GLACIAL LANDSCAPE PATTERNS
James S. Aber
|Lobate glaciation pattern||Continental glacial zones|
|Glacial landscapes on Mars||Related sites|
He continued: The superficial aspect of the formation is that of an irregular, intricate series of drift ridges and hills of rapidly, but often very graceful, undulating contour, consisting of rounded domes, conical peaks, winding and occasionally geniculated ridges, short, sharp spurs, mounds, knolls, and hummocks, promiscuously arranged, accompanied by corresponding depressions, that are sometimes even more striking in character.
On moraine loops: It is disposed in a series of great loops which give to the whole a peculiar arcuate expression. A remarkable peculiarity is the extraordinary fact that the lateral portions of these great loops do not simply join, forming re-entrant angles, but coalesce and are prolonged sometimes a hundred miles (160 km) or more between the lobate areas embraced by the loops, forming a peculiar morainic type, to which the term intermediate or interlobate moraines will be applied. (p. 313) Chamberlin recognized twelve such great loops (lobes) encompassing broad valleys in the northern United States--see Table 8-1.
|Eastern United States||Central United States|
|Champlain-Hudson lobe, N.Y.||Lake Michigan lobe, IL/WI.|
|Finger Lakes lobe, N.Y.||Green Bay lobe, Wisconsin.|
|Grand River lobe, Ohio.||Chippewa lobe, N. Wisconsin.|
|Scioto lobe, Ohio.||Superior lobe, MN/Wisconsin.|
|Maumee lobe, Ohio/Indiana.||Des Moines lobe, MN/Iowa.|
|Saginaw lobe, Michigan.||James valley lobe, N/S Dakotas.|
|Hudson River, New York: NASA space-shuttle photo, STS58-81-038, 70 mm format. Low-oblique view toward northwest over Long Island, New York City, Hudson River, and Catskill Mts. Reddish-brown color shows autumn foliage of forest in Catskills, October 1993. During the late Wisconsin glaciation, an ice lobe extended southward along the Hudson valley and reached a maximum position marked by end moraines on Long Island and Staten Island. NASA Johnson Space Center, Imagery Services.|
|Finger Lakes, New York: NASA space-shuttle photo, STS51B-33-028, 70 mm format, 4/85. Near-vertical view of Lake Ontario and New York Finger Lakes. Finger Lakes occupy deep, ice-carved valleys that diverge southward from the Ontario basin. South of the Finger Lakes, a network of river valleys marks the path of ice-marginal melt-water drainage related to the Ontario ice lobe. NASA Johnson Space Center, Imagery Services.|
|Lake Michigan and Green Bay, WI/MI: NASA space-shuttle photo, STS28-153-22, 5-inch format, 8/89. High-oblique view toward southwest over Lake Michigan and Green Bay (right of lake). Large ice lobes followed both basins toward the south; ice flow was split by bedrock high that forms Green Bay peninsula and islands to the north. NASA Johnson Space Center, Imagery Services.|
|Prairie Coteau, MN/SD: NASA space-shuttle photo, STS36-152-110, 5-inch format, 3/90. Near-vertical view of Coteau des Prairies, eastern South Dakota and western Minnesota. Lakes are frozen, but ground is snow free in this late winter scene. The prominent light-colored zone along the eastern edge is the Bemis Moraine, which formed along the western side of the Des Moines lobe during late Wisconsin glaciation. Small elongated lakes in the moraine occupy tunnel valleys that were eroded by melt water flowing out from beneath the ice lobe. NASA Johnson Space Center, Imagery Services.|
Chamberlin undertook detailed mapping and analysis of the glacial features associated with the Green Bay lobe. From this he developed three fundamental laws of glacier dynamics in ice lobes (Chamberlin 1886).
|Great Lakes, United States and Canada: NASA space-shuttle photograph, STS064-54-022, 9/94, 5-inch format. High-oblique view toward southwest with sunglint from lake surfaces; Lake Ontario to left, Lake Michigan to far right. Great Lakes basins were eroded by large ice lobes that spread southward from the Laurentide Ice Sheet. NASA Johnson Space Center, Imagery Services.|
|Memorial plaque to T.C. Chamberlin on a large erratic boulder, University of Wisconsin, Madison. Chamberlin's service to Wisconsin and geology are commemorated on this monument. Photo date 7/89; © by J.S. Aber.|
The various glacial landforms are usually arranged in systematic patterns relative to ice lobes--see Figs. 8-2 to 8-4. End moraines, ice-pushed hills, and kames define arcs or crescents that outline the margins of ice lobes. Landforms associated with melt-water erosion or deposition in streams, lakes, or seas are located beyond the edges of ice lobes. Subglacial landforms, such as drumlins, eskers, tunnel valleys, and ground moraine, are found within the areas occupied by ice lobes. The kinds of landforms created by an ice-lobe advance and retreat depend on many factors: rate of advance, substratum lithology, basal temperature, etc.
The shape, thickness, and rate of movement of a glacier lobe are controlled by many variables--substratum topography, ice supply from interior of ice sheet, basal melt-water conditions, nature of substratum sediment, permafrost, etc. Two general forms of ice lobes were developed along the southern margin of the Laurentide ice sheet.
Continental glacial landscape zones
The distribution of glacial erosion, deformation, and deposition for large
ice sheets is not random or haphazard, but follows certain broad patterns
(Dyke and Prest 1987; Aber 1992). Three zones of glacial landscape modification may
be recognized for continental ice sheets--see Figs. 8-8 and 8-9.
Thick sequence of glacial strata exposed in Golden Valley bluffs, near
Medicine Hat, Alberta. The bluffs reveal >80 m of glacial sediments that
form a blanket, which buries the preglacial landscape of southern Alberta.
Photo date 8/84; © by J.S. Aber. Streeter Moraine in Logan County, southeastern North Dakota. This moraine
is located on the Missouri Coteau; it was formed along the western margin of
the James ice lobe. The moraine contains many large ice-shoved hills, like
the example seen on the horizon. Photo date 7/90; © by J.S. Aber. Glacial sequence exposed in quarry near Freienwalde, northeastern Germany.
The light-colored mass at top is a bedrock raft (megablock) of Oligocene
glimmersand (micaeous sandstone) that was thrust over the brown-colored
glacial sediment underneath. Photo date 8/95; © by J.S. Aber. Cape Cod, Massachusetts: NASA space-shuttle photograph, STS071-708-040, 07/06/95,
70 mm format. Low-oblique view toward northwest. Cape Cod, Nantucket, Martha's Vineyard, and
other offshore islands consist of moraines and ice-shoved hills built along the margins of
ice lobes during the late Wisconsin glaciation, around 18,000 to 20,000 years ago.
NASA Johnson Space Center, Imagery
Services. View over crystalline shield terrain, Stockholm, eastern Sweden.
The hill top is exposed bedrock without any sediment or soil cover. Lower on the hill
sides, thin moraine cover is present--see next photo. This bedrock landscape is
typical of the intermediate zone of glaciation. Photo date 4/87; © by J.S. Aber. Closeup view of thin moraine cover on lower portion of bedrock
hill. The sediment is only one boulder-layer in thickness.
Photo date 4/87; © by J.S. Aber. Andersön, a small drumlin (background) within Storsjön lake,
central Sweden. Glacial sediment forms a near-continuous cover in foreground.
The drumlin and sediment cover are typical of the inner zone of glaciation--see
next photo. Photo date 6/87; © by J.S. Aber. Closeup view of exposure on side of Andersön drumlin.
Fine sand/silt is folded around a core of coarse gravel. Overriding ice
molded the ice-pushed hill into a small drumlin. Ice movement from right to
left; scale pole is 2 m long. Photo date 6/87; © by J.S. Aber.
|View over Egersund, southwestern Norway. The crystalline bedrock was stripped
of soft sediment cover during repeated ice-sheet glaciations. The erosion
surface is the only evidence for former glaciation, except for scattered
pockets of sediment that date mainly from final phases of the last
glaciation. Photo date 8/87; © by J.S. Aber.|
The Antarctic shelf shows the unusual effects of repeated glaciations. Most continental shelves are <200 m deep near land and become gradually deeper (1 km) toward the shelf margin. The Antarctic shelf is the opposite--see Fig. 8-10. The shelf is relatively deep (0.8-1.3 km) near shore, but becomes shallower toward the margin, where water depth is only 200-400 m (ten Brink and Schneider 1995). Greater depth of the inner shelf results from glacial erosion and/or non-deposition; whereas the outer shelf has built up a very thick sequence of glacially related deposits. The Greenland continental shelf exhibits similar development, in which outer portions have end moraines preserved in quite shallow water, and deep channels are cut across inner portions.
These landscape zones are related in general to the availability of erodible or deformable substrata, namely thick sediments or sedimentary bedrock. The three glacial landscape zones are the results of multiple glaciations and represent long-term, cumulative modifications of continental substratum by ice sheets. Complex patterns of ice movement exist, particularly in the inner and intermediate zones of glaciation, due to changing ice centers and flow lines--see Fig. 8-11. Thus, the landforms of older glacial movements may be overprinted, much modified, or completely obliterated by later ice flows--see Fig. 8-12. The results are cross-cutting geomorphic patterns over regions of former glaciation--see Fig. 8-13.
The three glacial landscape zones are apparently also related to thermal regimes developed at the base of the ice sheets, particularly during deglaciation--see Fig. 8-14. The glacial landscape zones in North America and northern Europe are arranged in symmetrical patterns with respect to the Atlantic Ocean--see Fig. 8-15. The general correspondence in glacial landscapes is most remarkable considering the geographic and climatic differences between the two continents. Repeated ice-sheet glaciations have imposed a common geomorphic pattern, in spite of these differences.
The infamous canals of Mars are really giant valleys eroded by immense water floods. These floods were triggered when episodic volcanism melted large volumes of permafrost in the Tharsis Bulge region--see Fig. 8-16. The floods scoured channel networks leading to a large basin in the northern hemisphere, in which Oceanus Borealis was formed. Oceanus Borealis covered up to 40 million km² with an average water depth of about 1700 m (1 mile). Such oceans may have formed repeatedly following major volcanic outbursts.
During each oceanic phase, a mild climate with moderate greenhouse effect came into being. Water vapor was circulated in the atmosphere around the planet. Snowfall and glacier formation occurred in the southern hemisphere resulting in a vast Austral Ice Sheet. The array of glacial landforms present on Mars bears a striking similarity to glacial features of the Earth--see Fig. 8-17. The mild climatic episodes on Mars evidently were short-lived, however, because volcanism was not continuous. The climatic history of Mars apparently consisted of long, cold intervals of permafrost punctuated by short, mild, oceanic/glacial episodes. The final glacial epoch took place late in martian history, not less than about 100,000 years nor greater than 20 million years ago (Kargel and Strom 1992).
The ultimate failure of Mars' episodic greenhouse climate can be attributed to the planet's small size. The planet's interior cooled early in its history, and volcanic emission of CO2 became intermittent. Weak gravity allowed much of the atmosphere to escape, so that a permanent greenhouse effect and mild climate could not become established.
This volcanic-climatic scenario along with the chance of martian life remain highly controversial subjects among planetary scientists and the public. The possibility of past life on Mars has major theological as well as scientific implications. A series of NASA missions are designed to collect data from martian atmospheric and surficial environments. These unmanned missions may culminate ultimately in manned expeditions by the mid-21st century. Only then can we firmly establish the evidence for past glaciation and the possibility of ancient life on Mars.
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