glaciation

Glacial erosion occurs in two principal ways: through the abrasion of surface materials as the ice grinds over the ground (much of the abrasive action being attributable to the debris embedded in the ice along its base); and by the quarrying or plucking of rock…

Glaciers are accumulations of snow transformed into solid ice. Important questions of glacial geology concern the climatic controls that influence the occurrence of glaciers, the processes by which snow is transformed into ice, and the mechanism of the flow of ice within glaciers. Other important…

At the height of the glacial epoch, especially about 20,000 years ago, most of the continental shelf in the Arctic Ocean was exposed to polar climates for thousands of years. These climates caused cold permafrost to form to depths of more than 700 metres (about 2,300 feet). Subsequently, within the…

glacier-transported rock fragment that differs from the local bedrock. Erratics may be embedded in till or occur on the ground surface and may range in size from pebbles to huge boulders weighing thousands of tons. The distance of transportation may range from less than 1…

…of the world that were glaciated during the Pleistocene Epoch (2,580,000 to 11,700 years ago). They cover vast areas in the tundra and boreal forest regions of Canada, northern Europe, and Russia. Areas of high rainfall farther to the south, such as the wetter parts of the British Isles, also…

Studies of glaciations during the Pleistocene Epoch (2.6 million to 11,700 years ago) indicate that drops in sea level caused by the removal of water from the oceans during glacial advances affected all coastal areas. During the last Pleistocene glacial period, the sea level is thought to…

…fluctuations that lead to major glacial advances and retreats—that is, ice ages and interglacial periods. Other causes that are not as well understood may include major mountain-building events and isostatic changes in crustal plates. When continental glaciers advance, as they did several times during the Pleistocene Epoch (which extended from…

…is compatible only with a glacial origin. It is assumed that the enormous, thick glaciers that formed in these valleys were so heavy that they could erode the bottom of the valley far below sea level before they floated in the ocean water. After the glaciers melted, the waters of…

…mass of perennial ice that originates on land by the recrystallization of snow or other forms of solid precipitation and that shows evidence of past or present flow.

The basin-forming mechanism responsible for the most abundant production of lakes, particularly in the Northern Hemisphere, is glaciation. The Pleistocene glaciers, which seem to have affected every continent, were especially effective in North America, Europe, and Asia. The retreat of ice sheets produced basins…

Temperate glaciers, however, with temperatures beneath the immediate subsurface constantly near the freezing (or the melting) point, can, like their associated snowfields, release large quantities of water during a given warm season. Their losses through evaporation are small.

The relationship between glaciation and depositional terraces constitutes the cornerstone of reconstructing geomorphic history in valleys that have been glaciated. The balance between load and discharge that ultimately determines whether a river will deposit or erode is severely altered during glacial episodes. An enormous volume of coarse-grained bed…

When glaciation was at its maximum, sea level was significantly lower than it is today because much of the precipitation falling on the continents was locked up in massive ice bodies rather than returning to the ocean. In response, rivers entrenched their downstream reaches as baselevel…

Glacial deposits, especially till sheets, can conceal the preglacial topography and provide initial slope systems for postglacial streams. Individual diversions occur at and near ice fronts, also where preglacial divides in mountain country are breached by the ice of caps or impounded mountain glaciers. The…

…waterfalls have been created by glaciation where valleys have been over-deepened by ice and tributary valleys have been left high up on steep valley sides. In the glacially gouged Yosemite Valley in California, the Yosemite Upper Falls tumble 436 m (1,430 feet) from such a hanging valley.

The processes of glaciation have served this same end. Mountain ranges that formerly were glaciated contain falls at the outlets of cirques, bowl-shaped depressions in the headwaters of drainage areas that were formed by the accumulation of ice and its erosive action on the underlying bedrock. In addition,…

…record reflects the continued Gondwanan glaciations, which produced the extensive coal cyclothems (repeated sequences of distinctive sedimentary rock layers) throughout the Northern Hemisphere. These cyclothems produced the most extensive coal deposits in the entire geologic record. The Pennsylvanian concludes with the Ouachita-Alleghenian

…allowed the formation of continental glaciers. These glaciations were similar to those occurring in the Northern Hemisphere during the Pleistocene Epoch. Coeval (parallel) continental glaciations did not occur in the high latitudes of the Northern Hemisphere, probably because the landmasses were too small to sustain large ice fields.

…Cenozoic was the development of glaciation on the Antarctic continent about 35 million years ago and in the Northern Hemisphere between 3 million and 2.5 million years ago. Glaciation left an extensive geologic record on the continents in the form of predominantly unconsolidated tills and glacial moraines, which in North…

…initiating the process of continental-scale glaciation in Antarctica. In addition, the cooler oceans of the early Oligocene may have been more productive than oceans of the late Eocene.

Furthermore, glacial deposits dating back to the Ordovician are also known from much of Africa, southern Asia, and parts of Europe. It had been thought the South Pole was glaciated for much of the Ordovician, but more-recent dating of these glacial deposits, plus isotopic evidence, suggests…

…because of the onset of glaciation over the African and South American portions of Gondwana. In many areas the interval of glaciation was accompanied by the invasion of cool-water brachiopod fauna, even into tropical latitudes, suggesting the onset of significant global cooling. A third phase of extinction occurred with the…

Extensive glaciation persisted from the Carboniferous Period into the initial stage of the Early Permian Epoch over vast areas of present-day southern India, Australia, Antarctica, and northeastern Siberia. Middle Permian climates generally were warmer and moist. Climates of the Late Permian (Lopingian) Epoch were typically hot…

…extensive ice sheets and other glaciers formed repeatedly on the landmasses and has been informally referred to as the “Great Ice Age.” The timing of the onset of this cold interval, and thus the formal beginning of the Pleistocene Epoch, was a matter of substantial debate among geologists during the…

…to build up to maximum glaciation, only to terminate abruptly and go from maximum glacial to full interglacial conditions in less than 10,000 years (see figure). A third problem is the synchronous nature of the climatic record between the Northern and Southern hemispheres, which one would not expect from the…

The presence of tillites (glacial sediments) indicates that extensive glaciations occurred several times during the Precambrian. Glacial deposits are not necessarily limited to high latitudes. In general, they are complementary to the carbonates, evaporites, and red beds that are climatically

The most distinctive changes seen during the Quaternary were the advances of ice into temperate latitudes of the Northern Hemisphere. The glacial landscapes were dominated by ice several kilometres thick that covered all but the highest peaks in the interior. Grounded ice extended onto…

Dramatic unconformities (interruptions in the deposition of sedimentary rock) between the Silurian and Ordovician systems indicate how extreme the glacially induced drawdown in late Ordovician sea level had been. The maximum global fall in sea level was on the order of 70 metres (about…

6 million years ago), glaciers flowed through these valleys, deepening and excavating the ground, and leaving moraines (deposits of waste material) when they shrank at the end of the glacial period. Water filled up the excavations or was dammed up by the moraines.

…during the Quaternary by Alpine glaciation and by expanding ice tongues, some reaching depths of nearly 1 mile (1.6 kilometres), that filled in the valleys and overflowed onto the plains. Amphitheatre-like cirques, arête ridges, and majestic peaks such as the Matterhorn and Grossglockner were shaped from the mountaintops; the valleys…

Antarctica provides the best available picture of the probable appearance 20,000 years ago of northern North America under the great Laurentide Ice Sheet. Some scientists contend that the initial glacier that thickened over time to become the vast East Antarctic Ice Sheet originated in…

…Appalachians were also affected by glacial forces. During the Pleistocene Epoch (about 2,588,000 to 11,700 years ago), continental ice sheets flowed down over North America, covering New England but reaching no nearer the southern Appalachians than the Ohio River valley. These moving tongues of ice stripped topsoil, ground and polished…

Although the Arctic is commonly thought to be largely ice-covered, less than two-fifths of its land surface in fact supports permanent ice. The remainder is ice-free because of either relatively warm temperatures or scant snowfall. Glaciers are formed when the annual accumulation of snow,…

The mantle of glaciation from the Pleistocene Epoch (i.e., about 2,600,000 to 11,700 years ago) embraced northwestern Asia only to latitude 60° N. East of the Khatanga River, which flows from Siberia into the Arctic Ocean, only isolated glaciation of the mantle debris and of the mountains occurred,…

Lastly, during the Pleistocene, small glaciers developed in the Mount Kosciuszko area of New South Wales and the central plateau of Tasmania. Small, ice-scoured hollows and small moraines (ridges of glacial debris) attest to those events, while over rather wider areas frost-shattered rocks that subsequently caused soils to flow down-slope…

…end of the Pleistocene Epoch glaciations. Some 14,000 years ago, ice covered all of northern Europe as far south as the present German-Polish coastline; by 7700 bc glacial meltwater had formed the Yoldia Sea, which stretched east from the present Skagerrak across what is now lake-strewn southern Sweden as far…

…years ago), the vast continental glaciers that covered northern North America had this region as a centre. The ice, in moving to the south, scraped the land bare of its overlying mantle of weathered rock. Some of this material was deposited on the shield when the ice melted, but the…

Early glaciation also affected both Mount Elgon and the Aberdare Range. More than 30 small glaciers on the Ruwenzori together cover a surface of approximately 1.5 square miles (4 square km), most of which is on Mounts Stanley and Speke; the lowest valley glacier descends to…

…withdrawal of water by the glaciers of the late Pleistocene Epoch (about 25,000 years ago), produced a sea level at least 300 feet lower than the present. Later the melting of the ice raised the sea level to its present mark, and the ecologically important land bridge across the Strait…

…is largely the product of glacial activity. The south is covered with moraine hills, and the central part of the country abounds in elongated hills with flat tops. Northern Estonia is characterized by long narrow swells consisting of deposits left by glacial rivers that formed during the melting of ice.…

…at a time of worldwide glaciation; they occur in northwestern Scotland (Islay Island), western Ireland, Norway (Finnmark and West Spitzbergen), Sweden, France (Normandy), and the Czech Republic (Bohemian Massif). Second is the occurrence of impressions of soft-bodied organisms, such as seaweed, jellyfish, and worms, which represent the beginnings of

Glaciers are the most powerful engines provided by nature for the transport—by plucking or quarrying—of large masses of rock, and certainly the European glaciers transformed the physique both of their source areas and of the lands to which they moved. Many physical forms of northern…

Valley glaciers emerging from the Alps and ice sheets from Scandinavia had some erosive effect, but they mainly contributed sheets of glacial deposits. Slopes outside the area of the actual ice sheets—those under tundra conditions and unprotected by vegetation—were rendered less steep by the periglacial slumping…

…the last (Weichsel, or Vistula) glaciation. The advancing ice sheets pushed up material that remains today as terminal moraines, stretching across the country in a generally southeast-to-northwest direction and rising to some 500 feet (150 meters) above the general level. Within the terminal moraines the decay of the ice sheets…

…experienced at least two general glaciations—one covering most of the country and the other extending as far south as a line linking Limerick, Cashel, and Dublin—and the characteristic diversity of Irish scenery owes much to this glacial influence. The large areas of peat bog to be found throughout the country…

The valleys were heavily scoured by glaciers in the Quaternary Period (the past 2.6 million years); there are still more than 1,000 glaciers left, though in a phase of retreat, more than 100 having disappeared in the past half century or so.

Glacial meltwater is a large part of the flow of larger rivers such as the Yukon and Tanana in Alaska and the Yukon territory. Glacial meltwater carries a heavy load of suspended sediment that deposits in riverbeds and causes frequent channel shifts. Glacial river floodplains…

As glaciation began much earlier in Antarctica (about 37 million years ago), it is suspected that a specific causal factor—presumably involving a change in ocean-atmosphere circulation—was involved in addition to the overall global cooling trend related to the emergence of greater continental landmass over the past…

…2,600,000 to 11,700 years ago), ice sheets advanced and retreated several times and deposited a thick layer of clay on the seafloor. At the time of the greatest advance, the ice covered all of the North Sea from a line joining the Thames estuary with the Dutch coast. The final…

Glaciation and other forces wore down the surface and created thick sandstone, conglomerate, and limestone deposits known as sparagmite. Numerous extensive areas called peneplains, whose relief has been largely eroded away, also were formed. Remains of these include the Hardanger Plateau—3,000 feet (900 metres) above…

The present Pyrenean glaciers, perhaps more frequent on the northern than on the southern slopes, have been reduced to high basins—cirques or hanging valleys—at elevations over 9,800 feet. During and after the great Ice Ages (i.e., within the past 2.5 million years), however, especially in the Central and…

…last Ice Age a large glacier, fed by several small ones, plowed down the valley floor of the Valais and, except for some harder rock obstacles found near the town of Sion, succeeded in widening and deepening the narrow valley floor. As it did so, it held back both the…

The gentler west-facing slope has been dissected by a series of streams, much longer than those of the eastern slope. Such rivers as the Yuba, American, Mokelumne, Stanislaus, Merced, and Kern originate in deep valleys carved largely by glaciers into the predominant granite and…

The glaciations that encompass most of the Pleistocene Epoch (i.e., about 2,600,000 to 11,700 years ago) began in southern South America as early as the late Miocene Epoch (i.e., about 9 million years ago), when ice caps first covered the Patagonian Andes. Maximum ice expansion was…

…after the last major Pleistocene glaciation (the Pleistocene Epoch occurred from about 2,600,000 to 11,700 years ago), which included the widespread adoption of farming and the establishment of permanent settlements from the 7th millennium bce. These laid the foundation for all future developments of European civilization.

…conditions of the last Pleistocene glaciation began to improve about 13,000 bce as temperatures slowly rose. The Scandinavian Ice Sheet itself started to retreat northward about 8300 bce, and the period between then and the origins of agriculture (at various times in the 7th to 4th millennia, depending on location)…

At that time glaciers began to absorb increasing amounts of water, causing global sea levels to fall by as much as 400 feet (120 meters). A complete connection between Asia and North America existed from about 28,000 to 10,000 bce, and, at its greatest extent, Beringia may have…