TheLast Glacial Period (LGP), also known as thelast glacial cycle, occurred from the end of theLast Interglacial to the beginning of theHolocene,c. 115,000 – c. 11,700 years ago, and thus corresponds to most of the timespan of theLate Pleistocene.[1] It thus formed the most recent period of what is colloquially known as the "Ice Age".
The LGP is part of a larger sequence of glacial andinterglacial periods known as theQuaternary glaciation which started around 2,588,000 years ago and is ongoing.[2] The glaciation and the currentQuaternary Period both began with the formation of theArctic ice cap. TheAntarctic ice sheet began to form earlier, at about 34 Mya (million years ago), in the mid-Cenozoic (Eocene–Oligocene extinction event), and the termLate Cenozoic Ice Age is used to include this early phase with the current glaciation.[3] The previous ice age within the Quaternary is thePenultimate Glacial Period, which ended about 128,000 years ago, was more severe than the Last Glacial Period in some areas such as Britain, but less severe in others.
The last glacial period saw alternating episodes of glacier advance and retreat with theLast Glacial Maximum occurring between 26,000 and 20,000 years ago. While the general pattern of cooling and glacier advance around the globe was similar, local differences make it difficult to compare the details from continent to continent (see picture of ice core data below for differences). The most recent cooling, theYounger Dryas, began around 12,800 years ago and ended around 11,700 years ago, also marking the end of the LGP and thePleistocene epoch. It was followed by theHolocene, the currentgeologicalepoch.
The LGP is often colloquially referred to as the "last ice age", though the termice age is not strictly defined, and on a longer geological perspective, the last few million years could be termed a single ice age given the continual presence ofice sheets near both poles. Glacials are somewhat better defined, as colder phases during which glaciers advance, separated by relatively warminterglacials. The end of the last glacial period, which was about 10,000 years ago, is often called the end of the ice age, although extensive year-round ice persists in Antarctica andGreenland. Over the past few million years, the glacial-interglacial cycles have been "paced" by periodic variations in the Earth's orbit viaMilankovitch cycles.
The LGP has been intensively studied in North America, northern Eurasia, theHimalayas, and other formerly glaciated regions around the world. The glaciations that occurred during this glacial period covered many areas, mainly in the Northern Hemisphere and to a lesser extent in the Southern Hemisphere. They have different names, historically developed and depending on their geographic distributions:Fraser (in thePacific Cordillera of North America),Pinedale (in theCentral Rocky Mountains),Wisconsinan orWisconsin (in central North America),Devensian (in the British Isles),[5]Midlandian (in Ireland),Würm (in theAlps),Mérida (inVenezuela),Weichselian orVistulian (in Northern Europe and northern Central Europe),Valdai in Russia andZyryanka inSiberia,Llanquihue inChile, andOtira in New Zealand. The geochronologicalLate Pleistocene includes the late glacial (Weichselian) and the immediately precedingpenultimate interglacial (Eemian) period.
Canada was almost completely covered by ice, as was the northern part of theUnited States, both blanketed by the hugeLaurentide Ice Sheet.Alaska remained mostly ice free due to arid climate conditions. Local glaciations existed in theRocky Mountains and theCordilleran ice sheet and asice fields andice caps in theSierra Nevada in northernCalifornia.[6] In northern Eurasia, theScandinavian ice sheet once again reached the northern parts of theBritish Isles,Germany,Poland, and Russia, extending as far east as theTaymyr Peninsula in western Siberia.[7]
The maximum extent of western Siberian glaciation was reached by about 18,000 to 17,000 BP, later than in Europe (22,000–18,000 BP).[8] Northeastern Siberia was not covered by a continental-scale ice sheet.[9] Instead, large, but restricted, icefield complexes covered mountain ranges within northeast Siberia, including the Kamchatka-Koryak Mountains.[10][11]
The Arctic Ocean between the huge ice sheets of America and Eurasia was not frozen throughout, but like today, probably was covered only by relatively shallow ice, subject to seasonal changes and riddled withicebergscalving from the surrounding ice sheets. According to the sediment composition retrieved from deep-seacores, even times of seasonally open waters must have occurred.[12]
Outside the main ice sheets, widespread glaciation occurred on the highest mountains of theAlpide belt. In contrast to the earlier glacial stages, the Würm glaciation was composed of smaller ice caps and mostly confined to valley glaciers, sending glacial lobes into the Alpineforeland. Local ice fields or small ice sheets could be found capping the highest massifs of thePyrenees, theCarpathian Mountains, theBalkan Mountains, theCaucasus, and the mountains ofTurkey andIran.[13]
In the Himalayas and theTibetan Plateau, there is evidence that glaciers advanced considerably, particularly between 47,000 and 27,000 BP,[14] but the exact ages,[15][16] as well as the formation of a single contiguous ice sheet on the Tibetan Plateau, is controversial.[17][18][19]
Other areas of the Northern Hemisphere did not bear extensive ice sheets, but local glaciers were widespread at high altitudes. Parts ofTaiwan, for example, were repeatedly glaciated between 44,250 and 10,680 BP[21] as well as theJapanese Alps. In both areas, maximum glacier advance occurred between 60,000 and 30,000 BP.[22] To a still lesser extent, glaciers existed in Africa, for example in theHigh Atlas, the mountains ofMorocco, theMount Atakor massif in southernAlgeria, and several mountains inEthiopia. Just south of the equator, an ice cap of several hundred square kilometers was present on the east African mountains in theKilimanjaro massif,Mount Kenya, and theRwenzori Mountains, which still bear relic glaciers today.[23]
Glaciation of the Southern Hemisphere was less extensive. Ice sheets existed in the Andes (Patagonian Ice Sheet), where six glacier advances between 33,500 and 13,900 BP in the Chilean Andes have been reported.[24] Antarctica was entirely glaciated, much like today, but unlike today the ice sheet left no uncovered area. In mainland Australia only a very small area in the vicinity ofMount Kosciuszko was glaciated, whereas inTasmania glaciation was more widespread.[25] An ice sheet formed in New Zealand, covering all of the Southern Alps, where at least three glacial advances can be distinguished.[26]
Local ice caps existed in the highest mountains of the island ofNew Guinea, where temperatures were 5 to 6 °C colder than at present.[27][28] The main areas of Papua New Guinea where glaciers developed during the LGP werethe Central Cordillera, theOwen Stanley Range, and theSaruwaged Range.Mount Giluwe in the Central Cordillera had a "more or less continuous ice cap covering about 188 km2 and extending down to 3200-3500 m".[27] InWestern New Guinea, remnants of these glaciers are still preserved atopPuncak Jaya andNgga Pilimsit.[28]
Small glaciers developed in a few favorable places inSouthern Africa during the last glacial period.[29][A][B] These small glaciers would have been located in theLesotho Highlands and parts of theDrakensberg.[31][32] The development of glaciers was likely aided in part due to shade provided by adjacent cliffs.[32] Variousmoraines and former glacial niches have been identified in the eastern Lesotho Highlands a few kilometres west of theGreat Escarpment, at altitudes greater than 3,000 m on south-facing slopes.[31] Studies suggest that the annual average temperature in the mountains of Southern Africa was about 6 °C colder than at present, in line with temperature drops estimated forTasmania and southernPatagonia during the same time. This resulted in an environment of relatively arid periglaciation withoutpermafrost, but with deep seasonal freezing on south-facing slopes. Periglaciation in the eastern Drakensberg and Lesotho Highlands producedsolifluction deposits andblockfields; including blockstreams and stone garlands.[29][30]
Scientists from the Center for Arctic Gas Hydrate, Environment and Climate at theUniversity of Tromsø, published a study in June 2017[33]describing over a hundredocean sediment craters, some 3,000 m wide and up to 300 m deep, formed by explosive eruptions ofmethane from destabilizedmethane hydrates, following ice-sheet retreat during the LGP, around 12,000 years ago. These areas around theBarents Sea still seep methane today. The study hypothesized that existing bulges containingmethane reservoirs could eventually have the same fate.
During the last glacial period, Antarctica was blanketed by a massive ice sheet, much as it is today. The ice covered all land areas and extended into the ocean onto the middle and outer continental shelf.[34][35] Counterintuitively though, according to ice modeling done in 2002, ice over centralEast Antarctica was generally thinner than it is today.[36]
Britishgeologists refer to the LGP as theDevensian. Irish geologists, geographers, and archaeologists refer to theMidlandian glaciation, as its effects in Ireland are largely visible in theIrish Midlands. The name Devensian is derived from theLatinDēvenses, people living by theDee (Dēva in Latin), a river on the Welsh border near which deposits from the period are particularly well represented.[37]
The effects of this glaciation can be seen in many geological features of England, Wales, Scotland, andNorthern Ireland. Its deposits have been found overlying material from the precedingIpswichian stage and lying beneath those from the followingHolocene, which is the current stage. This is sometimes called theFlandrian interglacial in Britain.
The latter part of the Devensian includespollen zones I–IV, theAllerød oscillation andBølling oscillation, and theOldest Dryas,Older Dryas, andYounger Dryas cold periods.
Alternative names includeWeichsel glaciation orVistulian glaciation (referring to the Polish RiverVistula or its German name Weichsel). Evidence suggests that the ice sheets were at theirmaximum size for only a short period, between 25,000 and 13,000 BP. Eightinterstadials have been recognized in the Weichselian, including the Oerel, Glinde, Moershoofd, Hengelo, and Denekamp. Correlation withisotope stages is still in process.[38][39] During the glacial maximum in Scandinavia, only the western parts ofJutland were ice-free, and a large part of what is today theNorth Sea was dry land connecting Jutland with Britain (seeDoggerland).
TheBaltic Sea, with its uniquebrackish water, is a result of meltwater from the Weichsel glaciation combining with saltwater from the North Sea when the straits between Sweden and Denmark opened. Initially, when the ice began melting about 10,300 BP, seawater filled theisostatically depressed area, a temporarymarine incursion that geologists dub theYoldia Sea. Then, aspostglacial isostatic rebound lifted the region about 9500 BP, the deepest basin of the Baltic became a freshwater lake, in palaeological contexts referred to asAncylus Lake, which is identifiable in the freshwater fauna found in sediment cores.
The lake was filled by glacial runoff, but as worldwide sea level continued rising, saltwater again breached the sill about 8000 BP, forming a marineLittorina Sea, which was followed by another freshwater phase before the present brackish marine system was established. "At its present state of development, the marine life of the Baltic Sea is less than about 4000 years old", Drs. Thulin and Andrushaitis remarked when reviewing these sequences in 2003.
Overlying ice had exerted pressure on the Earth's surface. As a result of melting ice, the land has continued to rise yearly in Scandinavia, mostly in northern Sweden and Finland, where the land is rising at a rate of as much as 8–9 mm per year, or 1 m in 100 years. This is important for archaeologists, since a site that was coastal in theNordic Stone Age now is inland and can be dated by its relative distance from the present shore.
The termWürm is derived fromWürm, a river in the Alpine foreland, roughly marking the maximum glacier advance of this particular glacial period. The Alps were where the first systematic scientific research on ice ages was conducted byLouis Agassiz at the beginning of the 19th century. Here, the Würm glaciation of the LGP was intensively studied.Pollen analysis, the statistical analyses ofmicrofossilized plant pollens found in geological deposits, chronicled the dramatic changes in the European environment during the Würm glaciation. During the height of Würm glaciation,c. 24,000 – c. 10,000 BP, most of western and central Europe and Eurasia was open steppe-tundra, while the Alps presented solid ice fields and montane glaciers. Scandinavia and much of Britain were under ice.
During the Würm, theRhône Glacier covered the whole western Swiss plateau, reaching today's regions of Solothurn and Aargau. In the region of Bern, it merged with the Aar glacier. TheRhine Glacier is currently the subject of the most detailed studies. Glaciers of the Reuss and the Limmat advanced sometimes as far as the Jura. Montane and piedmont glaciers formed the land by grinding away virtually all traces of the older Günz and Mindel glaciation, by depositing base moraines and terminal moraines of different retraction phases andloess deposits, and by the proglacial rivers' shifting and redepositing gravels. Beneath the surface, they had profound and lasting influence ongeothermal heat and the patterns of deep groundwater flow.
ThePinedale (central Rocky Mountains) andFraser (Cordilleran ice sheet) glaciation was the last of the major glaciations to appear in theRocky Mountains in the United States. The Pinedale lasted from around 30,000 to 10,000 years ago, and was at its greatest extent between 23,500 and 21,000 years ago.[40] This glaciation was somewhat distinct from the main Wisconsin glaciation, as it was only loosely related to the giant continental ice sheets and was instead composed of mountain glaciers, merging together to comprise the Cordilleran ice sheet.[41]
The Cordilleran ice sheet produced features such asglacial Lake Missoula, which broke free from its ice dam, causing the massiveMissoula Floods.USGS geologists estimate that the cycle of flooding and reformation of the lake lasted an average of 55 years and that the floods occurred about 40 times over the 2,000-year period starting 15,000 years ago.[42]Glacial lake outburst floods such as these are not uncommon today inIceland and other places.
TheWisconsin glacial episode was the last major advance ofcontinental glaciers in the North American Laurentide ice sheet. At the height of glaciation, theBering land bridge potentially permitted migration of mammals, including people, to North America fromSiberia.
It radically altered the geography of North America north of theOhio River. At the height of the Wisconsin episode glaciation, ice covered most of Canada, theUpper Midwest, andNew England, as well as parts ofMontana andWashington. OnKelleys Island inLake Erie or in New York'sCentral Park, thegrooves left by these glaciers can be easily observed. In southwestern Saskatchewan and southeastern Alberta, asuture zone between the Laurentide and Cordilleran ice sheets formed theCypress Hills, which is the northernmost point in North America that remained south of the continental ice sheets.
TheGreat Lakes are the result of glacial scour and pooling of meltwater at the rim of the receding ice. When the enormous mass of the continental ice sheet retreated, the Great Lakes began gradually moving south due to isostatic rebound of the north shore.Niagara Falls is also a product of the glaciation, as is the course of the Ohio River, which largely supplanted the priorTeays River.
With the assistance of several very broad glacial lakes, it released floods through thegorge of theUpper Mississippi River, which in turn was formed during an earlier glacial period.
In its retreat, the Wisconsin episode glaciation leftterminal moraines that formLong Island,Block Island,Cape Cod,Nomans Land,Martha's Vineyard,Nantucket,Sable Island, and theOak Ridges Moraine in south-central Ontario, Canada. In Wisconsin itself, it left theKettle Moraine. Thedrumlins andeskers formed at its melting edge are landmarks of the lowerConnecticut River Valley.
In theSierra Nevada, three stages of glacial maxima, sometimes incorrectly calledice ages, were separated by warmer periods. These glacial maxima are called, from oldest to youngest, Tahoe, Tenaya, and Tioga.[43] The Tahoe reached its maximum extent perhaps about 70,000 years ago. Little is known about the Tenaya. The Tioga was the least severe and last of the Wisconsin episode. It began about 30,000 years ago, reached its greatest advance 21,000 years ago, and ended about 10,000 years ago.[citation needed]
In northwest Greenland, ice coverage attained a very early maximum in the LGP around 114,000. After this early maximum, ice coverage was similar to today until the end of the last glacial period. Towards the end, glaciers advanced once more before retreating to their present extent.[44] According to ice core data, the Greenland climate was dry during the LGP, with precipitation reaching perhaps only 20% of today's value.[45]
The nameMérida glaciation is proposed to designate the alpine glaciation that affected the centralVenezuelan Andes during the Late Pleistocene. Two main moraine levels have been recognized - one with an elevation of 2,600–2,700 m (8,500–8,900 ft), and another with an elevation of 3,000–3,500 m (9,800–11,500 ft). The snow line during the last glacial advance was lowered approximately 1,200 m (3,900 ft) below the present snow line, which is 3,700 m (12,100 ft). The glaciated area in theCordillera de Mérida was about 600 km2 (230 sq mi); this included these high areas, from southwest to northeast: Páramo de Tamá, Páramo Batallón, Páramo Los Conejos, Páramo Piedras Blancas, and Teta de Niquitao. Around 200 km2 (77 sq mi) of the total glaciated area was in theSierra Nevada de Mérida, and of that amount, the largest concentration, 50 km2 (19 sq mi), was in the areas ofPico Bolívar,Pico Humboldt [4,942 m (16,214 ft)], andPico Bonpland [4,983 m (16,348 ft)]. Radiocarbon dating indicates that the moraines are older than 10,000 BP, and probably older than 13,000 BP. The lower moraine level probably corresponds to the main Wisconsin glacial advance. The upper level probably represents the last glacial advance (Late Wisconsin).[46][47][48][49][50]
The Llanquihue glaciation takes its name fromLlanquihue Lake insouthern Chile, which is a fan-shapedpiedmont glacial lake. On the lake's western shores, large moraine systems occur, of which the innermost belong to the LGP. Llanquihue Lake'svarves are a node point in southern Chile's varvegeochronology. During the last glacial maximum, the Patagonian ice sheet extended over the Andes from about 35°S toTierra del Fuego at 55°S. The western part appears to have been very active, with wet basal conditions, while the eastern part was cold-based.[51]
Cryogenic features such asice wedges,patterned ground,pingos,rock glaciers,palsas, soilcryoturbation, andsolifluction deposits developed in unglaciated extra-Andean Patagonia during the last glaciation, but not all these reported features have been verified.[51] The area west of Llanquihue Lake was ice-free during the last glacial maximum, and had sparsely distributed vegetation dominated byNothofagus.Valdivian temperate rain forest was reduced to scattered remnants on the western side of the Andes.[52]
| Region | Glacial 1 | Glacial 2 | Glacial 3 | Glacial 4 |
|---|---|---|---|---|
| Alps | Günz | Mindel | Riss | Würm |
| North Europe | Eburonian | Elsterian | Saalian | Weichselian |
| British Isles | Beestonian | Anglian | Wolstonian | Devensian |
| Midwest U.S. | Nebraskan | Kansan | Illinoian | Wisconsinan |
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