Northern Indiana was repeatedly covered by continental ice sheets during the past one million years or so, which produced a thick section of unconsolidated sediments that mantle the bedrock to depths as great as 500 ft. The bulk of the glacial sediments in Allen County were deposited during the most recent period of glacial activity, known as the late Wisconsin Stage. Older deposits are preserved in deeply buried bedrock valleys in northern Allen County, but are masked by the modern land surface, whose form is entirely the result of late Wisconsin glaciers and subsequent postglacial events. As far as is known, all of the important unconsolidated aquifers and confining units date from this most recent period of glacial activity.

The flow of continental ice sheets developed along distinct axes paralleling the Great Lakes basins, producing well-defined glacial lobes, each having a characteristic set of landforms and sediments (figs. 1 and 2).

Figure 1.
Map of Allen County showing selected glacial landforms and related features (from Fleming, 1994a).

Figure 2.
Surficial geologic map of Allen County as presented in the interactive map portion of this website. The map is presented in more detail as Plate 5 of Indiana Geological Survey Special Report 57 (Fleming, 1994a).

Three major late Wisconsin glacial episodes are recognized in Allen County, beginning with the advance of the Huron-Erie Lobe into the county from the northeast about 22,000 years ago (fig. 3). These lobes were not all present at the same time and individually advanced and retreated over different time periods. The Erie Lobe was the last glacial lobe to advance and then retreat from northeastern Indiana.

Figure 3.
Generalized map of the glacial lobes present in Indiana during the late Wisconsin age. This map is modified from Bleuer (1974b).

This event produced the most widespread sequence of glacial deposits, known as the Trafalgar Formation, which is found throughout the subsurface in the county and extends well south of Indianapolis and westward into eastern Illinois. This sequence is dominated by abundant fragments of bedrock units that crop out east of Allen County—Ordovician limestone from the Lake Ontario basin, and Silurian dolomite, Devonian limestone, and Devonian black shale from Ohio — indicating the direction of ice flow from east to west (fig. 4).

The sample shown in figure 4 is from a gravel unit between the Lagro and Trafalgar Formations, collected via downhole sampling of a well drilled in Jefferson Township, eastern Allen County. A second sequence, known as the Huntertown Formation and found only in the subsurface of northern Allen County, contains characteristic fragments of sandstone, coal, and red mudstone from the Michigan Basin (fig. 5), as well as distinctive metamorphic rocks from the north shore of Lake Huron, suggesting that the ice that deposited it came from the north. The sample shown in figure 5 is from a gravel unit in the Huntertown Formation, collected in a downhole sample set from a well drilled in Perry Township, northern Allen County. The ice sheet that deposited this material is called the Saginaw Lobe.

The late Wisconsin culminated in the readvance of the Erie Lobe into northeastern Indiana, which produced the third major sequence of deposits. Known as the Lagro Formation, this last sequence immediately underlies and is responsible for the form of the modern land surface throughout most of the county and is characterized by its extremely clay-rich composition, reflecting the fact that the ice sheet advanced across the bed of a vast glacial lake containing clays (the forerunner of Lake Erie) as it approached Allen County. As the Erie Lobe melted back out of northeast Indiana and northwest Ohio about 12,000 years ago, another large glacial lake, known as Glacial Lake Maumee, formed in front of the retreating ice front. The subsequent catastrophic drainage of the lake had a profound effect on the Allen County landscape, and triggered a series of events that led to the development of the modern continental drainage divide upon which Fort Wayne was founded. All three glacial sequences, as well as the post-glacial deposits associated with Glacial Lake Maumee, are important to a variety of groundwater, geotechnical, and environmental issues, and each is discussed briefly below. The origin, characteristics, and hydrogeology of all the units depicted on the surficial geology map are discussed in greater detail in Indiana Geological Survey Special Report 57 (Fleming, 1994a).

Figure 4.
A sample of an eastern source (Erie Lobe) gravel.

Figure 5.
A sample of northern source gravel presumably deposited by the Saginaw Lobe.

Trafalgar Formation

Figure 6.
Map of Allen County showing the political subdivisions and the geographic localities referred to in the text.

The Trafalgar Formation (Wayne, 1963; Bleuer, 1974a; Bleuer and Moore, 1972; 1978; Fleming, 1994a) is at the land surface in much of central Indiana (Gray, 1989); however, it is buried by younger glacial deposits and is only exposed in stream valleys and ravines in much of Allen County. The unit forms a prominent horizon throughout the county at depths ranging from as little as a few feet below downtown Fort Wayne and the Wabash-Erie Channel to more than 150 ft in the northern sections of the county (fig. 6). The Trafalgar Formation is a sequence of loam-textured till, sand and gravel, silt, and mud flows; however, it is composed chiefly of very hard, stony, loam-textured till (fig. 7).

This basal outwash is an important aquifer throughout the county. Other types of sand and gravel bodies are also abundant in the Trafalgar Formation. Most of these consist of small, localized lenses enclosed in the hard till, but much larger bodies do occur, especially in southwest Allen County, where a series of sizable, wedge-shaped bodies of channel-like form are prominent. Some of these can be traced for miles in the subsurface, and appear to mark the locations of earlier meltwater outlets active when the Trafalgar Formation was being deposited. Many of the channel deposits in Aboite and western Wayne Townships (fig. 8) are 50 to 100 ft thick and extend completely through the formation, where they allow groundwater recharge through the otherwise poorly permeable till.

Surface exposures of the Trafalgar Formation (shown as map units T and Tc on the surficial geology map) are limited to a few ravines and other low-lying areas near the Wabash-Erie Channel, where the Maumee Torrent or modern streams stripped off overlying sediments such as the Lagro Formation.

The till is severely overconsolidated and at places it is harder than some of the bedrock, which sometimes creates difficulties for shallow excavations and test drilling with small equipment. The same characteristic makes the top of the Trafalgar Formation a readily recognizable and mappable subsurface horizon throughout the county. The base of the formation is commonly marked by a robust sheet of sand and gravel greater than 30 ft thick, deposited by meltwater in front of the advancing ice sheet (fig. 8).

Figure 7.
This photograph shows a typical hard, stony, basal meltout till of the Trafalgar Formation. This unit has conspicuous sub-horizontal fabric produced by aligned platy clasts of dark Ordovician limestone, black Antrim (Ohio) Shale, and tan Paleozoic limestone and dolomite, and by a vague layering of till units separated by thin horizontal partings filled with sand. Stone Street Quarry, Marion Township, southern Allen County.

Figure 8.
This picture is of loamy debris flows embedded in the top of the basal outwash unit (gravelly sand) of the Trafalgar Formation. The debris flows are remobilized till thought to have flowed off of the advancing ice front, just before the glacier overrode the outwash. Stone Street Quarry, Marion Township, southern Allen County.

Huntertown Formation

The Huntertown Formation (map unit H) was named by Fleming (1994a) for a distinctive group of predominantly sandy sediments in the shallow subsurface of northern Allen County. The sequence contains abundant and commonly large, bodies of sand and some gravel (fig. 9) that account for as much as 50 to 75 percent of the apparent thickness of the unit in most locations. The granular units are associated with loamy to sandy till and other diamictons. A typical vertical sequence (fig. 10) consists of a thick, clean, basal sand, overlain by a massive sandy till with gravel bodies of various shapes and sizes in the upper parts. At least two sand-filled channels (tan color) are visible in the till. The Huntertown sector shown in figure 10 is a light gray sandy basal outwash at bottom of photo is overlain by massive, dark, sandy loam till. At least two sand-filled channels (tan color) are visible in the till. Borrow pit along Indiana SR 3, Perry Township, northern Allen County.

In Eel River and Perry Townships (Fig. 8), it is not uncommon for the till to be thin or absent, resulting in a thick composite section of sandy sediments. All of these lithologies contain features strongly suggestive of a northern (Saginaw Lobe) provenance or source: conspicuous clasts of coal, quartz sandstone, reddish mudstones, and vein quartz from the Michigan Basin; fragments of Gowganda Tillite and Loraine Conglomerate (two-billion-year-old rocks of glacial origin) from the north shore of Lake Huron; and enrichment of the clay fraction of the till with kaolinite. Bleuer and Moore (1974; 1978) noticed similar features and attributed the unit to the Saginaw Lobe. The formation has its maximum development in Eel, Perry, northern Washington, and western Cedar Creek Townships, where it locally attains thicknesses of 100 to 120 ft; it thins to the southeast, grading into increasingly abundant fine sands (fig. 11), silts, and loamy diamictons of apparent lacustrine origin (fig. 12) in the vicinity of the St. Joseph River, before pinching out entirely below the north shore of the Maumee Lacustrine Plain and the north side of Fort Wayne.

The Huntertown Formation constitutes a major aquifer system that is the dominant source of water in the northwestern third of the county (Fleming, 1998a, b), and which contributes as much as 14 million gallons per day of ground water discharge to the St. Joseph River in northern Allen County (Arvin, 1989).

Figure 9.
A large body of slightly gravelly sand (light gray) underlying thin, sandy loam till (dark colored). Borrow pit along Indiana SR 3, Perry Township, northern Allen County.

Figure 10.
A section of the Huntertown Formation with dimensions of the field of view approximately 30 ft wide by 20 ft high.

Figure 11.
Fine to medium-grained lacustrine sand at the distal edge of the Huntertown Formation. Collected from well drilled into the large sand unit underlying the Harlan area, Springfield Township, northeastern Allen County.

Figure 12.
Rhythmically layered graded beds in lake sediment underlying the Lagro Formation in the Maumee Lake Plain. These sediments were deposited from turbidity currents during the Erie Interstadial. Each bed consists of a lower, light-colored section of fine-grained sand and silt, grading up into the dark silty clay at the top. The sandy base of the next overlying bed is in sharp contact with the underlying bed. Woodburn Quarry, Maumee Township, eastern Allen County.

Lagro Formation

Figure 13.
Two till units of the Lagro Formation separated by a 1 to 2 in. layer of pebbles and sand. The upper unit is silty clay, the lower unit is clay loam and contains slightly more pebbles. Stone Street Quarry, Marion Township, southern Allen County.

The Lagro Formation (Wayne, 1963) encompasses the extensive group of clay-rich tills and glaciolacustrine sediments that blanket the surface over most of the county. These sediments were deposited by the Erie Lobe following the Erie Interstadial (Fleming, 1994a); the clayey textures were imparted by large amounts of silt and clay incorporated by the ice sheet as it advanced through the silt and clay-rich beds of the broad proglacial lake in western Ohio. The texture of the Lagro Formation is typically a silty clay within and to the east of the Wabash Moraine, whereas to the west of the moraine, it is consistently a clay loam (coarser-textured). The finer-textured till can be seen overlying the coarser textured till in some exposures just inside the Wabash Moraine (fig. 13).

These distinctly different textures are thought to reflect two different periods of ice advance, separated by a minor retreat of the ice front prior to a readvance to the Wabash Moraine. The hiatus was evidently of sufficient length to enable a proglacial lake to develop in the Erie Basin and begin accumulating silt and clay, which were picked up and incorporated into the ice when it readvanced. Large bodies of strongly deformed silt and clay are common at the base of the sequence (fig. 14).

Most of the Lagro consists of massive basal till (fig. 15) that contains relatively few large clasts, exhibits a strong microfabric produced by parallel orientation of sand grains and sheared lenses of silt and clay, and is pervasively fractured (fig. 16).

Figure 14.
Contorted unit of silt and sand under basal till of the Lagro Formation. The sediment was deformed as the ice overrode it. Stone Street Quarry, Marion Township, southern Allen County.

Most of the fractures occur in two near-vertical sets oriented at 60-degree angles to the local direction of ice flow, which is generally in a westward direction (Fleming, 1994a). This conjugate relation to the direction of principal stress suggests that the fractures are the result of glaciotectonic stress produced by the motion of the overlying ice sheet as the till was deposited. The thickness of the Lagro ranges from 20 to 50 ft in the lake and till plains (map unit Lt) to 60 to 100 ft in the moraines (map unit Lte). In the hummocky interlobate region of northwest Allen County (map unit Lt/H), the character of the Lagro is much more variable than elsewhere. There, the till is considerably less uniform in both texture and thickness (fig. 17), contains many more sand and gravel bodies than elsewhere, and shows little continuity across that landscape.

This is thought to reflect the variable bed conditions produced as the Erie Lobe attempted to override obstructions produced by preexisting hummocky topography and large blocks of stagnant ice as it climbed a sharp regional slope north of the Wabash Moraine and Eel River (Fleming, 1994a; 1998a, b).

The fine-grained Lagro tills are the main soil-forming parent material in most of the county, and they serve as the principal confining unit that protects underlying aquifers. The unit has a dual porosity: the fine-grained matrix exhibits very low hydraulic conductivities (10^-7 to 10^-8 cm/sec; Fleming, 1994a), whereas the fracture systems impart much greater bulk permeabilities and promote groundwater recharge to considerably greater depths (Ferguson, 1992; Ferguson and others, 1991, 1992). In outcrop, water is commonly observed to seep from large, open fractures (fig. 18), while the till adjacent to the fractures exhibits strong oxidation haloes up to 24 inches wide.

The fracture surfaces are characteristically encrusted with calcite, gypsum, and various iron and manganese oxides (fig. 16). These features indicate the presence of a dynamic shallow groundwater flow system associated with the fractures.

Figure 15.
Very clayey till of the Lagro Formation overlying sandy gravel. The gravel beds dip to the right (west), parallel to the flow direction of the meltwater that deposited them. The ruler for scale is 6 in.; StoneCo Quarry, Aboite Township, southwestern Allen County.

Figure 16.
Sets of large fractures in basal till of the Lagro Formation with the field of view approximately 20 ft wide by 15 ft high. One set of joints forms the broad, flat, gray surfaces subparallel to the photo, the other set is oriented toward the viewer. The two sets intersect at 60 and 120-degree angles about the local direction of ice flow. The surfaces of the joints are encrusted with groundwater deposited gypsum; some surfaces are stained with oxides of iron (orange) and manganese (black). Stone Street Quarry, Marion Township, southern Allen County.

Figure 17.
Irregular mudflow and muddy outwash exposed on the side of a hummock, Lagro Formation, Eel River Township, northwestern Allen County.

Figure 18.
Water seeping from an open fracture in the Lagro till. The iron staining is produced when iron dissolved in the ground water under reduced conditions is exposed to the atmosphere and is oxidized. The iron probably is derived from dissolution of pyrite in the till. Aboite Creek, Aboite Township, southwestern Allen County.

Outwash

Figure 19.
Photograph showing steeply dipping and faulted beds at the head of a large outwash fan composed of sandy gravel. The ice front off of which the sediment was deposited lay just to the right of the photograph. Klink Sand and Gravel, Steuben County.

Sizable deposits of sandy and gravelly outwash are associated with all three of the glacial sequences described above. Most of the outwash exposed at the modern land surface (map units O, OH) was deposited by the Erie Lobe in meltwater channels such as the St. Joseph and Eel Rivers, and within outwash fans (fig. 19) such as those visible at Waynedale and Bicentennial Woods in northwestern Allen County (fig. 20).

These surficial outwash bodies are part of the Lagro Formation, and tend to be relatively thin, generally less than 50 ft thick. Though thin, these bodies are typically water saturated to within 10 ft or less of the land surface, and are in hydraulic communication with the adjacent streams and wetlands. Much more robust thicknesses of outwash (50 to 100+ ft) are found beneath upper Cedar Creek (map unit Ot), the Eel River Valley (map unit OH), and portions of the Waynedale fan and Times Corners area (southwestern Allen County), where the relatively thin Erie Lobe outwash is superposed directly atop older sand and gravel bodies of the Trafalgar and Huntertown Formations. Such locations act as important conduits for groundwater recharge to deep aquifers that are under confined (artesian) conditions elsewhere.

Figure 20.
Photograph showing slightly mucky surface of the Cedar Creek outwash fan near the Eel River Valley, Perry Township, northern Allen County.

Postglacial History and Deposits

Following the retreat of the Erie Lobe, portions of the landscape were dotted with melting blocks of ice of various sizes, especially in the northwest part of the county, where the disappearance of these blocks created numerous depressions. Small lakes, swamps, and marshes developed in the depressions, eventually leading to the accumulation of various thicknesses of organic sediments (map unit p), chiefly sandy and silty muck, and sedge peat (fig. 21).

Similar wetland environments developed in abandoned meltwater channels and low-lying parts of plains (the Eel River Valley and parts of unit Lts in western Allen County). Such depressional wetlands were abundant when the first European settlers arrived here, but most were drained to make way for agriculture. Groundwater beneath depressional wetlands in and around the hummocky interlobate area and Eel Valley of northwest Allen County exhibit distinct geochemical signatures and strong downward hydraulic gradients (fig. 22), suggesting that these terrains serve as a crucial regional groundwater recharge area for the Huntertown aquifer system and, ultimately, the St. Joseph River, which is Fort Wayne's water supply (Fleming, 1994a, b; Fleming and Yarling, 1994).

Figure 21.
Photograph showing water standing in a small depressional peat bog following a heavy rain. Eel River Township, northwestern Allen County.

The signal event in the immediate postglacial landscape of Allen County, however, was the development and catastrophic collapse of Glacial Lake Maumee (Dryer, 1889; Leverett and Taylor, 1915; Bleuer and Moore, 1972, 1978). The lake formed between the retreating front of the Erie Lobe and the Fort Wayne Moraine, and at its maximum extent is thought to have held a volume of water comparable to or larger than modern Lake Erie. The bed of the old lake is essentially level and blanketed by stratified lake muds up to 15 ft thick (map unit Ltl); the mud was derived from meltwater debouching from the retreating ice margin and by waves whipping up sediment off the shallow parts of the lake and depositing in deeper parts of the lake below wave base. The margins of the lake are marked by small beach ridges, bars, and spits (map units b and s) composed chiefly of fine- to medium-grained sand, with some coarse sand and fine gravel. The largest beach ridge now serves as the route of SR 37 between I-469 and Hicksville, Ohio. Old Lincoln Highway (US 30) follows a smaller set of beach ridges along the south margin of the lake. The bluff at the southeast corner of the intersection of I-469 and US 30 offers a commanding view eastward across virtually the entire portion of the lake plain in Allen County.

Perhaps owing to a minor readvance of the ice front, the lake overtopped a sag in the moraine near what is now downtown Fort Wayne, causing a catastrophic outburst of water known as the Maumee Torrent, whose impacts can be seen for hundreds of miles down the Wabash River Valley (Fraser and Bleuer, 1988). The torrent scoured out the mile-wide outlet now known as the Wabash-Erie Channel (fig. 23), which extends from near New Haven (the Fort Wayne outlet) westward through downtown Fort Wayne and southwest Allen County, to Huntington. The floor of the outlet (fig. 24) was scoured down well into the hard till of the Trafalgar Formation, stripping off vast quantities of sediment and leaving a local boulder lag in the process.

Figure 22.
Photograph showing the same peat bog several day