A
glacial erratic is a piece of
rockIn geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word
errare, and are carried by
glacial iceA glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (15000 metric tons (16,534.7 ST)) in
AlbertaAlberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...
.
Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics are significant because:
- Since they are transported by glaciers, they are 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. This allows quantification of the extent of glacial flooding resulting from ice dam failure which release the waters stored in proglacial lake
In geology, a proglacial lake is a lake formed either by the damming action of a moraine or ice dam during the retreat of a melting glacier, or by meltwater trapped against an ice sheet due to isostatic depression of the crust around the ice...
s such as Lake Missoula. Erratics released by ice-rafts that were stranded and subsequently melt, dropping their load, allow characterization of the high water marks for transient floods in areas like temporary Lake LewisLake Lewis was a temporary lake in the Pacific Northwest region of North America, largely formed by the Missoula Floods in about the 14th millennium B.C....
.
- 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. These can be correlated with ocean temperatures and levels to better understand and calibrate models of the global climate
A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...
.
Formation of erratics
The term "erratic" is commonly used to refer to erratic blocks, which 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 erosionErosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
resulting from the movement of ice.
GlacierA glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s erode by multiple processes: abrasion/scouring,
pluckingGlacial plucking exploits pre-existing fractures in the bedrock. This plays a key role in opening and creating new fractures but has only provided small segments of loose material. This is then followed by the entrainment of the loosened rock by the ice. During the process of entrainment, loose...
, ice thrusting and
glacially-induced spallingAn Ice Lens or ice lenses are formed when moisture, diffused within soil or rock, accumulates in a localized zone. The ice initially accumulates within small collocated pores or pre-existing crack, and, as long as the conditions remain favorable, continues to collect in the ice layer or ice lens,...
. Glaciers crack pieces of bedrock off in the process of plucking, producing the larger erratics. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. Glacially-induced spalling occurs when
ice lensAn Ice Lens or ice lenses are formed when moisture, diffused within soil or rock, accumulates in a localized zone. The ice initially accumulates within small collocated pores or pre-existing crack, and, as long as the conditions remain favorable, continues to collect in the ice layer or ice lens,...
formation with the rocks below the glacier spall off layers of rock, providing smaller debris which is ground into the glacial basal material to become till.
Evidence supports another option for 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
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...
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
A terminal moraine, also called end moraine, is a moraine that forms at the end of the glacier called the snout.Terminal moraines mark the maximum advance of the glacier. An end moraine is at the present boundary of the glacier....
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
PleistoceneThe Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
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 & 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 3000 kilometres (1,864.1 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
MontanaMontana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
) and temporary lakes (e.g.,
Lake LewisLake Lewis was a temporary lake in the Pacific Northwest region of North America, largely formed by the Missoula Floods in about the 14th millennium B.C....
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.
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 metres (4.9 ft) diameter boulder can be carried by a 3 metres (9.8 ft) high iceberg and could be found stranded at higher elevations than a 2 metres (6.6 ft) boulder which requires a 4 metres (13.1 ft) high iceberg.
Large erratics
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
allochthonthumb|right|250px|Schematic overview of a thrust system. The hanging wall block is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a [[window |window]]...
us, 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 kilometres (247.1 acre) in area and 30 metres (98.4 ft) in thickness can be found on the Canadian prairies, Poland, England, Denmark and Sweden. One erratic megablock located in
SaskatchewanSaskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....
is 30 by 38 km (18.6 by 23.6 mi) (and up to 100 metres (328.1 ft) 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 (186.4 mi) 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
kelpKelps are large seaweeds belonging to the brown algae in the order Laminariales. There are about 30 different genera....
holdfasts, which have been documented to transport rocks up to 40 centimetres (15.7 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.
History
Erratics were once considered evidence of a vast flood approximately 10,000 years ago, similar to the legendary floods described in the texts of ancient civilizations throughout the world. Ancient
legendA legend is a narrative of human actions that are perceived both by teller and listeners to take place within human history and to possess certain qualities that give the tale verisimilitude...
s of an epic flood come from many cultures including Mesoamerican,
SumerSumer was a civilization and historical region in southern Mesopotamia, modern Iraq during the Chalcolithic and Early Bronze Age....
ian (
Epic of GilgameshEpic of Gilgamesh is an epic poem from Mesopotamia and is among the earliest known works of literature. Scholars believe that it originated as a series of Sumerian legends and poems about the protagonist of the story, Gilgamesh king of Uruk, which were fashioned into a longer Akkadian epic much...
),
HebrewHebrews is an ethnonym used in the Hebrew Bible...
(
Old TestamentThe Old Testament, of which Christians hold different views, is a Christian term for the religious writings of ancient Israel held sacred and inspired by Christians which overlaps with the 24-book canon of the Masoretic Text of Judaism...
) and
IndianThe Indus Valley Civilization was a Bronze Age civilization that was located in the northwestern region of the Indian subcontinent, consisting of what is now mainly modern-day Pakistan and northwest India...
culture. In the 19th century, many scientists came to favor erratics as evidence for the end of the
last glacial maximumThe Last Glacial Maximum refers to a period in the Earth's climate history when ice sheets were at their maximum extension, between 26,500 and 19,000–20,000 years ago, marking the peak of the last glacial period. During this time, vast ice sheets covered much of North America, northern Europe and...
(
ice ageAn ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
) 10,000 years ago, rather than a flood.
GeologistA geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
s have suggested that
landslideA landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...
s 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.
Charles DarwinCharles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
published extensively on geologic phenomena including the distribution of erratic boulders. In his accounts written during the voyage of , Darwin observed a number of large erratic boulders of notable size south of the
Strait of MagellanThe Strait of Magellan comprises a navigable sea route immediately south of mainland South America and north of Tierra del Fuego...
,
Tierra del FuegoTierra del Fuego is an archipelago off the southernmost tip of the South American mainland, across the Strait of Magellan. The archipelago consists of a main island Isla Grande de Tierra del Fuego divided between Chile and Argentina with an area of , and a group of smaller islands including Cape...
and attributed them to ice rafting from
Antarctica. Recent research suggests that they are more likely the result of glacial ice flows carrying the boulders to their current locations.
Glacier-borne erratics
The largest known Glacial erratic is Big Rock near
Okotoks, AlbertaOkotoks is a town situated on the Sheep River, south of Calgary, Alberta, Canada. The town is a member of the Calgary Regional Partnership, a cooperative of communities within the Calgary Region. Okotoks has become a popular bedroom community for the City of Calgary...
, Canada. Other examples of glacial erratics include:
- Bleasdell Boulder, southern Ontario was described "glacial" in origin by Reverend William Bleasdell in 1872
- A fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...
-covered erratic the size of a garage is found on Cougar MountainCougar Mountain Regional Wildland Park is a regional park in King County, Washington, near the towns of Bellevue and Issaquah. The park was established in June 1983 to protect the central core of Cougar Mountain...
near Seattle.
- White Rock, British Columbia
White Rock is a city in British Columbia, Canada, that lies within the Metro Vancouver regional district. It borders Semiahmoo Bay and is surrounded on three sides by the City of Surrey, British Columbia. To the south lies the Semiahmoo First Nation, which is within the city limits of Surrey...
gets its name from a coastal erratic the size of a garage found on the beach at Semiahmoo BaySemiahmoo Bay is the southeastern section of Boundary Bay on the Pacific coast of North America. The name "Semiahmoo" is a Coast Salish word for "half moon".From the north to south, the following communities and places are located on its shore:...
, right at the borderBorders define geographic boundaries of political entities or legal jurisdictions, such as governments, sovereign states, federated states and other subnational entities. Some borders—such as a state's internal administrative borders, or inter-state borders within the Schengen Area—are open and...
with Washington.
- The Foothills Erratics Train
The Foothills Erratics Train is a landscape feature of Alberta which is composed of thousands of boulders in a 644 kilometre chain ranging in size from pebbles to immense boulders like Big Rock. These boulders are composed of pink and purple quartzites that are not native to this region of Alberta...
is a deposit of rocks of many sizes. These deposits stretch in a narrow belt for about 600 kilometres (372.8 mi) from Alberta's Athabasca RiverThe Athabasca River originates from the Columbia Glacier of the Columbia Icefield in Jasper National Park in Alberta, Canada...
Valley to the southwest of the province.
- Doane Rock
Doane Rock is a glacial erratic located in Eastham, Massachusetts on the grounds of the Cape Cod National Seashore. It is the largest exposed boulder on Cape Cod. It stands 18 feet high and extends below the ground an estimated 12 feet ....
, the largest exposed boulder in Cape CodCape Cod, often referred to locally as simply the Cape, is a cape in the easternmost portion of the state of Massachusetts, in the Northeastern United States...
.
- Madison Boulder, a 5000 short tons (4,535.9 MT) glacial erratic the size of a large house in Madison, New Hampshire
Madison is a town in Carroll County, New Hampshire, United States. The population was 2,502 at the 2010 census. Madison includes the village of Silver Lake.- History :...
.
- The northern portion of the town of Waterville, Washington
Waterville is a town in and the county seat of Douglas County, Washington, United States. It is part of the Wenatchee–East Wenatchee Metropolitan Statistical Area. The population was 1,138 at the 2010 census.-History:...
has a large number of large basalt erratics, particularly along the moraine running east-west from McNeil Canyon.
- The Merton Stone, Merton
Merton is a civil parish in the English county of Norfolk.It covers an area of and had a population of 113 in 50 households as of the 2001 census. For the purposes of local government, it falls within the district of Breckland...
, NorfolkNorfolk is a low-lying county in the East of England. It has borders with Lincolnshire to the west, Cambridgeshire to the west and southwest and Suffolk to the south. Its northern and eastern boundaries are the North Sea coast and to the north-west the county is bordered by The Wash. The county...
, EnglandEngland is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, the largest in the UK.
- Rollstone Boulder, a 110 ton porphyritic granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
boulder that was originally located at the summit of Rollstone Hill in Fitchburg, MassachusettsFitchburg is the third largest city in Worcester County, Massachusetts, United States. The population was 40,318 at the 2010 census. Fitchburg is home to Fitchburg State University as well as 17 public and private elementary and high schools.- History :...
. It was carried by the last glaciation from Mount MonadnockMount Monadnock, or Grand Monadnock, is the most prominent New England mountain peak south of the White Mountains and east of the Massachusetts Berkshires, and is the highest point in Cheshire County, New Hampshire...
in New HampshireNew Hampshire is a state in the New England region of the northeastern United States of America. The state was named after the southern English county of Hampshire. It is bordered by Massachusetts to the south, Vermont to the west, Maine and the Atlantic Ocean to the east, and the Canadian...
. Threatened by quarrying operations, it was moved to Litchfield Park in downtown Fitchburg in 1929-1930.
- Ehalkivi (Sunset Glow Boulder) near Letipea
Letipea is a village in Viru-Nigula Parish, Lääne-Viru County, in northeastern Estonia, on the coast of Gulf of Finland. It's located on the Letipea cape...
in EstoniaEstonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...
, 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.
Flood-borne erratics
In the event that glacial ice is "rafted" by a flood such as that created when the ice dam broke during the
Missoula FloodsThe Missoula Floods refer to the cataclysmic floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age. The glacial flood events have been researched since the 1920s...
, 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 SiteErratic Rock State Natural Site is a state park in the Willamette Valley of Oregon, United States. Featuring a glacial erratic from the Missoula floods, the small park sits atop a foothill of the Northern Oregon Coast Range in Yamhill County between Sheridan and McMinnville off Oregon Route 18...
just outside
McMinnville, OregonMcMinnville is the county seat and largest city of Yamhill County, Oregon, United States. According to Oregon Geographic Names, it was named by its founder, William T. Newby , an early immigrant on the Oregon Trail, for his hometown of McMinnville, Tennessee...
. The park includes a 40 short tons (36.3 MT) specimen, the largest erratic found in the
Willamette ValleyThe Willamette Valley is the most populated region in the state of Oregon of the United States. Located in the state's northwest, the region is surrounded by tall mountain ranges to the east, west and south and the valley's floor is broad, flat and fertile because of Ice Age conditions...
.
See also
- Wedgwood Rock
Wedgwood Rock is a glacial erratic near the neighborhood of Wedgwood in Seattle, Washington. Prior to the establishment of the Wedgwood neighborhood, the erratic was known first as Lone Rock and later simply as Big Rock...
- Precarious boulder
- Grannie stone
The Grannie or Granny stone is either the only surviving part of a stone circle or a simple glacial erratic. It lies in the River Irvine below the Rivergate Centre in Irvine, North Ayrshire, Scotland. The Grannie Stane is clearly visible when the water is low...
External links