Glaciers

Foreword Glacial Processes Glacial Landforms Illinois Glacial History

Foreword

In approximately 4.6 billion years of Earth history, there have been three major episodes of glaciation. The Varangian glaciation occurred during the Proterozoic part of the Precambrian period about 700 million years ago (m.y.a.). The next one happened in the late Paleozoic Era around 300 m.y.a. The most recent period of glaciation started approximately 1.6 m.y.a. and ended about 10,000 years ago.

Today, glaciers contain nearly 75% of the world’s fresh water supply in ice that covers about 10% of land area. In contrast, ice covered as much as 30% of total land area during the most recent ice age. The largest concentration of ice today is the Antarctic ice sheet, up to 4,200 meters thick in some areas, and in the Greenland ice sheet. The remainder of glaciers is located in montane regions and in ice caps in polar seas. If climate were to suddenly warm enough to melt all land ice, there would be a eustatic sea level rise of about 70 meters. Sea level has risen about 100 meters since the last glacial maximum 20,000 years ago. Potential causes of Ice Ages and glacial cycles are variations in lithospheric plate configurations, changes in atmospheric and seawater circulation patterns, changes in atmospheric composition, and Milankovitch orbital variations

A glacier is defined as a body of ice that flows under its own mass due to gravity. They are categorized by morphological, thermal and dynamic characteristics. The two main morphologic categories of glaciers are continental and alpine. The label is dependent on where the glacier has the most influence on shaping the landscape. Continental glaciers include ice caps and ice sheets. Alpine glaciers include cirque and valley glaciers and ice fields. Ice shelves are extensions of calving glaciers. Within the two main categories of glaciers, there are two types of glaciers based on thermal properties. The first type is a warm glacier; the second type is a cold glacier. The former usually has a layer of meltwater lubricating the ice/surface contact that facilitates slippage in addition to internal flow causing basal movement. Both, meltwater and internal flow paths transport sediment. In contrast, cold glaciers have little or no meltwater and only flow internally. These glaciers move over the rocky basement as bottom ice grinds the bedrock surface beneath. In these, sediment erosion occurs at the base, and material transported is embedded in the ice. Classification of glacier by their dynamism depends on amounts of ice movement. Active glaciers are characterized by high rates of internal movement from the accumulation zone to the terminus. Passive glaciers show low rates of ice movement between its accumulation and ablation zone. The last type, a dead glacier, exhibits no discernible internal ice flow.

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Glacial Processes

Glacial ice forms by compaction and re-crystallization of snow. Snowflakes are melted preferentially at their edges first because it is the region where the surface area most closely equals the volume. Snow that survives (usually in shaded areas) at least one full season is converted to a granular type of ice called firn. With enough snow accumulation and time, pressure from overlying snow may be build up enough to convert firn into crystalline ice and cause flow. Ice under sufficient pressure behaves plastically and deforms under its own weight at the crystal level. However, the upper 40 meters of ice behaves rigidly, riding on ice layers behaving plastically beneath. Valley glaciers flow plastically and by basal sliding. Continental glaciers usually remain attached at its base to the substrate, moving plastically, downward and outward from a central high area toward its margins (see Figure, Plummer,1999). In both, snow and ice accumulation causes a transfer of mass thru the ice and results in glacier advancement. In any case, the perennial survival of firn and glaciers depends on the glacial budget: Glacier mass balance = accumulation – ablation. Glacial mass balance is positive when accumulation > ablation (ice advances). It is negative when accumulation < ablation (ice front recedes). Cool summers and wet, mild winters are favorable to the survival of ice. This happens for two reasons, respectively: Snow from the previous winter does not entirely melt, and because there is enough moisture in the air for snow accumulation. The portion of a glacier on to which snow accumulates is the zone of accumulation, whereas the zone of wastage refers to the area where ice begins to melt and evaporate. The snowline (firnline) divides the two zones. The equilibrium line is usually located near the firnline. However, the glacial mass balance may show year-end accumulation only in the proximal portion of glaciers, though snow may have fallen over the whole area.

Glaciers are powerful enough to transport vast quantities of sediment of all sizes, though there is differential erosion and sedimentation through the glacier profile. Increases in physical and chemical weathering intensify erosion potential. Examples of physical processes that help break down the sediment that the glacier transports are frost wedging and shattering. Frost wedging occurs when water freezes in tiny fractures and forces out sediment particles. Frost shattering is similar. Water seeps into cracks and freezes, creating a closed system. If pressure from the ice growth exceeds the tensile strength of the rock, it will cause fracturing. Then, additional pressure may be released from the rock when outer layers become removed or when there is a decrease in hydrostatic pressure. In headwalls, pressure release may cause dilation joints to form parallel to the wall, thus increasing the rocks susceptibility to future erosion. A lot of sediment is transported away from the source point by glacial quarrying and plucking. If aided by physical weathering, the plucking process is called nivation. Mass wasting is an indirect effect of glaciated or once glaciated areas.

Glaciers sculpt the underlying landscape by abrasion. The rate of it depends on three criteria: ice thickness, the presence of water and the hardness of basal debris relative to bedrock. Ice thickness is related to pressure phenomena and deformation of the land. Greater pressures result in more notable abrasion. The presence of water reduces friction at the ice/bedrock interface. In one sense, lubrication reduces erosion; in another, greater ice movement velocity increases shear stress on the bedrock and subsequent surface alteration. Greater hardness in basal debris relative to the bedrock hardness results in more prominent the defacing of the surface. Glacial polish, striations, and grooves are examples of abrasion. Surface grinding also creates rock flour. Often times, these features reveal the direction of ice movement. Lastly, in addition to subsurface alterations, glacier movement produces a number of distinct erosional and depositional landforms and structures. In alpine regions, for example, glaciers may create U-shaped valleys, arêtes, horns, hanging valleys, cirques, col depressions, and tarns. In contrast to the rugged, angular nature of glaciated mountainous regions, continental glaciers tend to produce rounded topography with abrasional surfaces. Glacial deposits in both systems are referred to as glacial drift.

Glacial Landforms

Drift deposits are either stratified or non-stratified (diamicton). The first type is usually fluvioglacial in origin. In these, sediment deposits are well-sorted and well-rounded (mature) bedload. Landforms include outwash plains, or sandars. Secondary landforms include dunes, loess blankets, and desert pavement. Most tills are diamictons. Types of tills include lodgment (basal), and ablation till (flow till and melt-out till.) Till deposits are in the form of moraines (lateral, medial, ground, and end moraines- recessional or terminal) (see Figures, from ISGS). End moraines form when the ice margin remains in the same place for a relatively long time (tens to hundreds of years). As the ice front thins and melts, debris transported by the glacier is deposited in a heap. Ideally, end moraines assume a rather narrow ridge-like shape although the form and size is directly related to the amount of glacial load, the mass budget, and the volume of meltwater circulating in the system (Ritter et al. 1995). Ground moraines form as the ice front “retreats” and consist of more flat, low-lying till deposits. Till plains, as well, result from glaciation. Other proglacial and supraglacial deposits include kames and kettle hole A kame is formed when meltwater plunges into crevasses on the ice surface or at the ice front and deposits its load of sediment (see Kame Formation Diagram, Killey, 1998). A kettle is formed when large blocks of ice became detached from the ice front and are covered by insulating glacial till. As climate gradually warms, the ice front melts and forms kettles within the holes. Small faults result from the collapse of sediment into the depression (see Kettle Formation Diagram, Killey, 1998). Subglacial deposits include flutes, drumlins, and eskers. Eskers form when a stream of meltwater carrying sediment and rock beneath the glacier slows enough that enough that it can no longer transport coarser sediment. As it winds toward the ice front, sand and gravel is deposited are deposited first. It is possible that the stream channels become clogged and cause debris of all sizes to backstep toward the source or it may have filled up gradually with lags and other coarse sediments (see Esker Formation Diagram, Killey, 1998). Other glacier-related remnants include erratics, dropstones and varves. One other intriguing feature associated with some glaciers, particularly in a narrow wastage zone along the Transantarctic Mountains and the Antarctic ice sheet are meteorite concentrations. The meteorites landed on the ice at various times, became buried, and were transported through the ice along internal flow lines to the edge of the ablation surface where they are exposed.

Glacial deposits have an important impact on land use. Some land is most appropriate for crop farming due to the mineral-rich nature and uniformed texture of till. Other plots of land are suitable for grazing or road aggregate deposits. Sand and gravel deposits are used for concrete aggregate, road gravel, building sand, molding sand and construction fill material. Glacial deposits also serve as storage areas for natural resources such as water, natural gas, and peat. In Illinois, groundwater supplies the water need for 50 percent of the population. Much of this supply is found in aquifers within glacial deposits. Occasionally, natural gas is found in porous layers of sand and gravel capped by till, or it is found in buried soils, peat’s and organic-rich silts as drift gas. Northeastern Illinois has several examples of bogs. Peat deposits can form in bogs and in other poorly drained environments. Peat is used as a soil conditioner to increase organic content, to make clayey soils more friable and to increase moisture retention in soils.

Illinois Glacial History

During the ice ages of the Pleistocene Epoch, ice sheets up to 8000 feet thick advanced and retreated several times in North America and Eurasia. With the exception of Alaska, only 20 cubic miles of glacial ice remain in the continental United States, the majority of which can be found in the Rocky Mountains. The evidence for glacial and interglacial stages can be found in till deposits and in soils formed during interglacial stages. Deep-sea core samples show that there were eight or nine recognizable changes in climate over the last 850,000 years, and twenty-six oscillations between cold and warm climates (see Figure, Killey, 1998).

In North America, there were at least four major glaciations separated by warm interglacial periods during the Great Ice Age beginning 1.6 m.y.a. Glaciers spread from two centers of ice accumulation in Canada into northern sections of the United States (see Figure, Killey, 1998). These advances include the Nebraskan, the Kansan, the Illinoian, and the Wisconsinan glacial cycles and the Aftonian, Yarmouth, the Sangamon and the present interglacial. Newer studies reveal that there may have been at least seven continental glaciations before the Illinoian, collectively called pre-Illinoian. No one is certain on the number because more recent glaciations destroyed many of the remnants left by older ones.

Unlithified glacial drift, up to 400 feet thick in some areas of Illinois, blankets approximately 90 percent of the state (see Glacial Drift Map). This overlies 2,000-15,000 feet of sedimentary rocks and the igneous and metamorphic basement. Tills are widespread in areas once covered by glaciers, and they are also the most easily traced and correlated type of glacial deposit (see Surface Geology Map). Unfortunately, most of the surface features from pre-Illinoian glaciers are unidentifiable because later glaciers moved over the same area and removed them. However, drill hole studies in sediments other than tills are useful for revealing probable methods of deposition. There are many beds of clay, silt, sand and gravel interspersed within and between layers of till filling many parts of the bedrock valleys (see Bedrock Geology Map). These sediments are indicative of fluvial sorting and deposition. It is hypothesized that deposition took place in subglacial meltwater streams that drained into moulins or crevasses. Areas in which till lies above and below major sand and gravel beds are prime locales for locating water supplies. In addition to shaping topography, glaciers rearranged drainage patterns in major river systems (see Figure, Killey, 1998). Many river patterns that came to be before the Illinoian Glacial Episode were rearranged again during the Illinoian and Wisconsinan advances (see Figures, Killey, 1998).

Evidence from the Illinoian and Wisconsinan glacial periods are easier to interpret than older ones because sediment deposits are directly accessible on the glacially shaped landscape. At the end of the Yarmouth Episode, climate cooled and continental ice again advanced into the Midwest. At its maximum extent, the ice covered nearly 90 percent of Illinois, extending farther south than any other continental glacier in the United States. Deposits from this episode lie directly beneath the loess that covers much of the state. It is possible that climate was warm enough around the Shawnee, Illinois area to halt the ice advance, or the ice did not have enough energy to override the Shawnee Hills. During glacial retreat in the second of three advances, the flat Illinoian till plain was lain down in the lower third of the state north of Shawnee Hills. Similarly, the driftless zone in northwest Illinois and southwest Wisconsin may have also been unglaciated because of highlands to the north (see ISGS Shaded Relief Map).

In contrast, more recent till deposits in Illinois show morainal topography and have greater relief because of stream dissection. Glaciers entered Illinois out of the Lake Michigan basin during the most recent Wisconsinan glacial advancement as they did during the Illinoian. This orientation is shown by striations in exposed bedrock, the mineralogical composition of the till, the pebble fabric and texture within the till, the predominant bedrock type, and the northeast-southwest trending end moraines. The clay mineral illite, found in Lake Michigan basin shale of similar age to the Wisconsinan glacial advance, dominates the clay portion of all of the Wisconsin Episode tills. The oldest bundles of till are generally sandy and exhibit a pinkish brown color; the next bundle of till is gray and siltier; the last bundle is lacustrine, gray and very clayey. Markers such as the black, organic Robein silt, and the greenish-gray, poorly- drained Sangamon Soil on which it rests separate Illinois and Wisconsin Episode deposits in outcrops and in the subsurface. In northeastern Illinois, some of these markers were truncated or removed by glacial advances. The series of Wisconsinan end moraines in Illinois represent a more pulsating glacier history during the Wisconsin Episode (see Quaternary Moraine Maps, Killey 1998). Studies of Wisconsinan till tell us that dense, massive, over consolidated tills were deposited directly at the base of the glacier. Non-massive, less consolidated till may have flowed off the ice; fine-grained, bedded sediments are probably lakebed deposits. Rhythmites indicate cyclic deposition, and vertically graded and sorted sediments signify fluvial deposition. More locally, glacial melt water shaped the Kankakee and Fox River valleys and left rubble in the current floodplains.

The Wisconsinan glaciation is separated into five different stages. The first stage, the Altonain, arrived in the Chicago area about 60,000 years ago. There were several advances and retreats of this glacier before it ended, about 28,000 years ago. After the Altonian glacier completely retreated from the Chicago area, the time period is called the Farmdalian interval. This period lasted from 28,000 – 22,000 years ago. During this time, climate was still rather cold because the Lake Michigan Basin remained partly frozen. This is also the time when the Farmdale Soil formed from the erosion of the Altonian deposits. The next time period is called Woodfordian; this stage left most of the glacial drift in the Chicago area. It also was the time of maximum Wisconsinan glaciation. This period lasted from 22,000 –12,500 years ago. During this time, Chicago was buried by 2,000 to 3,000 feet of ice. There were several advances and retreats of this glacier during this time, an interval in which nine major moraines were deposited. Several of these will be seen on this trip (Barlina, Marengo, Gilberts, Fox Lake, Cary). The deposition of each of these moraines took up to approximately 1900 years. The Marengo Moraine, one of the most prominent in the area, was deposited last in the Woodfordian stage. The Valparaiso Moraine, overlaps some of the older moraines in this area, before the ice front retreated and re-advanced to the position of the Tinley Moraine. As the ice retreated from the Tinley Moraine, meltwater pooled behind it and created Glenwood Lake about 14,000 years ago. This lake was later renamed Lake Chicago. During this stage, the glacier deposited the five moraines that make up the Lake Border Morainic System. Drainage along the front of the Lake Border glacier established the Des Plaines Valley. After the Lake Border Moraines were deposited, the ice withdrew into the Lake Michigan Basin and exposed the Highland Park Moraine and Zion City Moraine to erosion. Woodfordian glaciation ended with the retreat of the glacier and established an eastward outlet for Lake Chicago. Even after the ice retreated from the area, glacial meltwater continued to pond between moraines and the lakefront and occasionally catastrophically drained. This type of flood carved or deepened many of the large meltwater valleys found in once glaciated regions. Lake sediments were deposited across what is now the greater Chicagoland area. Former beachfronts, sand bars and spits in this area attest to former lakeshores. The village of Blue Island, Illinois is built on high ground that was once an island in a lake, and the city of Chicago is built on a glacial lake plain.

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