Introduction to GlaciersReading Assignments
Introduction Living in New England one becomes accustom to glacial terranes. Outcrops are relatively fresh, lacking the mantle of weathered rock, or saprolite, encountered further south. The regolith is transported, laid down by glaciers or their meltwater products. Soils are typically stony except in valleys where sandy or clayey soils may prevail. Rocks in the regolith are commonly varied in composition, they are erratics, unlike the underlying bedrock. Our coastlines are jagged lacking the backing of a depositional coastal plain, composed of stratified sediments accumulated over millions of years. Drainage basins are locally deranged, streams seem to disappear in swamps and bogs, later reappear down valley. All of these subtle characteristics are the product of glaciation. Formation and flow of Glaciers Glaciers form when the climate is such that more snow and ice accumulate than can be melted. They can range in size from huge continental ice sheets to small alpine cirques. Once the accumulated ice reaches a thickness of 40 meters or greater it will begin to flow by slow plastic deformation and basal sliding, driven by gravity acting the glacier's mass.
Effects on landscape erosion erosion and relief Whether a glacier moves purely by creep or by creep and basal sliding (discussed below) depends on the presence of water between the ice and the bed. Basal water will occur beneath glaciers where the basal ice is at or near the pressuring melting temperature. Basal water therefore will exist where the geothermal gradient is high, such as over a volcanic terrain, where very thick ice insulates the basal ice from extremely low surface temperatures, or when meltwater from the surface works it's way to the bed. Temperate glaciers are typically warm-based, and large ice sheets may have zones of both warm-based and cold-based ice; the boundary of which can be an area of extreme subglacial excavation.
Glacial flow is largely accommodated by internal plastic deformation (creep) and basal sliding. Under the right conditions ice at the bed of warm-based glacier will move around an obstacle by melting and refreezing; a process known as regelation slip. Where stresses exceed the ability of ice to plastically deform it will do so by fracturing. For this reason the ice surface is fractured as it moves down an ice falls or surges forward in response to a large influx of basal meltwater. The shear stress that drives glacial movement is determined using equation 1 below. The rate of plastic deformation, or strain rate (equation 2) depends not only on shear stress but also the internal properties (i.e. size and orientation of the ice crystals) and temperature of the ice. Basal sliding is largely influenced by shear stress, basal meltwater, and bed friction.
For several reasons the rate of internal deformation is greatest near the bed. First, this is where geothermal heat is trapped and the ice is warmest; second, the overlying ice is thickest at the bed, hence shear stress is greatest; and third, ice crystals near the base are larger, tabular, and oriented to accommodate flow by gliding perpendicular to the c-axis. Although the rate of deformation is greatest at the base, deformation occurs throughout, and each layer is carried on the back of the layer below. The net result is a steady, but not necessarily linear, increase in velocity toward the surface. Glaciers erode by mechanically plucking and abrading their bed. There can also be substantial subglacial meltwater erosion. These processes occur beneath warm-based glaciers, where freeze-thaw activity and glacial sliding occur, and where basal meltwater is present. In New England plucked and abraded surfaces are abundant, particularly in rocky areas of thin drift. In terms of rates of erosion, glacial quarry operates at a rate that is approximately 10x greater than abrasion. Such surfaces attest to the fact that the glacier eroding them was not frozen to its base. Read this summary of Glacial Erosion by Rob Chambers Geography Department at St Ivo School, Cambridgeshire, UK.
Explore these features in Central Park, NY
Glacial drift is the common name given to sediments left behind by glaciers. The term comes from the old belief that these sediments and the erratic boulders associated with them were deposited by the biblical flood. (Just another application of the dilluvial "theory".) Charles Lyell (1797 - 1875) know to many as the father of modern geology by popularizing Hutton's Principle of Uniformitariansim in his Principles of Geology (electronic edition if your interested), initially resisted Louis Agassiz's (1807–1873) glacial theory in the 1830's. Lyell developed the "drift theory" believing that erratics and other glacial sediment were flood dilluvium dumped from icebergs as marine waters flooded the landscape. A popular hypothesis among those who still supported the biblical flood. However in 1840, after viewing evidence in the Alps and coming to the realization that the unstratified deposit know as till could not be produced by marine sedimentation, Lyell became an ardent supporter of the glacial theory. A major component of "drift" is glacial till (figs. 7 and 8), which is deposited directly from the ice. Till is
a poorly sorted mixture of rocky debris with a noticeable
bimodal character of fine and coarse sediment, such as one gets when
crushing rock. Till can be sheared off or melted from the base
of the glacier (lodgement till) or laid down from the body or surface
of the glacier by melting (ablation till). Typically lodgement
till is more compact, more clay rich, impermeable, and has a fabric
of microlithons separated by small shear planes, and poorly aligned
clasts. Ablation
till is typically looser, lack a fabric, and often contains less clay,
having been partially removed during the melting process. Technically
till is deposited directly from glacial ice.
However, till-like material can also be produced by the shearing
of water-rich sediment beneath the glacier. Similar deposits are produced
by mass-wasting (flowtill), so where the origin is vague the term diamicton is
used, which now seems to be most of the time. It's not
uncommon for accumulated debris concentrated on melting ice to
flow from the margin onto proglacial deposits. Therefore a till-like
deposited sandwiched between proglacial sediments could indicate a
readvance or just a glacially-derived debris flow, so such deposits
need to be carefully evaluated before any interpretation can be made. Moraine Moraine is the geomorphic expression of debris released directly from the ice, either subglacially as lodgment till (fig. 5) or from the melting body of ice as ablation till. Ground moraine is any blanket till deposit having no noticeable topographic expression, unless you consider stonewalls, and shallow ponds caused by poor drainage. All other moraine deposits are organized in various types or ridges that are classified by where they were formed (e.g. lateral or end moraine), how they were formed (dump, push, or ice-contact moraines), or when they were formed in the progression of events (terminal or recessional moraines.) End moraines can be produces when previous deposits are bulldozed by into ridges by advancing ice, or formed by the dumping of debris along the ice margin during a period of stability. Some moraines may exhibit elements of both processes. Imbricated Cretaceous and Pliocene sediments (image) produce the brightly colored bluffs on Gay Heads where the Late Wisconsin terminal moraine forms the northeast corner of Martha's Vineyard, MA. Elsewhere the moraine is composed of till, dumped by the glacier while it sat in equilibrium over 2000 years.
Drumlins: Drumlins are elongate hills that are characteristically streamlined in the direction of ice flow. They are a subglacial bedform of sorts. Very long linear drumlins are believed to indicate higher glacial flow velocities. Drumlins often have the shape of an inverted spoon with the up-ice slope steeper than the down-ice slope. Drumlins can be composed of a variety of materials, such as till and glaciofluvial sediments, and even rock, depending on the process that formed them. Nevertheless, drumlins are largely composed of glacial sediment, which is why they are typically classified as depositional features. However, many are in fact created by the erosion of older drift deposits. The Boston Harbor drumlins are a good example.
Marginal moraines (refer to figure 2)
When glacial ice melt the water produced transports and deposits the sediment in a variety of environments. Some of theses are outlined below. Glaciofluvial sediments are laid down by streams. Sediment can be deposited in and against the ice (ice-contact deposit) or beyond the ice margin (proglacial). The closer the sediment is release to the ice the coarse and more heterogeneous it tends to be. Glaciofluvial ice-contact deposits Because they are laid down in channels and hollows founded in melting ice, ice-contact deposits exhibit a variety of forms. Sinuous ridge, called eskers (fig. 12, 13), are formed in channels that flowed through stagnant ice, whereas conical hills, called kames, were formed where sediment was dumped into depressions (fig 14). When the final ice melted, these deposits became positive features; channels became ridges and depressions became hills. Hummocky moraine, formed over a large region of stagnant ice (fig. 14) may contain a mixture of irregular and interstratified till and meltwater deposits. Kettles, which form the cranberry bogs of southeastern Massachusetts, were stranded ice block where deposition was prevented (fig. 14). Melting produced depressions, or negative relief feature.
Lakes dammed by sediment deposits, detached ice blocks, and stray ice lobes were commonplace during deglaciation. Such temporary lakes filled with water draining from a melting glacier are called a glacial lakes. Sediment-laden braided streams poured into these lakes creating Gilbert-type deltas and depositing fine-grained seasonal layers (varves) on the lake bed. It's not uncommon to find glacial deltas in New England's large valleys. "Gravel pits" typically expose sandy foreset with their gravelly topsets removed. That critical contact reveals the elevation of the now drained glacial lake. The varved bottomset bed, which are in fact the distal part of the delta, give information on the length of time the lake existed and the waning and waxing climatic conditions. Major glacial lakes were formed against the north side of Cape Cod, which was above sea level during the early stages of ice retreat, and in the Merrimack and Nashua River Valleys and tributaries. However, the largest glacial lake of all was Glacial Lake Hitchcock in the Connecticut Valley. Analysis of varves, originally done by Ernst Antevs and carried on by others reveals that the lake existed for over 4000 years during 17,800-13,100 calibrated yr BP. Read the history of varve chronology in the Northeast and export the North American Glacial Varved Project run by Jack Ridge at Tuffs University. Learn more about glacial deltas from the Maine Geological Survey: Maine's Glacial Deltas Throughout the Pleistocene there is evidence in the marine record of multiple glacial events. However, in New England there is evidence that only two events extended into southern New England. The evidence lies exposed in a number of drumlins in Boston Harbor where Late Wisconsin till overlies a till interpreted as Pre-Wisconsin till with an upper weathered profile. The youngest and most notable glaciation occurred during the Wisconsinan (~110,000-12,000 ypb). This was the time of the Laurentide Ice Sheet which grew slowly, its margin oscillating in southern Quebec before finally advancing into New England around 40,000 ypb, around the beginning of the Late Wisconsinan phase of glaciation. The ice sheet reached its terminal position along Long Island, Nantucket and Martha's Vineyard around 24,000 were it remained stable until 20,000 ybp. By ~14,000 ybp the margin had retreat to Salem and central Massachusetts, leaving New England entirely by 11,000 bp. The Late Wisconsinan was accompanied by a 150 meter drop in sea level, commensurate with the volume of water trapped in the various glaciers world wide. The weight of the 2-4 km thick Laurentide Ice Sheet isostatically depressed the land .85 - .96 meters/kilometer from the ice margin northwestward towards the central Canadian dome. Retreat of the glacier was accompanied by a complex series of events owning to asynchronous sea level rise and isostatic rebound. Coastal valleys deepened by glaciation were flooding by rising sea level and the development of our modern estuaries. Beyond the glacial limit streams flowing through the Coastal Plain deepened their channels to meet the glacially lowered sea level. These valleys also drowned giving rise to dendritic digitate estuaries such as Chesapeake Bay. Glacial rebound progressed as the ice retreated. Valley floors depressed near the ice margin were rising farther south where the ice had retreated years earlier. Reverse valley gradients and mounds of debris left behind by the melting ice sheet trapped glacial meltwater forming glacial lakes in most of the major river valleys. Their evidence lies along the valley walls where Gilbert-type deltas form high depositional terraces, clearly graded to a higher baselevel, and where deposits of varved lake sediments form unstable valley slopes. The longest and most persistent lake was Glacial Lake Hitchcock, discussed above. As the ice retreated the lake extended northward. Geologists studying its deposits have not only confirmed and refined the extent of isostatic tilt but have also been able detail climatic conditions and retreating ice marginal positions as the ice sheet deteriorated. (Read Ridge, 2004, and explore the North American Glacial Varved Project) Glacial deposits, specifically sand, gravel and clays deposits are New England's most valuable mineral resource. Sand and gravel are required for roads and nearly all construction projects. Clay from New England's glacial lakes is use for brick manufacturing. Most high yield surface aquifers are in permeable glacial meltwater deposits, and the well drained surfaces of outwash and delta plains form the foundation of many town airports (i.e. Beverly airport). "God make eskers to build roads on," was a common phrase among loggers in Maine (figure 12). Eskers, which used to be common relict glacial features are today seen only on old maps or where protected. Most of their highly prized gravel has been mined and placed in other more preferred road locations. As the Laurentide Ice Sheet withdrew from North America large expanses of glacial outwash and lake sediments were laid bare and exposed to strong winds which winnowed the fine silt and deposited elsewhere as loess deposit. Read: Eolian History of North America (USGS)
Bibliography Andrews, J.T., and Fulton, R.J., 1987, Inception, growth and decay of the Laurentide Ice Sheet: Episodes, v. 10 n. 1, p 13-15 (pdf) Benn, D.I. and Evans, D.J.A.,1998, Glaciers and Glaciation: Edward Arnold, London, 734 pp. Bloom, Arthur. 2004, Geomorphology, A systematic analysis of Late Cenozoic Landforms, (4th edition): Waveland Press Inc., Longe Grove , IL 482 p. Chorley, R.J., Schumm, S.A., Sugden, D.E., 1984, Geomorphology: Methuen and Co. Ltd., London, 605 p. Easterbrook, D.L., , 1992, Surface Processes and Landforms: Prentice Hall, Inc., Upper Saddle River, NJ, 546 p. Newman, W.A. and Mikelson, D.M., 1994, Genisis of Boston Harbor drumlins, Massacuhsetts: Sedimentary Geology, v. 91, p. 333-343. Palmer, Douglas, 2005, Earth Time - Exploring the deep past from Victorian England to the Grand Canyon: Wiley and Sons, West Sussex, England, 436 p. Ridge, John, 2004, The Quaternary glaciation of western New England with correlations to surrounding areas, in Ehlers, J. and Gibbard P.L., eds. Quaternary Glaciations - Estent and Chronology, Part II: Elsevier B.V., p. 168- Rosen, P. and FitzGerald, D.M., The drowning of Boston Harbor and the development of the shoreline, in Pena, A. and Wright, C.E., eds., An Environmental history of the city and its environments, University of Pittsburgh Press. (pdf) Summerfield, M.A., 1991, Global Geomorphology: John Wiley and Sons, New York, NY, 536 p.
 Lindley
Hanson/Department
of Geological Sciences/Salem
State College/Geomorphology/GeoIndex/QkRef |
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