.[7][8]

Yeager Rock, a 400-metric-ton (440-short-ton) 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 of 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 at 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 3,000 kilometres (1,900 mi) distant from the point at which the ice floes originally broke free.[7]

The location and altitude of ice-rafted boulders relative to the modern landscape has been used to identify the highest level of water in proglacial lakes (e.g., Lake Musselshell in central Montana) and temporary lakes (e.g., Lake Lewis in Washington state. Ice-rafted debris is deposited when the iceberg strands on the shore and subsequently melts, or drops out of the ice floe as it melts. Hence all erratic deposits are deposited below the actual high water level of the lake; however the measured altitude of ice-rafted debris can be used to estimate the lake surface elevation.

Angular glacial erratic on Lembert Dome.

This is accomplished by recognizing that on a fresh-water lake, the iceberg floats until the volume of its ice-rafted debris exceeds 5% of the volume of the iceberg. Therefore, a correlation between the iceberg size and the boulder size can be established. For example, 1.5-metre (4.9 ft) diameter boulder can be carried by a 3-metre (9.8 ft) high iceberg and could be found stranded at higher elevations than a 2-metre (6.6 ft) boulder which requires a 4-metre (13 ft) high iceberg.[9]

Glacial erratic Ehalkivi with overground volume 930 cubic metres (1,220 cu yd) (weight approximately 2,500 metric tons or 2,800 short tons) in Estonia

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, 20-foot (6.1 m)-high piece of gneiss. Bering Glacier, Alaska flows through Wrangell-Saint Elias National Park

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 1 square kilometre (250 acres) in area and 30 metres (98 ft) in thickness can be found on the Canadian Prairies, Poland, England, Denmark and Sweden. One erratic megablock located in Saskatchewan is 30 by 38 kilometres (19 mi × 24 mi) (and up to 100 metres or 330 feet 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 300 kilometres (190 mi) from their source.[10]

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 40 centimetres (16 in) in diameter, rocks entangled in the roots of drifting logs, and even in transport of stones accumulated in the stomachs of pinnipeds during foraging.[11]

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

Examples of erratics include:

• 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.

• Big Rock near Okotoks, Alberta, Canada. It is the largest erratic in the Foothills Erratics Train.
• Bleasdell Boulder, southern Ontario was described "glacial" in origin by Reverend William Bleasdell in 1872[16]
• The Foothills Erratics Train is a deposit of rocks of many sizes. These deposits stretch in a narrow belt for about 600 kilometres (370 mi) 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.

• Ehalkivi (Sunset Glow Boulder) near Letipea, Estonia is the largest erratic boulder in the glaciation area of North Europe. Height 7m, circumference 48,2m, a volume of 930m3 and a mass of approx 2500 tonnes.

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

• 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

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

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

Devil Stone, Kashubia, Poland

• Devil Stone, Kashubia

• The Crosby Erratic, Coronation Park, Liverpool, England; unearthed in a field nearby in 1898.[17]
• The Great Stone of Fourstones, at the county border between North Yorkshire and Lancashire, has fifteen steps carved in its side to enable it to be climbed.

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.[18]
• 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.[19]

• Jim Crow Rock, a controversial glacial erratic in Hunters Quay, situated on the foreshore of the Firth of Clyde.

• 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.

Bubble Rock, Acadia National Park, Maine

• 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 at 570 short tons (520 t) 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 5,000-short-ton (4,500 t) 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.
• Plymouth Rock, the site in Plymouth, Massachusetts that 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.

In the event that glacial ice is "rafted" by a flood such as that created when the ice dam broke during the Missoula Floods, 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 40-short-ton (36 t) specimen, the largest erratic found in the Willamette Valley.