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 errare ("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
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.
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. Angularity – the supraglacially transported rocks tend to be rough and irregular, with no sign of subglacial abrasion. The sides of boulders are roughly planar, suggesting that some surfaces may be original fracture planes. Great size – the size distribution of the boulders tends to be skewed toward larger boulders than those produced subglacially. Surficial positioning of the boulders – the boulders are positioned on the surface of glacial deposits, as opposed to partially or totally buried. Restricted areal extents – the boulder fields tend to have limited areal extent; the boulders cluster together, consistent with the boulders landing on the surface of the glacier and subsequently deposited on top of the glacial drift. Orientations – the boulders may be close enough that original fracture planes can be matched. Locations of the boulder trains – the boulders appear in rows, trains or clusters along the lateral moraines as opposed to being located on the terminal moraine or in the general glacial field.
Glacier-borne erratic
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.
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.
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 3,000 kilometres (1,900 mi) distant from the point at which the ice floes originally broke free. 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.
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