Now on to the main snow and ice features that are associated with glaciers. In the last 2 million years, ice centers in high latitudes and in high mountains several times have undergone expansion and, particularly in the northern hemisphere, ice miles thick moved from the polar regions into Siberia, northern Europe, and in the U.S as far south as Cincinnati and into the northern Great Plains. This map summarizes the distribution of continental ice coverage in the northern hemisphere at the height of the Pleistocene age:
The last major advance of Pleistocene continental glaciers over North America is the so-called Wisconsin event, shown in this map:
In North America, there have been several (perhaps 6) significant advances and retreats (stages) during the Pleistocene. Generally, the last advance (Wisconsin) overrode glacial deposits and landforms of the earlier glaciations, wiping them out in some places. This excerpt from Wikipedia summarizes the current status of the nomenclature used to specify these stages: "The major glacial stages of the current ice age in North America are the Illinoian, Sangamonian and Wisconsin stages. The use of the Nebraskan, Afton, Kansan, and Yarmouthian (Yarmouth) stages to subdivide the ice age in North America have been discontinued by Quaternary geologists and geomorphologists. Those stages have all been merged into the Pre-Illinoian Stage in the 1980s."
Glacial landforms are widespread in the northern hemisphere, being found in lowlands of Europe, parts of Asia, and North America, all of which were visited by continental glaciers in the Pleistocene. Likewise, glaciers are found outside these continental regions wherever mountain conditions permit extended cold climates.
Glaciers appear wherever it is cold enough year round for periods of extensive snowfall. As the snow compacts, pressure aids in its melting and refreezing as ice. Almost all modern day glaciers appear either as extensive ice sheets in the cold high latitudes of both global hemispheres or more localized where mountains rise to heights that remain cold much of the year (glaciers occur near the top of Mount Kilimanjaro in Kenya near the Equator).
There are many specific landforms associated with both mountain and continental glaciation. Some are detectable in satellite imagery. Others are hard to recognize except in high resolution images or aerial photos. The next two illustrations depict most of the common landforms (some have French names, since Louis Agassiz and other pioneer glaciologists were frenchmen); they are taken from Geomorphology from Space:
Before we embark on a series of satellite images showing glaciers and glacial terrain, lets illustrate some of the above landforms depicted in the preceding diagrams as they are visible from the ground or air. Some are erosional while others are depositional; some occur mainly/only in mountain glaciers while others are more typical of continental glaciers.
This ground view is typical of a mountain glacier, with its medial and terminal moraines (see below), and the jagged topography of adjacent mountains, all brought about by glaciation:
Glaciers tend to have black streaks on their surfaces - these are medial moraine deposits formed as rock falls loose at the head of the glacier and continues this process over the years, so that the debris are strung out as streaks that move over the years. Here is an aerial oblique view that shows this morainal pattern in the Steele Glacier of Alaska:
In the upper reaches of mountain glaciers, the ice plucks away at the bedrock. At the head (top) of a glacier, an amphitheaterlike erosional depression called a cirque is produced.
Where multiple cirques coalesce, knife edgelike ridges called aretes are the result:
Glacial ice is a powerful agent of erosion. At the contact surface where the glacier meets the bedrock of the mountains it occupies, water from melt mixes with the rock and plucks away, grabbing the rock and incorporating it in the ice. Other rock gets on the surface by means of avalanches or gravitational weathering. Deposits of the incorporated rock make up "moraines", a major feature of glaciers - both mountain and continental. These deposits are usually dark (most rocks are nonwhite) and stand out in contrast to the white tones of the ice. Some rock is located at the glacier's base (basal moraine); some rock is found within the rock (englacial); some rock is built up along a glacier's sides (lateral); some rock is collected on the glacier's surface. Where glaciers meet, surface deposits coalesce to form medial moraines. As glaciers melt from the limit of their farthest advance, they produce terminal (alternately called end) moraines; as they retreat they leave behind morainal deposits that are spread out as ground or recessional moraines. Two general terms refer to unsorted, heterogenous glacial deposits: drift and till.
Next, consider these photos that focus on the deposits of unsorted loose rubble (with a wide range of sizes) that comprise moraines
Moraines can become vegetated as soils develop on them. Here is a ridge made up of a terminal moraine deposit.
What does a landscape once dominated by valley glaciers and ice sheets look like when the ice cover has melted? In mountainous terrain, a glaciated valley is both deepened and widened, giving rise to a characteristic U-shaped cross-section:
The sequence of stages that end in a U-shaped valley after the glacial ice has melted is shown in this diagram:
In the lowlands beyond a retreating mountain glacier, some of the landforms that develop appear in this oblique aerial photo (note: several also are characteristic of continental glaciation):
This photo shows how each landform is interrelated to one another through glaciation. (1) Glacier, (2) ice cored hummocky end moraine, (3) fluted till, (4) esker, (5) fan and (6) braided streams. (Martini, 2001; Canadian Landform Inventory Project)
One of the more expansive landforms is the outwash plain, seen in this example from Alaska; beneath it is a braided stream within an outwash plain:
Eskers form when a subglacial ice tunnel is filled to choking with rock debris, followed by melting of the glacial ice leaving sinuous low hills that are distinctive. Here are two examples:
Drumlins are elongated hills of glacial till molded by ice sheets as these paste the subglacial load against obstructions. They have distinctive shapes, as indicated in the aerial photo of a drumlin field in Manitoba:
This cross-section and plan view of a drumlin show its streamlined shape; compare this with an aerial shot of a single drumlin:
Another aerial view of a swarm of aligned drumlins emphasizes the distinctive shape of this intriguing glacial landform.
Kames form as till is washed into holes in retreating ice. Low, moundlike hills result:
Kettles are depressions in till plains that form where large chunks of ice melt, leaving a "hole" in which water may fill into a lake.
Now, on to examples of glacial features as seen from space, from the air, and the ground. Glaciation is commonplace in cool regions of the globe, at high mountain elevations and in high (polar) latitudes. Ice is conspicuous from space because of its whiteness and high reflectivity (albedo); many of the landforms ice movements produce are also distinctive. Ice can cover nearly an entire continent, such as Antarctica, or a huge island such as Greenland. When ice begins to flow by gravity to lower elevations, usually along distinct, often narrow paths, it becomes a glacier. Here are an example of ice accumulation and glacial movement in an icecap in the south of Iceland:
The above image is a special, false-color image, using yellow, red, and blue filters, to enhance various types of ice and glacial zones. It shows the thick ice cap (defined as a dome-shaped ice mass with radial flow) known as Vatnajökull, in south-central Iceland which has 43 outlet glaciers (light blue), many with lobate termini. Areas in yellow-orange are vegetated, while reds associate with basaltic rocks and sparse vegetation. Green in the ocean is sediment, and black around the ice cap is a zone of ground soaking from glacial meltwater.
On a smaller scale, Bylot Island in the Arctic Sea of northern Canada is capped by a small permanent icefield, from which mountain glaciers are flowing downslope on both the north and south sides:
These glaciers have been shrinking steadily in the last 40 years. Here is an aerial oblique view of several:
The image mosaic below shows typical alpine or mountain glaciers developed from snow fields covering the higher elevations of parts of the Wrangell and Chugach mountains of southeast Alaska. The first range rises to 5,043 m (16,541 ft) at Mount Kennedy. Among the large glaciers are Nabesna (to the north), Kennecott and Rohn sending meltwater into the Chitina River, and Russell and Barnard coming off the eastern segment of the Wrangell group. In the Chugach mountains, within the scene, the highest peak is Mount Tom White (3270 m; 10630) but higher peaks are found both to the west and east. The Bering glacier is the largest in the Chugach group, although not as broad as the Malaspina glacier off the image to the east. Note the dark streaks in some of these piedmont glaciers. These are medial and lateral moraines (glacial debris that may become rock deposits). The reds in the scene are mostly tundra vegetation.
Along the same southern coast of Alaska is a famed active glacier known as the Malaspina Glacier. The Landsat subscene below shows a series of internal moraines on the glacier and a prominent wide lateral moraine to the west (left).
The perspective view of this glacier as it creeps towards the Gulf of Alaska, made by combining Landsat and STRM data, shows it in context with its mountain source:
Mountain glaciers occur in groups largely owing to the preglacial prevalence of multiple, often parallel river valleys, all filling with ice when the climate turns cold enough to support extensive ice accumulation. Several glaciers can occupy tributary valleys that coalesce into a single larger valley, as seen here:
The Bear glacier in the Denai Peninsula of Alaska shows a prominent medial moraine. As it reaches an ocean inlet, ice breaks off to form small icebergs that could become a hazard if they reach the open sea. This IKONOS image has high enough resolution to show crevasses formed by differential movement of the ice mass:
Note that part of the above glacier is almost completely covered with rock debris (eventual till). Although uncommon, this degree of coverage is unusual. An example is the Imja Glacier flows through eastern Nepal, part of a glacier network that ultimately feeds the Ganges River. On October 4, 2010, the Advanced Land Imager (ALI) on NASA�s Earth Observing-1 (EO-1) satellite captured this natural-color image of Imja Tsho and surrounding glaciers. Dirt and debris coat these rivers of ice; like the glaciers feeding it, Imja Tsho appears dull gray-brown.
Dark streaks in advancing ice can have other causes and in other modes of ice movement. This EO-1 ALI image shows the edge of the Barnes ice sheet on Baffin Island in the Canadian arctic. The dark lines are produced by seasonal dust accumulating as the sheet grows.
This JERS-1 SAR (radar) image of an Alaskan glacier brings out the details in the cracking of the ice as the entire mass moves slowly down gradient.
The largest and longest glacier in the world is the Lambert Glacier, in northeastern Antarctica (Australian sector), which meets the sea at the Amery Ice Shelf. Its length is given as 403 km (250 miles); it width reaches to 64 km (40 miles). Below are three space images: the top shows part of the glacier in a Landsat image; the center is a perspective view made using DEM data; the bottom displays rates of flow determined from radar data taken over an extended period: