The Pleistocene Ice Age

The central Indiana landscape is primarily a product of the Pleistocene Epoch of the Quaternary Period, or Ice Age, a period of widespread continental glaciation in which the temperate northern latitudes were repeatedly invaded by large ice sheets. The Pleistocene began about 2.6 million years ago and was characterized by a cooler, wetter, and presumably cloudier climate than today. These conditions led initially to the development of ice caps in the vicinity of Hudson Bay and Labrador. Over tens or hundreds of thousands of years the ice caps gradually expanded, eventually coalescing into the massive Laurentide Ice Sheet that flowed southward into the temperate latitudes of North America. During the course of the Pleistocene, the glaciers carved out the basins of the Great Lakes, which helped direct the flow into Indiana from three principal directions: (1) the northwest (Lake Michigan Lobe); (2) the north-northeast (Saginaw Lobe); and (3) the east-northeast (Huron-Erie Lobe). The deposits of each lobe are characterized by a distinctive suite of rocks and minerals derived from the particular bedrock each lobe flowed over. As far as is known, all of the glacial deposits in Marion County are from the Huron-Erie and Saginaw Lobes.

The Ice Age was punctuated by several prolonged warm periods during which the glaciers disappeared entirely from the temperate latitudes and a climate similar to today or even warmer prevailed. The interglacial landscape was modified by typical terrestrial processes, such as erosion by streams to produce hills and valleys, and deep soil development; it also supported abundant vegetation, including hardwood forests and wetlands similar to those present today. These warmer periods are known as interglacial stages (fig. 1), and they separate several major stages of glacial advance that are recorded by the Pleistocene deposits in the Midwest.

Indirect geophysical evidence suggests that the core of the Laurentide Ice Sheet in the vicinity of southern Canada and the Great Lakes was up to 2 miles (3.2 km) thick, similar to the modern Greenland ice cap. The thickness of the glacial lobes that affected central Indiana is less clear, but was probably several thousand feet at times, with progressively thinner ice toward the margin of the glacier. In general, deposition occurs near the margin of glaciers, whereas erosion is the dominant process further back up-ice — especially beneath the cores of large ice sheets, where the substrate beneath the glacier may be severely scoured, completely removing any earlier glacial deposits that may have been present, and deeply eroding the underlying bedrock. Topographic characteristics of the substrate also greatly affect where erosion and deposition occur, with hills and other obstructions often experiencing intense erosion, while valleys and lowlands typically are sites of deposition where earlier deposits tend to be well preserved (see How Glaciers Work).

Unconsolidated Deposits

The modern landscape of Marion County, and the unconsolidated deposits within about 30 to 50 ft (9 to 15 m) of the surface, are chiefly the product of the most recent stage of glaciation, called the late Wisconsin, which affected the county between about 22,000 and 17,000 radiocarbon years B.P. At many places, however, the late Wisconsin deposits comprise a relatively thin veneer that mantles a thick series of deposits from earlier ice advances, the oldest of which may approach one million years in age. Marion County lay near the southern terminus of ice sheets throughout the Pleistocene, a position that helped protect older deposits from erosion during younger ice advances and contributed to the preservation of a fairly robust, though complex and locally incomplete, record of glacial events (fig. 2). The total thickness of unconsolidated deposits in the county is commonly between 100 and 200 ft (30.5 and 61 m), and locally exceeds 300 ft (91 m) in Lawrence and Franklin Townships, where a thick sequence of ancient pre-Illinoian deposits is preserved in a series of deep bedrock valleys and lowlands. In contrast, the glacial deposits are extremely thin in parts of Decatur Township, where large bedrock hills associated with the buried northern extension of the Knobstone Escarpment obstructed ice flow and stand within a few feet of the modern land surface.

These examples demonstrate the close relationship between the thickness of glacial deposits and the underlying bedrock topography: glacial deposits are almost invariably thicker and better preserved in low areas on the bedrock surface, such as buried valleys, whereas they are usually much thinner over bedrock highs, such as that in the southwestern part of the county (fig. 2). Certain kinds of glacial deposits, such as end moraines and kames, are associated with elevated, irregular topography. Crown Hill (fig. 3) and the large ridges at Glenns Valley are good examples, where glacial action has produced conspicuous topographic high points underlain by very thick glacial deposits. On the other hand, erosion by streams both during and after glaciation has produced low-lying valleys, below which the glacial deposits are generally thinner than beneath adjacent uplands.

Pleistocene History and Glacial Terrains of Marion County

The landscape of Marion County is made up of a series of glacial terrains, each characterized by a specific set of landforms and underlying sedimentary sequences that reflect a particular geologic history and set of depositional (or erosional) processes local to that region of the landscape. These terrains are primarily the result of late Wisconsin glaciers active in the county from about 22,000 to 17,000 radiocarbon years B.P., and the meltwater they produced (fig. 4). Late Wisconsin sequences average about 50 to 75 ft (1.5 to 22.9 m) thick in the county, but are locally much thicker or thinner, depending on the type of terrain they form and the relief on underlying deposits and bedrock. These deposits are widely exposed at the surface, cropping out in bluffs, along stream banks, and in many excavations almost anywhere in the county, thus they are readily observed and relatively well known. Among other things, they form the parent materials for the surface soil, act as the foundation for most infrastructure, and contain major groundwater resources, hence their character is of immediate relevance to everyday life. Therefore, most of this section focuses on the late Wisconsin glacial history and depositional sequences of the county.

At many places, however, the Late Wisconsin sequences make up only a fraction of the total glacial deposits present above bedrock, and are draped over thick sequences of Illinoian and pre-Illinoian age sediments that collectively are referred to as "pre-Wisconsin deposits." By virtue of their greater depth, the pre-Wisconsin deposits are poorly exposed at the modern land surface, cropping out only along the sides of several deeply entrenched valleys, such as Eagle Creek, Fall Creek, White River, and their major tributaries. Hence, most of our understanding of the pre-Wisconsin sequences comes from water wells and other boreholes that penetrate below the late Wisconsin deposits. But even though they are not typically close to the surface, the pre-Wisconsin deposits are of immense scientific and practical importance. They contain vast groundwater resources that are used at many places in the county, as well as evidence crucial to understanding the history of the Ice Age and the behavior of climate over the past million years or so.

Pre-Wisconsin History and Sequences

The oldest glacial deposits in the county (fig. 5) consist of a series of reddish-brown, clay-rich lake sediments, tills, and minor sand and gravel deposits that fill the deep bedrock valleys in the northeastern part of the county. These deposits contain clasts of Jurassic mudstone ("redbeds," which impart the distinctive color) and other rock types from the Michigan Basin, and are thus thought to have been deposited by an early version of the "Saginaw Lobe" that advanced into Indiana from the north. They closely resemble deposits found in large buried valleys north of Marion County, which were named the "West Lebanon" Member by Bleuer (1991) and mark the first incursion of Pleistocene ice into Indiana. Lake sediments of the West Lebanon Member are magnetically reversed, meaning they were deposited during the last polar reversal and are older than 780,000 years. The West Lebanon glaciation disrupted the pre-glacial drainage pattern throughout the northern half of the state by damming up major drainages and creating large glacial lakes that became filled with sediment. Its diagnostic red-brown color makes the West Lebanon Member the most readily recognizable pre-Illinoian unit in the subsurface of Marion County.

At least two other major pre-Illinoian glaciations are recognized in the deposits beneath Marion County. One, represented by a series of pinkish tills and associated sand and gravel, was deposited by ice flowing into Indiana from the northeast and probably correlates with the so-called "Hillery Till Member" of eastern Illinois (Johnson and others, 1972; Bleuer, 1991). In contrast, the top of the pre-Illinoian section consists of a thick sequence of weathered sand and gravel deposits interbedded with olive-gray sandy till. This interval is an important deep aquifer at many places in the county and is commonly recognizable by the strong interglacial weathering profile formed on its surface (fig. 6). Although evidence of weathering profiles locally occurs within and between all of the pre-Illinoian sequences, none are as pronounced or as widely recognizable as the one at the top of the section, which is marked by a reddish or olive-colored buried soil profile more than 20 ft (6.1 m) thick in some boreholes. This weathering horizon is believed to have formed mainly during the Yarmouth interglacial stage, prior to 200,000 years ago, and represents the interglacial landscape that existed just prior to the Illinoian glaciation.

Much of the pre-Wisconsin section beneath Marion County consists of a series of at least four, hard, gray-brown, loam-textured Illinoian tills deposited between 200,000 and 130,000 years ago (fig. 7). The tills are mostly similar in appearance and difficult to distinguish without detailed chemical and mineralogical analyses, which suggest that at least two of the tills were deposited by glaciers that came from the northeast, while another was deposited by ice that advanced out of Michigan. These till sheets are, however, locally separated from one another by variably eroded weathering horizons that exhibit loss of carbonate minerals, strong jointing, and olive-brown paleosols a few feet thick. Illinoian glaciers advanced as far as the Ohio River valley and northern Kentucky — further south than any other Pleistocene glaciation in Indiana — resulting in significant erosion of earlier deposits by each successive ice advance that came over Marion County. The Illinoian tills are also separated at places by thin, discontinuous lenses and some larger bodies of sand and gravel, the largest of which form extensive sheet-like bodies in the southern part of the county, where they serve as important groundwater sources.

The tendency of later ice advances to modify and erode the older deposits they advanced over has produced complex subsurface relations among the various pre-Wisconsin sequences, characterized by numerous cutouts of older sequences and paleosurfaces by younger ones. Large, outwash-filled buried valleys localized within the glacial section are fairly common, and may or may not exhibit any relationship to buried valleys or other topographic features associated with the bedrock surface. These relations, along with the limited number of surface exposure, hinder systematically sorting out the character, continuity, and history of these ancient deposits at any except the most local scale. More extensive exposures of pre-Wisconsin deposits elsewhere in the state (for example, fig. 8) offer additional clues as to their character and complex history, and serve as a useful analogue for the subsurface of Marion County. A more comprehensive treatment of the pre-Wisconsin deposits of Marion County can be found here.

The Pre-Wisconsin Surface

Despite these complications, or perhaps because of them, the most readily recognizable horizon associated with the pre-Wisconsin deposits below Marion County is the paleosurface (fig. 9) that developed during the Sangamon interglacial stage between 130,000 and 22,000 years ago and was subsequently modified by late Wisconsin glaciers and their meltwaters. The pre-Wisconsin surface represents the landscape initially encountered by late Wisconsin glaciers advancing into the county, and upon which their sediments were deposited. This horizon appears to have contained a considerable amount of topographic relief (fig. 10), which exerted a major influence on the behavior of late Wisconsin ice sheets and served to focus meltwater streams and outwash deposition in valleys and other low areas along the paleosurface. Although the pre-Wisconsin surface experienced a considerable amount of erosion and valley incision at places during late Wisconsin glaciation, it nevertheless preserves a variety of features that make it a fairly recognizable horizon that can be mapped in the subsurface throughout the county. Among the most interesting of these features (fig. 9) are the presence of buried wood, organic silt deposited in interglacial wetlands, and other evidence of the terrestrial habitats that existed about 22,000 years ago, just before they were obliterated by the late Wisconsin glaciation.

Figure 10 shows a simplified map of the major physiographic features of the pre-Wisconsin surface beneath Marion County. The configuration of the surface was identified by analyzing thousands of water well records, along with hundreds of samples and gamma-ray logs collected from boreholes, and numerous exposures along streams and in deep excavations. This key buried horizon is a composite feature having as much as 300 ft (91.4 m) of relief, and it truncates many pre-Wisconsin sequences whose ages span the entire range of Illinoian and older glacial events that affected this part of the state. The respective terrain regions have experienced differing geologic histories, consequently the age of the surface varies from place to place. In general, the pattern of terrains and distribution of underlying sequences and paleosols suggest that the pre-Wisconsin surface in eastern and northwestern Marion County is, in fact, representative of the Sangamon paleosurface. In contrast, the pre-Wisconsin surface in the central part of the county near the White River valley appears to have been extensively modified by late Wisconsin meltwater, and most buried soils or other evidence of a paleosurface present along the pre-Wisconsin surface in that area are likely to be exhumed pre-Sangamon paleosurfaces. (Diagram from Brown and Laudick, 2003, fig. 5, adapted from Fleming and others, 1993.)

Late Wisconsin History and Terrains: Origin of the Modern Landscape

The late Wisconsin Laurentide Ice Sheet began to affect Marion County and surrounding areas approximately 22,000 years ago with the arrival of large volumes of meltwater that were carried down the already well-established White River valley from the ice front, which lay north of the county. Over the next 1,000 years, the ice sheet advanced into and eventually covered all of Marion County as it established a terminal position well to the south in Morgan and Johnson Counties. Radiocarbon ages from wood at the base of the Wisconsin section in northwestern Marion County indicate that glacial conditions were well established in that part of the county no later than 21,000 radiocarbon years B.P.

All of the late Wisconsin sequences in Marion County are members of the Trafalgar Formation (Wayne, 1963), which is named for the small Johnson County village where these deposits were first described. The Trafalgar Formation is the principal surface unit throughout the central till plain and was deposited by ice flowing out of the Huron-Erie Basin. It has a distinctive eastern source mineralogy, characterized by a high concentration of calcite (limestone) in the silt and sand fraction, as well as abundant fragments of Paleozoic limestone and dolostone, and dark colored shale (fig. 11), which crop out along the path of the Erie Lobe and were incorporated into the glacier as it advanced into Indiana.

The Trafalgar Formation, as currently defined, encompasses several different Huron-Erie Lobe events and till sheets, which are difficult to correlate with one another, in various parts of northern and south-central Indiana. All these eastern-source tills are strikingly similar and frequently give way to bodies of outwash and lake sediment, making it highly problematic to trace individual events or deposits across the major region affected by the lobe. Thus, interpretations regarding the sequence of events as well as the character and continuity of the deposits they left, are best made at a more local scale, such as a county, and are often based as much on differences between landscapes affected by various events as they are on sediment properties. The following discussion simply highlights the major late Wisconsin events in Marion County, with a particular emphasis on natural and historical areas in the county where the impact of these events can readily be seen in the modern landscape. Additional details about the late Wisconsin deposits and events in the county, how they are manifested and identified, and the methods used to map them, can be found here.

Beginning between 21,000 and 22,000 years ago, late Wisconsin glaciers appear to have been active in or near Marion County almost continuously for the next several thousand years, producing three major depositional sequences of somewhat different character. The initial incursion of the ice sheet produced a robust till sheet, referred to herein as "T1," which can be traced in the subsurface throughout the county, along with large outwash fans in and adjacent to the White River valley. This till sheet, and its associated outwash, overlie the pre-Wisconsin surface at most places in the county, and are distinguished from the older deposits at and below that surface by their fresh, calcareous character and gray color, which typically contrast with the brown or olive, carbonate-depleted, weathering features associated with deposits immediately below the pre-Wisconsin surface. In general, all the till deposited by late Wisconsin ice sheets has a loam texture, in which the matrix consists of roughly two parts each of sand and silt, and one part clay. Local textural variations are fairly common, however. This is particularly true near the base of the T1 sequence (fig. 12), where the till is commonly more silty or sandy at places where the glacier overrode and incorporated silt and clay from older interglacial silt units, or sand and gravel from its own outwash.