Marion County
Glacial Geology — How Glaciers Work
Anthony H. Fleming

Glaciers can be divided into two broad classes: mountain glaciers (also called valley glaciers), and continental glaciers, or ice sheets. The former chiefly occur at high elevations where snow accumulation exceeds melting. Although their behavior tends to be influenced by the same global climate cycles that cause continental glaciation, it more closely reflects local climate. Valley glaciers and associated ice caps occur in many locations throughout the globe. Continental glaciers, on the other hand, generally appear in response to major global cooling and shifts in precipitation patterns, and are vastly larger than valley glaciers. The Greenland and Antarctic ice sheets are the only modern examples similar in scale to the Pleistocene ice sheets that affected the Midwest; their presence is largely related to the cold climate associated with high latitudes.

Continental ice sheets can, in turn, be divided into two types, based on ice temperature and how they move: (1) polar glaciers, such as the Antarctic ice sheet, in which the base of the ice is below 32 degrees F and is thus frozen to the bed; and (2) temperate ice sheets, where the base of the ice is at the melting point (freezing) and not attached to the bed. Polar glaciers move almost entirely by internal deformation of the ice. They produce little meltwater and carry a relatively small amount of debris. The landscape around them is dominated by widespread, deep permafrost . Temperate glaciers, on the other hand, move chiefly by sliding along their base, often lubricated by a thin film of meltwater. They tend to erode bedrock and older glacial deposits as they slide along, especially further back under the ice sheet where the ice is thickest. As a result, they also tend to incorporate large amounts of rock and sediment eroded from the bed, and they generate large volumes of meltwater. The landscape around a temperate glacier may have minor permafrost features, but most areas are not permanently frozen, and the area immediately in front of the glacier may be heavily forested by spruce and other boreal plants. This sliding style of motion results in a patchy record of older deposits in areas that were repeatedly scoured by thick ice, such as the Canadian Shield region north of the Great Lakes. In contrast, older deposits tend to be better preserved in areas near the ice margin, where the ice is thinner and erosion is less severe. The ice sheets that affected central Indiana were all temperate based, as far as is known, because they clearly generated large quantities of both sediment and meltwater, and no obvious large-scale permafrost features appear to be associated with the landscapes they produced.

Temperate glaciers can also move by internal deformation, particularly when they encounter obstructions, such as bedrock knobs, or climb regional slopes. In such cases, the base of the glacier may become "stuck" on the obstruction, causing the overlying ice to simply shear off and continue moving forward over the immobilized lower ice. Such shear planes commonly curve upwards and help to transport debris from the base of the ice towards the surface of the glacier. In extreme cases, large amounts of material can accumulate on the surface of the ice via this process, helping to insulate the ice from melting, and allowing large ice-block depressions to form later when the glacier melts. The same mechanism is partially responsible for producing hummocky topography, such as that found along end moraines and ice-contact deposits, such as kames . Good examples of hummocky topography can be seen in southeastern Marion County, particularly in the vicinity of the Bunker Hill Moraine, which produces the rolling landscape near Southeastway Park.

Deposition of most sediment takes place at or out in front of the margins of active temperate glaciers. Most sediment is carried within a few feet of the base of the ice. Sediment released directly from the ice and deposited in-situ with little or no reworking by water or gravity is called till, and typically consists of a completely unsorted mix of particles ranging from microscopic clay sizes up to large boulders. The stones in till are commonly shaped like flatirons, with distinct pointed and blunt ends. This shape results from the abrasion of the stone by sand and other stones frozen into the ice sliding over it. Such glacial stones are sometimes described as having a "bullet" shape. Many of the stones and boulders are transported great distances from their sources, and are referred to as "erratics," because they are not native to their place of deposition. Granite and metamorphic rocks derived from the Canadian Shield are common types of glacial erratics in Indiana.

Large volumes of meltwater are also released at the ice margin and carry sediment away from and parallel to the glacier. Finer particles, such as silt and clay, are typically winnowed out and carried away, leaving a coarse deposit of sand and gravel known as outwash . Till and outwash are the two most abundant glacial deposits in Marion County. Each may occur in thick, unbroken sections, or the two may be complexly interbedded. The latter arrangement commonly occurs in places where the ice margin was fluctuating back and forth, overriding its own outwash. Outwash may also be deposited up against the front of the ice sheet, often forming large fans that appear to radiate from a central point, similar to the alluvial fans seen today in mountainous regions of the western United States. The front of the glacier may actually climb up the head of the fan as the fan grows, producing large hummocky ridges, such as the one seen today near Glenns Valley. When meltwater becomes ponded in basins and depressions, silt, clay, and fine sand are commonly deposited as various types of glaciolacustrine deposits.

Virtually all deposition of sediment associated with temperate glaciers occurs at or in front of the ice margin. Sediment derived from erosion further back beneath the glacier is transported to the ice margin, where it is released by melting ice (A). In this way, a glacier can be thought of as a "conveyor belt" of sorts, constantly transporting sediment to the ice margin. Most of the sediment load of a glacier is concentrated near the base of the ice (B). Whether the sediment ends up being deposited as till (C, D) or reworked into outwash (E, F) depends on the processes operating near the ice margin. If there is little reworking by meltwater, the sediment melts out in place (C, D) and remains unsorted. Such sediment is known as glacial till, and it commonly preserves the fabric (C; ice flow is from right to left) imparted during sediment transport, as particles orient themselves parallel to the direction of ice flow, and stones and boulders are abraded into bullet and flatiron shapes by the ice flowing over them. When meltwater is present, the freshly melted-out sediment is reworked. The fine sand, silt, and clay are winnowed out and carried away, leaving the coarser sand and gravel to be deposited as outwash, both in small streams up close to the ice margin (E) as well as in vast outwash plains (F) in front of the glacier, where streams draining the ice margin coalesce into large sluiceways. The amount of sediment delivered to such outwash plains is commonly so great that it exceeds the capacity of the streams to carry it, resulting in a "braided" appearance from the air (F) as larger streams split into smaller anastomosing channels winding through the outwash. Photos by A. H. Fleming, from modern glaciers in Alaska and northern Norway.

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