Geology

From the air, the Wabash-Erie Channel resembles a huge scar on the landscape, extending southwestward for about 30 miles from the western apex of the Maumee Lake Plain, through downtown Fort Wayne and southwest Allen County, to the confluence with the main stem of the Wabash River at the forks. The eastern continental drainage divide, which separates the Great Lakes and Mississippi River drainages, crosses the channel just west of the St Marys River. The Little River occupies that portion of the Wabash-Erie Channel west of the divide. In historical times, the floor of the Little River Valley was dubbed “The Great Marsh" because it comprised a vast wetland complex covering some 25,000 acres.

Near the St Marys River, the valley has two distinct, sub-parallel segments: 1) the wide, northern segment, which includes Eagle Marsh, is referred to as the Fort Wayne Outlet, because it forms the head of the Wabash-Erie Channel at the Maumee Lake Plain; 2) the narrower southern segment originates at the St Marys River near Waynedale. The two segments are separated by the rolling, sandy upland historically known as the Sand Point, and by a smaller upland just to the west of that, known as Midway Island. The two channel segments gradually coalesce in the vicinity of Fox Island, and continue downstream as one broad channel to Huntington.

One of the first things the attentive visitor notices is the absence of any large stream commensurate with the broad, flat valley bottom, which is up to 2 ½ miles wide and lies between 75 and 125 feet lower than the adjacent uplands. This is the quintessential example of what geologists call an “underfit" valley: the modern stream within it (the Little River) is miniscule in comparison to the size of the valley, meaning that the valley must have been cut by a vastly larger river that existed at an earlier geologic time. Such a relationship implies a complex geologic history characterized by dramatic, even catastrophic, events. As it turns out, the geologic story of the valley is a fascinating tale with many twists and turns, involving enormous discharges of icy meltwater, constantly shifting drainages, and the repeated filling and exhumation of the valley with sediment over time. These events ushered in a remarkable series of rapid (by geologic standards) environmental changes, which explain why the Little River Valley possesses such a distinctive and ecologically valuable natural environment. In fact, it would be accurate to say that the ecological significance and the rich human history of the Great Marsh are both indelibly linked to the geological evolution of the valley.

Continental glaciers first entered Indiana more than 700,000 ybp (years before present), but the record of these early ice sheets is extremely fragmentary because each succeeding glaciation tended to remove the deposits left by earlier ones. Long interglacial periods characterized by temperate climates and much erosion also aided this process. As a result, all of the glacial deposits and associated landforms in and around the Little River Valley date from the most recent period of glacial activity, which is known as the Late Wisconsin Stage.

Two major episodes of Erie Lobe glaciation are recognized in the Late Wisconsin deposits in the vicinity of the Little River Valley. During the first of these, the ice sheet advanced as far as south-central Indiana and eastern Illinois before slowly receding. This episode produced the hard, stony tills and associated sand and gravel units known as the Trafalgar Formation, which occupy a large part of the surface of central Indiana and occur throughout Allen County in the subsurface. These deposits occur beneath the floor of the Little River Valley at many places. About 17,000 ybp, the Erie Lobe receded far back into the eastern Great Lakes, allowing a large glacial lake to form in front of the ice margin where Lakes Erie and Ontario now lie. This period is called the Erie Interstadial, and significant amounts of silt and clay accumulated throughout the basin of the interstadial lake. When the Erie Lobe subsequently readvanced towards Indiana during the second episode of glaciation, the ice sheet incorporated a large volume of the silt and clay into its sediment load as it overrode the lake bed. The result is the distinctive series of clay-rich glacial tills-known in Indiana as the Lagro Formation-that cover most of the northeast part of the state, and are the primary soil-forming material on uplands. At its maximum extent, this second glacier reached as far south as Muncie and as far west as Logansport. The ice margin receded slowly back towards the Erie basin, its retreat punctuated by the series of 5 looping end moraines that encircle northeast Indiana: the Union City, Mississinewa, Salamonie, Wabash, and Fort Wayne Moraines. The age of the Lagro Formation in Indiana is thought to be between 16,000 and 14,000 years. The clayey till from this event is not present in the bottom of the Little River Valley, because of later erosion, but it does comprise most of the valley walls and adjacent uplands.

Geologic evidence obtained in the subsurface of southwestern Allen County and further downstream in the Wabash Valley suggests that a major west-bound drainage existed in the vicinity of Little River Valley throughout the Late Wisconsin Stage. At times, the drainage took the form of a major, subaerial sluiceway draining away from the ice margin, but at other times, it appears to have consisted of one or more large, ice-walled channels within the ice sheet-in other words, streams that were bounded by walls of ice hundreds of feet high, and which acted as a sort of internal plumbing system within the glacier. In any event, by the time the Erie Lobe began to retreat from Indiana, the stage was set for a remarkable series of events that would shape the Little River Valley into one of the most unique and history-rich natural areas in the state, and which continue to influence many aspects of modern life in the ‘Summit City’.

The Fort Wayne Moraine is the innermost moraine of the series, and forms the rolling uplands just inboard of the St Marys and St Joseph Rivers. The two rivers originated as ice-marginal channels that drained meltwater emanating from the melting ice front; together, they outline the arcuate shape of the terminus of the Erie Lobe as it stood at Fort Wayne. During this time, drainage was southwest, directly down the Little River Valley. In other words, the St. Marys and St Joseph Rivers were part of the Wabash River drainage, a relationship marked by terraces of sand and gravel that hang on the walls of the Little River Valley from Fort Wayne to Huntington, and comprise remnants of a much larger body of outwash that filled the valley. It was also during this time that the upland known as the “Sand Point" formed: most of this upland is an outwash fan-a landform analogous to an alluvial fan and composed of sand and gravel-deposited in the head of the valley by meltwater issuing from the ice front. It is likely that the outwash fan extended completely across the upper part of the valley, but later erosion by meltwater stripped away most of the fan, leaving only a fragmental record. The head of the fan lies along what is now the west bank of the St Marys River, and is exposed in a series of former gravel pits east of Waynedale, where very coarse, clean gravel is present.

The Erie Lobe subsequently retreated far back into the Lake Erie basin, and a large glacial lake formed between the retreating ice front and the Fort Wayne Moraine, which acted like a dam to impound the lake. At times, “Glacial Lake Maumee" may have held a volume of water comparable to or even greater than modern Lake Erie. Major beach ridges that parallel Indiana State Highway 37 northeast of Fort Wayne mark the high stand of the lake, at about 780-800 feet above sea level. Glacial Lake Maumee and its successor lakes had several stages that coincided with the waxing and waning of the ice sheet and the opening and closing of different outlets; the history of these early forerunners of Lake Erie is a fascinating story (see “The History of Lake Erie" by Michael Hansen, in the Fall, 1989 Ohio Geology Newsletter). As Glacial Lake Maumee slowly grew in size behind the Fort Wayne Moraine, the St. Joseph and St. Marys Rivers continued to drain southwest to the Wabash River via the Little River Valley. The Little River was not so little, and the valley bottom was characterized by an active alluvial environment of shifting sandbars, islands, floodplains, and river channels as the river reworked the outwash that had been left by earlier glacial meltwaters. The predominant vegetation on the more stable portions of the freshly deglaciated terrain was probably a spruce-alder-birch forest, punctuated by boreal wetland meadows on the active floodplains. Incidentally, it was during this time that Cedar Creek captured the drainage from the upper Eel River, which markedly increased the size of the St Joseph River and the volume of water it delivered to the Little River Valley.

Approximately 14,000 ybp, the surface of Lake Maumee reached its highest level at about 800 feet elevation. The waters overtopped a sag in the Fort Wayne Moraine near what is now downtown Fort Wayne and began draining westward down the Little River Valley. There is some evidence that the final rise in lake level that caused it to overtop the moraine was caused by a minor readvance of the glacier further east in the basin. In any event, the soft, saturated till of the moraine was no match for the power of the water, which quickly eroded a large outlet, unleashing a flood of unimaginable proportions, known as the “Maumee Torrent". A secondary outlet also opened at the same time along what is now Six-Mile Creek, causing lake water to spill into the St Marys River and thence down the southern segment of the Little River Valley. A large part of Glacial Lake Maumee rushed down the Wabash-Erie Channel in a matter of days or weeks-billions of gallons of frigid water filled the channel from wall to wall and scoured the bottom clean. The force of the torrent was concentrated in the Fort Wayne Outlet, which was scoured down to some 20-30 feet below the present-day floor of the valley. In fact, the entire Little River Valley was severely scoured; the torrent stripped away nearly all of the earlier outwash and alluvial sediment, leaving the valley floored by a barren surface on the hard lower till, with scattered boulders littering the surface. Most of the large outwash fan deposited earlier in the head of the valley also was scoured away, leaving only the Sand Point itself and a few remnant gravelly terraces high on the valley walls to stand as mute testimony to the earlier alluvial environment. The impact of the torrent was felt tens, or even hundreds of miles downstream. Boulder gravels found along the Wabash River in Cass, Miami, and Wabash Counties, for example, exhibit sedimentary structures indicative of immense power.

This initial outburst did not entirely empty Glacial Lake Maumee. Shallow limestone bedrock underlying the Little River Valley in the vicinity of Huntington acted as a hard sill that limited the depth of scouring by the Maumee Torrent; moreover, it is likely that the outlet of the lake became hung up on the much harder, over-consolidated tills of the Trafalgar Formation, which are widely present at shallow depth along the modern river valleys in Fort Wayne. There is abundant evidence, in the form of well-developed beach ridges in Ohio and far eastern Allen County, of a series of progressively lower lake levels. The existence of the beach ridges at lower elevations clearly indicates that some of the later and lower stages of Lake Maumee were relatively long lived.

The Fort Wayne Outlet continued to act as the primary outlet for the lake for a considerable period of time following the Maumee Torrent. The St. Joseph and St Marys Rivers also continued to drain to the southwest, but now they flowed through a vastly altered Little River Valley, a nearly featureless, wide trough stripped clean of loose sediment and punctuated by shallow depressions where the torrent had scooped out softer areas in the underlying glacial sediment. In fact, it is entirely possible there could have been sizable potholes in this landscape, cut by giant boulders that became trapped in swirling eddies in the torrent. Any semblance of a typical river valley was gone, and the waters flowing through this strange landscape spread out over the bottom of the valley, forming irregular depressional lakes and occupying poorly integrated sloughs and channels. Currents were probably slight in this environment, hence sediment carried in by the rivers upstream quickly began filling in irregularities in the floor of the valley. Most of the sand probably fell out near the head of the valley, while silt and clay dominated downstream. In this environment of small paludal lakes and sheetlike water flow, an incipient marsh began to develop. The climate was still relatively cold, at least in the beginning: after all, at 13,000 ybp, the ice front still stood only a couple of hundred miles north and east of Fort Wayne. No fossil pollen record has been recovered from the Great Marsh, but similar records from this period elsewhere in the eastern Great Lakes suggest that the region around the Little River would have been vegetated by species more characteristic of the far north today: boreal sedges and forbs in the marsh, and spruce-fir forest on adjacent uplands.

Continental glaciers finally receded back from the Great Lakes basin some 10,000-11,000 ybp, eventually allowing the St. Lawrence drainage to open eastward to the Atlantic Ocean. Glacial Lake Maumee was a thing of the past, its edges having receded eastward to a position approaching the modern shoreline of Lake Erie, and leaving behind a vast, flat lake plain in northwestern Ohio. Sometime after the Great Marsh started to develop in the Little River Valley, the lower reaches of the Maumee River became established near its present mouth at Toledo. The river began to erode headward across the lake plain, eventually breaching the Fort Wayne outlet, where it ultimately “captured" the drainages of the St Joseph and St Marys Rivers in a classic act of stream piracy. The abrupt development (in terms of geologic time) of east-bound drainage associated with the Maumee River basically robbed the Little River Valley of the major part of its surface watershed, and left behind the low divide just west of the St Marys River; this divide is part of the much larger eastern continental divide that separates Great Lakes drainage from the Mississippi River-Gulf of Mexico watershed. The exact timing of this signal event is poorly known-the piracy of the St. Joseph and St. Marys Rivers and the advent of the modern continental divide could, in theory, have occurred anytime between about 10,000 and 6,000 ybp-but it is clear that the floor of the upper Little River Valley had mostly filled in to its present level by the time the Maumee River arrived, because the elevation (750 feet) of the “three rivers" confluence is only a few feet less than the highest point (757 feet) where the continental divide crosses the north channel. But regardless of the timing of this event, it ultimately led to a much-diminished flow of surface water in the channel, and the creation of a slackwater type of environment in which a few small streams meandered sluggishly through small oxbows and expansive wetlands. At this point, the discharge of ground water became a major source of water supporting the wetlands in the upper reaches of the Little River Valley in Allen County, and accumulation of organic sediment (peat and muck) became an important type of sedimentary process in the valley bottom. The cumulative effect of these post-glacial events is that the sequence of post-glacial sediments below the valley floor exhibits a fining-upward pattern at most places, though there typically is no sharp break in the sedimentary record that marks the change in flow regime associated with the stream piracy.

Incidentally, the same processes also created a local drainage divide in Six Mile Creek (now known as Trier Ditch). This huge valley has two very unusual features: 1) it is underfit, meaning that Six Mile Creek is much smaller than expected for the size of its valley; and 2) it flows in two directions, part of it flowing north and northeastward for ~7 miles, where it empties into the Maumee River, and the rest of it flowing south and southwestward for ~2.3 miles to the St. Marys River. Thus (by a circuitous route) the southern part of Trier Ditch water also eventually empties into the Maumee River – at the headwaters in downtown Fort Wayne!

Several small fields of sand dunes dot the valley bottom in a linear chain of ‘islands’, so named by early settlers because they stood above the vast marshy bottomlands and offered the only consistently dry ground to farm and establish homesteads in the valley. The most familiar of these are located at Fox Island, Midway Island (on whose northern edge Eagle Marsh Woods is situated), and the Sand Point, while less prominent dune fields occur near Aboite and east of New Haven, in the head of the Fort Wayne Outlet. Most of the dunes are called “longitudinal dunes"-so named because their long dimensions parallel the prevailing winds and the direction of sand transport-and their orientations indicate that the wind blew the sand in from the southwest. The tallest dunes stand about 25-30 feet above the floor of the valley, and the largest dune fields (at Fox Island and Sand Point) occupy no more than a square mile or so. All of the dune fields are found near sources of sand. For example, the dunes at Sand Point are proximal to the remnant outwash fan that underlies that area, which could easily have furnished the source of sand. Similarly, a large body of sand and gravel that underlies the valley floor near the mouth of Aboite Creek could have furnished the sand that makes up the dunes in that area and, perhaps, at Fox and Midway Islands.

The age and origin of the dunes are speculative. The dunes are clearly younger than most of the mucky and peaty marsh sediments that floor the valley, because borings made at Fox and Midway Islands show that the sand overlies these deposits. This relation rules out the possibility that the dunes might have formed on the barren landscape that was left immediately after the Maumee Torrent. But whether the dunes were deposited before or after the Maumee River drainage became established in Fort Wayne is an open question. And their presence prompts an even larger question: ‘What caused the dunes to form in the first place?’ After all, sand generally does not blow around on a wet, vegetated landscape such as the Great Marsh. In other words, the presence of the dune fields would seem to indicate that the sand sources they came from were largely, if not entirely, unvegetated. Since this is scarcely the case today, the logical extension of this line of inquiry is that the dunes mark a period characterized by a vastly different and drier climate than the current one, when forest communities receded from the region and sparsely vegetated prairies were the dominant ecological community in the Little River Valley. Such a condition existed throughout much of eastern and central North America some 6,000-7,000 ybp, during a period when the geologic record is dominated by plant species a nd fossil pollen indicative of a much warmer and drier climate. This interval is variously termed the ‘altithermal’ or ‘post-glacial thermal optimum,’ and seems to be the most likely time when the dunes in the valley could have formed. Such a relatively recent origin for the dunes is further supported by the fact that they are little modified by erosion, and still retain their original, depositional form. During the altithermal period, the Little River Valley would have at times been a distinctly inhospitable place, as blowing sand and dust storms blanketed the landscape! Once this comparatively brief interval ended, however, a humid climate quickly returned, and by the time the early explorers arrived, the dunes were blanketed by a rich hardwood forest.