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. , 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. , 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 , with overground volume of (weight approximately ), in
Estonia '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==