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Marion County
  
Glacial Geology — Late Wisconsin Glacial Deposits

by:
Anthony H. Fleming

Adapted from Fleming and others (1993) and Brown and Laudick (2003)

General Characteristics

The glacial sediments between the modern land surface of Marion County and the buried pre-Wisconsin surface were deposited during at least three ice advances (fluctuations of the Huron-Erie Lobe) that occurred after about 22,000 years before present (yr B.P.), and are thus of late Wisconsin age. For brevity, the age of these deposits may be referred to simply as "Wisconsin" in places in this section, inasmuch as early or middle Wisconsin deposits are not known to be present in central Indiana. The thickness of the Wisconsin deposits in Marion County ranges from as little as a few feet to more than 200 ft (61 m), but is typically between about 30 and 50 ft (9 and 15 m).

The Wisconsin sediments include the massive outwash complex in the White River valley, which constitutes the most prolific aquifer system within the county. A variety of smaller sand and gravel bodies that form localized aquifers are also widely distributed, as are till-confining units of regional extent. Because the Wisconsin sediments directly underlie virtually all the land surface, where most potential contaminants are likely to originate, a clear understanding and representation of their geology is essential to the management and protection of the groundwater resource.

Regional Relations and Nomenclature

Till units of Wisconsin age in this part of Indiana have previously been included as members of the Trafalgar Formation (Wayne, 1963), and that usage is continued here. In the past, all the sand and gravel units directly below, within, or above the Trafalgar Formation have been referred to as "Atherton Formation" or "Martinsville Formation" (Wayne, 1963; Hartke and others, 1980). This division is arbitrary and not particularly useful, because most, if not all, of these sand and gravel bodies are simply facies of the Trafalgar Formation that record depositional environments where meltwater was the predominant agent, rather than ice. Therefore, we include the sand and gravel units above the pre-Wisconsin surface under the umbrella of the Trafalgar Formation. We also use the term "Trafalgar megasequence " interchangeably with "Trafalgar Formation," because the former connotes a greater sense of variation, especially in the many places where individual depositional sequences within the formation cannot be readily distinguished, or where the entire megasequence contains a preponderance of sediments other than till, such as outwash, ice-contact stratified deposits, and debris flows.

In central Indiana, the Trafalgar Formation has generally been regarded as being composed chiefly of two distinct till units, the lower Center Grove Till Member and the upper Cartersburg Till Member (Wayne, 1963; Hartke and others, 1980). This interpretation was predicated on the belief that the two till members were the products of two separate, regionwide advances of the Huron-Erie Lobe, and that these two glacial episodes constituted the entirety of the Wisconsin glaciation in central Indiana. Recent work elsewhere in central and west-central Indiana by Bleuer (oral commun., 1993) and during this study indicates that this model is probably not correct, and that the actual record is far more complex. Bleuer (oral commun., 1993) has suggested that as many as four distinct pulses of the Huron-Erie Lobe occurred during the late Wisconsin in west-central Indiana, and we have found strong evidence suggesting that ice advanced over part or all of Marion County at least three different times during the past 22,000 years. The relationship of these three advances to the classic "type" sections of Cartersburg and Center Grove till is not known. Consequently, those names are not employed in this report, and the depositional sequences and landscapes believed to be associated with each of the three ice advances are referred to informally as "T1" (oldest), "T2," and "T3" (youngest), respectively. (The terms "T1," "T2," and "T3" used in this report are not necessarily equivalent to similar terms of Bleuer (oral commun., 1993.) These three sequences collectively constitute the Trafalgar megasequence.

Throughout much of central Indiana, including Marion County, tills and other sediments deposited during the Trafalgar ice advances appear to be virtually identical to one another in many respects. Consequently, the physical characteristics of these materials (for example, grain-size distribution, color, and clay mineralogy) have been of limited value in distinguishing the different sequences from one another. Instead, the key evidence for multiple ice advances is in the form of differences in gross sequence characteristics, as expressed primarily in gamma-ray log signatures (fig. 1), as well as by contrasts in surface morphology between regions affected by successively younger ice advances (fig. 2). Widely scattered radiocarbon ages associated with the different glacial terrain regions of central and west-central Indiana also appear to corroborate the interpretation of multiple ice advances (Bleuer, oral commun., 1993), although many of the details of these events remain to be resolved.

Figure 1.
Left — Gamma-ray log signatures of late Wisconsin depositional sequences in and west of the White River valley: (a) located along the feather edge of T3 landscape and showing a very thin veneer of T3 ablation sediments plastered atop repetitive stack of T2 tills in end moraine , Bridgeport quadrangle; (b) located atop T2 end moraine about 2 miles (3.2 km) south of (a) and showing T2 till overlying thick channel deposit, Bridgeport quadrangle; (c) located just west and above Eagle Creek, and showing veneer of T3 ablation sediments and till(?) atop monotonous stack of T1 and T2 tills, Zionsville quadrangle; (d) located in the White River valley and showing outwash and T2 fan deposits atop thick T1 fan-till sequence, Maywood quadrangle. Right — Gamma-ray log signatures of late Wisconsin depositional sequences east of the White River valley: (a) located just west of the Bunker Hill Moraine and showing very thin T1 and T2 till-like sediments, Beech Grove quadrangle; (b) located near the crest of the Bunker Hill moraine and showing comparatively thick section of T3 till atop T2 and T1 tills, Beech Grove quadrangle; (c) located within typical T3 disintegration topography and showing sequence of T3 ablation sediments and till(?) atop T2 and T1 tills, Cumberland quadrangle; (d) located along crest of hummocky ridge and showing prominent T3 ice-contact deposits and till atop robust-looking T2 and T1 tills, Acton quadrangle.
Figure 2.
Map of Marion County showing the relationships of selected late Wisconsin glacial features and generalized terrain regions to different glacial episodes.

Radiocarbon ages have been obtained from wood at the base of the Trafalgar Formation in two localities in Marion County (Table 1). Two ages were obtained from spruce logs in a bed of peat exposed at the American Aggregates Quarry on South Harding Street (sec. 33, T. 15 N., R. 3 E., Maywood quadrangle). The peat is capped by 1 to 2 ft (0.3 to 0.6 m) of organic silt. These sediments are immediately below the lowest part of the outwash complex in the White River valley, which in turn underlies T1 till. The apparent age of the top of the peat indicates that deposition of Wisconsin outwash began in this part of the valley after about 22,000 yr B.P. The sand and gravel at this locality is believed to be part of an outwash apron that was deposited not far from the advancing ice front, therefore, ice probably overrode this site not long after the outwash fan was deposited.

Table 1.

Site location Stratigraphic Position Age (yr B.P.)
  • American Aggregates Harding Street Quarry
  • sec. 33, T. 15 N., R. 3 E. Maywood quadrangle
Peat bed between top of bedrock and the base of late Wisconsin outwash 22,070+210 (top of peat)

23,340+240 (bottom of peat)
     
  • Eagle Creek Park Confluence of Fishback and Eagle Creeks
  • sec. 28, T. 17 N. , R. 2 E. Zionsville quadrangle
Log in organic bed between paleosol and base of first late Wisconsin till unit 21,030+180
     
  • East edge of Plainfield (Hendricks County), just south of US 40
  • sec. 25, T. 15 N., R. 1 E. Bridgeport quadrangle
Logs and wood fragments in bed of organic silt below T3 sequence 17,650+190


One age was obtained from a spruce log exposed in a large bluff at the confluence of Fishback Creek and Eagle Creek Reservoir (sec. 28, T. 17 N., R. 2 E., Zionsville quadrangle). The log was found pressed into the top of a bed of organic silt immediately below basal till (T1) at the base of the Trafalgar megasequence. The age of this log indicates that Wisconsin ice first arrived at this locality sometime shortly after about 21,000 yr B.P.

The apparent ages at the two localities, although somewhat different, are generally comparable, and seem to indicate that the onset of Wisconsin glaciation in Marion County took place between 21,000 and 22,000 yr B.P., which is generally compatible with age relations elsewhere in central Indiana (Bleuer, oral commun., 1993). The latest ice movements in this area, however, appear to have occurred at a considerably later date. At Plainfield (Hendricks County), just west of Marion County, several radiocarbon ages from wood taken from an organic bed immediately beneath a thin, upper till sequence are between 17,000 and 18,000 yr B.P. Based on gross geomorphic evidence and sequence characteristics, it seems likely that the till above the organic unit at Plainfield is equivalent to sequence "T3" in Marion County. If this interpretation is correct, ice may have been active in Marion County as recently as 17,000 yr B.P., and outwash deposition in the White River valley probably continued for a long duration after that.

Nature of Late Wisconsin Events and Sequences

Till and till-like sediment are the most common constituents of the Wisconsin depositional sequences recognized in Marion County, but sand and gravel bodies, some of which are of considerable size, also occur both within and between the sequences. In addition, the massive complex of Wisconsin outwash sand and gravel that occupies the White River valley and its major tributaries constitutes a related, but different, type of sequence. The outwash appears to have had a complicated history. Its deposition occurred at several distinctly different times and was associated with all three of the ice advances, as well as with meltwater discharges that occurred after active ice margins had retreated far upstream from Marion County.

The first two Wisconsin ice advances appear to have been more robust than the third, consequently their associated sequences are generally thicker and have greater continuity. The ice sheets associated with both of the first two episodes advanced nearly to Martinsville (Morgan County), well south of Marion County, and probably completely covered the White River valley in Marion County. Relatively thick (15 to 30 ft [4.6 to 9 m] or more) till units and other till-like sediment are widely associated with both of these sequences. Although the till units are commonly irregular and discontinuous, and, in places, broken by large sand and gravel bodies, their widespread distribution and thickness suggests that the ice remained active for a considerable period during both events. Both events contributed large quantities of sand and gravel to the White River valley, chiefly in the form of extensive outwash aprons deposited in front of each advancing ice sheet.

The second episode (T2) also resulted in end moraines that formed as the ice front became stationary at certain times during its general retreat. The arcuate shapes of the moraines suggest that large reentrants were present in the ice front along major meltwater channels, such as the White River (fig. 2). The tall, hummocky ridges in the Glenns Valley area (Maywood quadrangle) are part of a feature known as the Greenwood moraine. These ridges are composed chiefly of sand and gravel, and their up-ice (northeast) sides are capped by a thin veneer of T2 till. They represent the heads of one or more large, coalescing ice-contact fans formed along this part of the moraine. The body of the fan(s) formerly occupied what is now the modern valley of the White River, but was subsequently eroded away by massive meltwater discharges during later events. The meltwater erosion left only a few small, streamlined mounds of sand and gravel projecting above the general floor of the valley, along with the isolated ridges near Glenns Valley to mark the heads of the fan(s). Interestingly, the river channel in this immediate area follows an unusually circuitous route that appears to have a vaguely radial symmetry about the ridges at Glenns Valley (fig. 2). This pattern strongly suggests that the river channel migrated to the west along the toe of the growing fan during T2 time.

In contrast to the first two glacial episodes, the latest (T3) advance appears to have been much less robust. The depositional sequence associated with this event is quite unlike the earlier ones, and generally does not contain a persistent till sheet or other readily definable units, and may not even be recognizable as a distinct sequence in many places. Where recognizable, the T3 sediments form a heterogeneous assemblage of debris flows, small granular units, lacustrine sediments, and very thin (generally less than 5 ft [1.5 m]), discontinuous till units. Collectively, these materials make up an ablation complex; that is, a diverse mass of sediments deposited in, on, against, and locally under, stagnant, downwasting ice. This latest assemblage typically forms a veneer less than 20 ft (6 m) thick, draped atop sediments and landforms of the two earlier sequences. It is characteristically accompanied by a well-developed, low-relief, hummocky disintegration landscape (fig. 3), which contains numerous small depressions, ablation hummocks, disintegration rings, and very small ridges composed of till-like sediments. This particular landscape occurs nowhere else in Marion County, and appears to be uniquely associated with the T3 event.

T3 ice evidently did not extend south of Marion County. Instead, its terminal position lay across the central part of the county and, at least to the east of the White River, is quite clearly marked by an abrupt change in surface morphology (figs. 2 and 3). The most prominent features associated with the T3 event are Crown Hill and the Bunker Hill moraine. The latter is a tall, hummocky ridge in the south-central and southeastern parts of the county that is more or less paralleled by Shelbyville Road (fig. 3). The Bunker Hill moraine appears to be the terminal moraine of the T3 event, as it marks the sharp break between the characteristic T3 disintegration landscape to the north, and older T2 landscapes to the south. Interpretations of gamma-ray logs from wells in the Beech Grove quadrangle suggest that the sediments within the moraine contain a greater proportion of till than does the typical T3 assemblage elsewhere (fig. 1). The hummocky terrain of Crown Hill may also mark the terminal position of T3, and is separated from the discontinuous northwestern end of the Bunker Hill moraine only by the valley of Fall Creek near its confluence with the White River.

Figure 3.
Section of the Beech Grove 7.5-minute topographic quadrangle, T. 14 N., R. 4 E., illustrating landscapes associated with different Wisconsin glacial events. The characteristic hummocky disintegration topography associated with T3 ice is well displayed in the northeastern part of the figure, especially in sec. 12. This type of landscape extends over most of northern and eastern Marion County, and contrasts sharply with the smoother landscape to the southwest (for example, in secs. 14 and 15), which was formed on T2 till. East of the White River valley, the two landscapes are separated by the Bunker Hill moraine, the high, hummocky ridge that trends between the northwest and southeast corners of the figure, parallel to Shelbyville Road.

It is also possible that the T3 sequence is simply draped over older T2 topography in these areas, giving the appearance of a T3 terminal moraine. This is certainly the case west of the White River, especially in the area between Indianapolis International Airport and Eagle Creek, where a very thin T3 ablation sequence is plastered atop older T2 ridges and ice-marginal positions. In that area, the feather edge of the T3 advance is not clearly defined, and can be identified only as lying within a very broad zone. This situation is in marked contrast with the relatively strong topographic expression of T3 east of the river, however. The contrasting morphology implies that there may have been a fundamental difference in the behavior and depositional mechanics of the T3 ice sheet on opposite sides of the river. In any event, regional relations tend to favor the interpretation of Crown Hill and Bunker Hill as disjunct parts of a T3 end moraine, although there is no firm proof of either possibility.

The primary effect of the T3 event appears to have been the release of massive volumes of meltwater into the White River drainage, along with the cutting of several major sluiceways that drained the disintegrating ice sheet (fig. 2). The gorges of Fall Creek, Mud Creek, and some segments of Eagle Creek and its tributaries were apparently formed as tunnel valleys or other types of subglacial channels during the T3 event, for they cut sharply through all of the other sequences. The straight, narrow segment of the White River west of Crown Hill may have also been initially cut in a similar fashion, for it is generally gorgelike in character and completely unlike the rest of the river valley in terms of its morphology and underlying sequences. This segment of the river also experienced a variety of later events, however, both as an ice-marginal channel and as the main sluiceway for meltwater released much further upstream, consequently its history is much more complex. In any event, all these subglacial channels may have been the sources of episodic, and probably catastrophic, outbursts of meltwater as the ice sheet disintegrated en masse. In southern Marion County, the presence of large-scale streamlined landforms within the White River valley, together with nearby outwash terraces atop T2 till and as much as 50 ft (15.24 m) or more higher than the main outwash plain, attest to the great volume of meltwater that must have filled the valley as the Huron-Erie Lobe disintegrated. These meltwater outbursts scoured the valley, and undoubtedly contributed to the strongly streamlined appearance of many parts of the outwash plain. They were also probably responsible for eroding the body of the Glenns Valley fan complex.

Based on the above-mentioned characteristics, it seems likely that the ice associated with the T3 event may have been a relatively thin slab that advanced very rapidly to its terminal position, before abruptly melting in place, as suggested by Bleuer (oral commun., 1993) for similar sequences in west-central Indiana. This model would best explain the existence of a discontinuous terminal moraine, the predominance of ablation deposits in the T3 sequence, and the late, and apparently catastrophic, meltwater outbursts that affected this part of the White River valley.

Nature of Mapped Sequences and Features

The amalgamation of the Wisconsin depositional sequences has resulted in a diverse suite of glacial terrains across the county, which, in turn, contain an equally varied range of hydrogeologic settings . The complex and extremely variable nature of these sequences create uncertainties in interpretation that make it impractical to map each particular hydrostratigraphic element (for example, discrete aquifers and confining units) on an individual basis throughout the county. In addition to the variation inherent within the sequences themselves, the nonuniform distribution and widely ranging quality of subsurface data pose several difficulties for systematic three-dimensional mapping. For example, in the absence of abundant gamma-ray logs or unusually detailed well records, it is commonly problematic to determine the true lateral extent, stratigraphic position, or depositional sequence (namely, T1, T2, or T3) of many of the small- and medium-sized sand and gravel bodies commonly present within the Trafalgar megasequence. Despite their apparently limited areal extent (typically less than 1/4 square mile [0.4 square km]), some of these bodies can locally be up to 50 ft (15.24 m) thick, and thus have obvious hydrogeologic importance because they may physically interconnect sand and gravel aquifers above and below. It is similarly difficult to systematically relate all the local till-confining units above and below these aquifers to the three depositional sequences.

To counter these difficulties, two parallel approaches were employed to depict the Wisconsin glacial deposits. First, a purely terrain-based approach was utilized to identify specific glacial terrain regions. Each region is generally characterized by a similar range of conditions with respect to the presence, continuity, size, and geometry of sand and gravel bodies and fine-grained units, and their relationship to surface morphology. Second, the largest and (or) most persistent sand and gravel units were mapped on an individual basis; additionally, the thickness and extent of fine-grained confining units were mapped, but only in relation to mappable sand and gravel bodies, rather than to any one particular depositional sequence. In general, a flexible mapping methodology was used to emphasize different kinds of sequence characteristics in different terrains.

This dual approach resulted in the construction of up to four types of maps for each of the twelve quadrangles in Marion County. The different maps must be used together to effectively represent the geology of the Wisconsin deposits. That is, no single map represents the complete geology of the late Wisconsin sequence. Specific features shown on each of the four types of maps are:

  1. glacial terrains, including the general nature of the underlying depositional sequences, and the nature and relationship of the landscape to sequences and major events;
  2. the thickness of all till units and associated fine-grained sediments that overlie mapped sand and gravel bodies within the Trafalgar megasequence. These units generally correspond to the T2 and (or) T3 events, but for simplicity, all the fine-grained units and sand and gravel bodies shown on this map are referred to generically as the "upper sequence";
    1. the thickness of all till units and associated fine-grained sediments that underlie mapped sand and gravel bodies of the upper sequence (above), and/or that overlie outwash aprons at the base of the Trafalgar megasequence. These units generally correspond to the T1 and (or) T2 events; or
    2. the aggregate thickness of the entire late Wisconsin sequence where it appears to be composed chiefly of till and related fine-grained materials. For simplicity, these packages are referred to as either (a) the "lower sequence," or (b) the "total sequence," respectively;
    3. the thickness of unconfined sand and gravel in the White River valley and in nearby reaches of major outwash-bearing tributaries (for example, Eagle Creek, Fall Creek), and the nature of the substrate the outwash was deposited on. This type of map was compiled only for parts of the Maywood, Indianapolis East, Indianapolis West, Clermont, Carmel, McCordsville, and Fishers quadrangles.

    Each map is explained further in the following sections, which also provide an expanded description of the origin and geometry of the various mapped features and sequences. The relationship of map units to sequence configuration is shown on the schematic cross-section diagram on plate 2. Reference to this diagram may be useful during the following discussion.

    Glacial Terrain Maps

    The depositional sequences and their attendant landscapes collectively form a group of glacial terrains. Each terrain is characterized by a particular association of landforms that, at least in part, reflects the particular succession of events and style of glacial deposition that produced the vertical sequence of sediments below it. Stated differently, the landscapes and materials encompassed within each glacial terrain region share a common local geologic history, and are, therefore, expected to generally be characterized by a similar range of both surface and subsurface geologic conditions.

    As a whole, each glacial terrain map provides a regional overview of surface composition and morphology, subsurface sequence, and the nature of the different depositional and erosional processes that have affected each landscape. In this light, a terrain map acts as a "road map" for guiding the user through the more sequence- and materials-oriented geologic units presented on the other types of maps. A terrain map can also be used to make general assessments about the geology of an area where subsurface data are sparse or where the continuity, geometry, and other characteristics of individual units are particularly uncertain.

    Each terrain map contains three overlays of information that, when combined, collectively form basic terrain units. Each overlay represents a specific category of terrain features or elements that differ from the other categories in terms of its basic definition and the scale of the defining features. Progressing from the largest to the smallest included features, these three overlays are:

    1. the regional distribution of three generalized types of Wisconsin vertical sequences. Each of the three basic vertical sequence types so defined reflects a predominance of a particular process or group of related processes that deposited the sequence (for example, processes related to large-scale channelized meltwater). This category is therefore defined simply as "Vertical sequence" in the map explanation;
    2. the general morphology, composition, and history of the landscape overlying each vertical sequence. This category consists of landscapes of intermediate to regional scale, and is defined as "Morpho-terrain";
    3. individual, distinct landforms or groups of landforms that signify deposition and (or) modification by a particular process or succession of processes. This category consists of local-scale features within each landscape, and is defined in the explanation as "Individual terrain elements."

    The style and complexity of map construction are largely controlled by the presence, geometry, and continuity of mappable sand and gravel units of Wisconsin age in the different terrain regions identified on the glacial terrain maps. In general, regions identified on the terrain maps as being underlain by "till-dominated sequences" present the least complicated situations, and in many cases, can be adequately characterized by one or, in some cases, two maps. In contrast, regions identified as having "mixed" or "sand and gravel-dominated" sequences are considerably more complicated, and require a somewhat different mapping approach using two or three maps to effectively characterize the range of sequences. All three sequence types are present in each of several quadrangles, necessitating the use of both mapping approaches on the same maps. On all such maps, the major terrain boundaries that mark this change in mapping style are prominently identified.

    Late Wisconsin Sequence Maps — Till-Dominated Sequences

    Till-dominated sequences are characteristic over nearly all the upland areas within Marion County. In the simplest case, the Trafalgar megasequence consists chiefly of till and till-like sediment between the land surface and the pre-Wisconsin surface. In this setting, the entire megasequence can be represented on a single map that uses a 20-ft (6-m) contour interval to show the total thickness of all till units within it. Very small lenses of sand and gravel are typically present in these situations, but are too thin (typically less than 5 ft [1.5 m]) and of far too limited extent (reported in widely scattered logs) to show at the map scale. This type of setting and the corresponding map are referred to as a "total sequence." It is important to note that some areas are mapped this way by default, because few or no data exist to map sand and gravel units and because some drillers simply do not report all the sand and gravel units.

    In some upland areas, the Trafalgar megasequence contains somewhat larger, but discontinuous bodies of sand and gravel along various horizons between and within the till and other fine-grained materials. They are regarded as discontinuous because they are typically not reported in a significant percentage of the wells in the given area in which they generally occur. Some units are reported to be as much as 10 to 20 ft (3 to 6 m) thick, and in some areas they appear to be localized along persistent horizons. Because of their uncertain continuity, however, they are simply mapped as "discontinuous" sand and gravel within the total late Wisconsin till-dominated sequence. Map units of this type are numerous, and are intended to call attention to areas where sand and gravel bodies of presumably small size may be concentrated, without denoting any particular geometry or specific stratigraphic position within the Trafalgar megasequence.

    In other places within the till-dominated sequences, sand and gravel units within the Trafalgar megasequence appear to be relatively larger, more abundant, and perhaps more continuous, and thus can be mapped as separate bodies, albeit with an uncertain degree of precision. Map units of this type may be of any size, but the sand and gravel is typically reported in more than 75 percent of the data points over an area of 100 acres (40 hectares) or more, and is consistently greater than 5 ft (1.5 m) thick. The great majority of these bodies are mostly or entirely enclosed within till, and geographically separate bodies commonly occur at different depths. In such instances, mapping the entire thickness of late Wisconsin till is not particularly useful for hydrogeological purposes, because that would give little indication of the settings of these bodies within the total megasequence. Instead, we judged that mapping the thickness of the till above the bodies separately from the thickness of the till below is most appropriate. This approach also provides a convenient means for highlighting places where till is locally absent above or below parts of these bodies. All these situations involving mappable sand and gravel bodies within the till-dominated sequence are best represented using two different maps, as follows.

    Mappable sand and gravel bodies that occur within the Trafalgar megasequence, as well as the thickness of any overlying till, are shown on one map. These units are collectively and informally referred to as the "upper sequence," and the map as the "upper sequence map." Discontinuous sand and gravel units are also shown on the upper sequence map, but the thickness of the till above the discontinuous units cannot be reliably contoured, because no clear break in the total Wisconsin till sequence is discernable where these units are absent. The units shown on the upper sequence map generally correspond to the T2 and (or) T3 events outlined previously. A second map shows the outlines of the mappable sand and gravel bodies plus the thickness of any till units that occur below. The units shown on the second map collectively constitute an informal "lower sequence," and, in general, then correspond to the T1 and (or) T2 events described earlier. In places where there are no till units present below the sand and gravel (namely, the bottom of the sand and gravel body is on the pre-Wisconsin surface), the sand and gravel unit is shown on the lower sequence map as a solid color, enclosed by a zero till-thickness contour. Map units showing the lower sequence are combined on the same map with the "total sequence" contours described above.

    Late Wisconsin Sequence Maps — Mixed Sequences

    Mixed sequences typically contain abundant sand and gravel bodies in addition to till units. Although some parts of these sequences consist primarily of till, the sand and gravel generally constitutes a roughly equal or somewhat greater proportion of the total late Wisconsin sequence in most places. The regions underlain by mixed sequences mostly border the White River valley and are widest in the southern half of the county.

    Most of the sand and gravel units within the mixed sequences form a relatively extensive complex of fans, aprons, and channels. The largest individual bodies cover several square miles and are up to 100 ft (30.5 m) thick, although most are between 20 and 60 ft (6 and 18 m) thick. We interpret these bodies to have been deposited during the T1 and T2 ice advances, and they are closely associated with the two till sheets believed to have been deposited during those advances. The fans and aprons grade laterally into, and form part of, the outwash complex within the White River valley.

    The mixed sequences are represented using a two-map format that is generally similar in concept to that employed for mappable sand and gravel bodies in the till-dominated uplands, with the presence and thickness of an "upper" and a "lower" till sequence being shown on separate maps, using 20-ft (6-m) contours. The major difference is that the thicknesses of the granular units below each till sheet are shown on the appropriate map utilizing a color scheme that corresponds to 20-foot (6-m) thickness intervals. At some places, data were sufficiently concentrated to allow the use of a 10-foot (3-m) contour interval for the sand and gravel and fine-grained units. Either of the till sequences may be absent in some places within these terrain regions. In places where the lower till sequence is missing, the sand and gravel units above and below have coalesced, resulting in composite granular units that are quite thick. The thickness and extent of these composite units are shown below the capping till (T2 + T3) on the "upper sequence map," inasmuch as there is no practical way to delineate any boundary between the "upper" and "lower" sequences within the thick section of sand and gravel. The boundary between terrain regions where this mapping style is used versus adjacent till-dominated regions where the previous format is employed, is indicated clearly on the appropriate quadrangles.

    Late Wisconsin Sequence Maps — Sand and Gravel Dominated Sequences

    Along the White River valley and adjacent parts of its major tributaries, the mixed sequences described above are usually overlain by a variable thickness of younger, unconfined outwash. In addition, the underlying till units become increasingly thin and discontinuous toward the axis of the valley, while the fans and aprons become progressively thicker. Along the margins of the valley, there also are numerous "cut-outs" of the till units, resulting in composite sand and gravel thicknesses that range from less than 10 ft (3 m) to well over 100 ft (30 m). Sand and gravel typically accounts for a substantial majority of the vertical sequences within this area, which generally corresponds to the "sand and gravel dominated sequences" shown on the terrain maps.

    The same two-map format described above for the mixed sequences is extended into the sand and gravel dominated sequences of the White River valley; that is, the thicknesses of large, laterally extensive sand and gravel bodies and their overlying till units are shown using a combination of colors and contours, respectively. It was necessary to construct a third type of map for these areas, however, to accommodate the increased degree of complexity associated with the unconfined outwash that caps the vertical sequence. These "unconfined outwash maps" are conceptually very similar to the other two types of maps, in that they utilize a range of colors to show the total thickness, in 20-ft (6-m) increments, of the outwash complex from the land surface down to the top of the first till unit in the vertical sequence or, if no till unit is present, to bedrock. Different line patterns indicate the type of fine-grained unit (for example, late Wisconsin upper or lower sequence, or undifferentiated pre-Wisconsin till) present at the base of the contoured outwash. The outwash itself is regarded as a complex, because in numerous places it appears to represent the coalescing of latest surficial outwash with one or more fans, aprons, and (or) other granular bodies that crop out from beneath the edges of adjacent late Wisconsin (and in some places, pre-Wisconsin) till units.

    References

    Brown, S. E., and Laudick, A. J., eds., 2003, Hydrogeologic framework of Marion County, Indiana — a digital atlas illustrating hydrogeologic terrain and sequence: Indiana Geological Survey Open-File Study 00-14, CD-ROM.

    Fleming, A. H., Brown, S. E., and Ferguson, V. R., 1993, Hydrogeologic framework of Marion County, Indiana: Indiana Geological Survey Open-File Study 93-05, 67 p.

    Hartke, E. J., Ault, C. H., Austin, G. S., Becker, L. E., Bleuer, N. K., Herring, W. C., and Moore, M. C., 1980, Geology for environmental planning in Marion County, Indiana: Indiana Geological Survey Special Report 19, 53 p.

    Wayne, W. J., 1963, Pleistocene formations in Indiana: Indiana Geological Survey Bulletin 25, 85 p.



 
 
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