Physiography of the Charleston Area
This paper was presented to the Faculty at Eastern as one of the AAUP lecture series in March, 1953.
The city of Charleston is located on the back slope of the terminal moraine of the Wisconsin ice sheet. To the south for approximately eight miles lies the complex pattern of ridges, composed of glacial debris, deposited as the ice front existed at this location. To the north one finds a slightly undulating plain, made up of similar material laid down as the ice retreated during a period of moderation in the Pleistocene (Ice Age) climate. Although the present upland surface is largely a response to the depositional forms created by the advance and retreat of the Wisconsin ice sheet, the Embarras River and its tributaries have resulted from an extremely complex physiographic history covering the last 200,000,000 years of geologic time.
Since the lowland features of this area are in part, at least, a response to the pre-glacial landscape it is necessary to consider the development of the bedrock topography. The limestones and shales, exposed wherever cuts through the glacial drift have occurred, are deposits of the Pennsylvanian period. The sediments from which these rocks were formed were laid down in part as deep water deposits and in part as deposits in the coastal waters of seas which existed at that time. Since no geologic record is found in this area from the close of the Pennsylvanian period to the beginning of the Pleistocene Period, crustal movement must have forced the surface to an elevation above sea level soon after the Pennsylvanian sediments were consolidated. During the vast period of time the Pennsylvania surface was subjected to more or less severe erosion and the present bedrock topography was produced.
In this vicinity the Embarras River was establishing a drainage pattern in many respects quite similar to the present stream pattern, flowing to the south generally, eventually emptying into the Wabash, while drawing its major tributary waters from the east and west. The main valley of the pre-glacial Embarras River was cut to a depth of 100 to l50 feet below the upland and was several miles wide. This ancient valley lies east of the present valley and is located just west of the town of Hutton. A major tributary of this Pennsylvanian river existed in a location approximately that of the present channel of the Embarras south and east of Charleston. Neither the main stream nor this tributary extended north beyond the present boundary of Coles County, thus Charleston is located immediately above a divide between the pre-glacial drainage systems of the Illinois and Wabash Rivers. This upland surface was a low rolling plain which increased slightly in elevation to the north and east.
It was upon this surface that glacial debris of the Kansan ice sheet was deposited. The Kansan was the second glacial stage of the Pleistocene Period, debris from the first, the Nebraskan although represented in western Illinois is not found in this region. The deposits of Kansan till are found in isolated areas, mostly in depression in the bedrock surface and are not continuous over the region. Presumably the Yarmout, inter-glacial stage following the Kansan was sufficiently long (some 300,000 years) to permit erosive forces to remove the mantle of debris laid down by this ice sheet.
The second ice invasion to affect the Charleston area was the Illinoian and in most of the area Illinoian drift lies immediately upon the bedrock formations. This ice sheet extended as far south in Illinois as Carbondale before sufficiently high temperatures were encouraged to produce a rate of melting which equaled the rate of forward motion of the ice. With the ice front of the Illinoian glacier l25 miles or more to the south, undoubtedly there was a rather deep layer of ice over the present location of Charleston. Pressures exerted by this depth of ice were sufficient to cause considerable scouring, as evidenced by the glacial grooves found in the upper surface of the Pennsylvanian rock. This scouring might well have removed some of the Kansan drift which had been protected from removal by the erosive agents active during the Yarmouth inter-glacial stage.
In the immediate vicinity of Charleston, outcroppings show little variation in the elevation of the Illinoian surface except near the major streams. On this gently undulating plain a well developed soil formed during the Sangamon interglacial stage, which was 100,000 to 120,000 years in length, this soil horizon, where preserved, may be used to identify the boundary between the Illinoian and Wisconsin drift shoots.
The Illinoian landscape of this vicinity showed only slight differences from the present landscape. Much of the deep pre-glacial Embarras valley had been filled by deposits of both Kansan and Illinoian glacial debris and now drainage lines were being established. Some of those new drainage ways were established along the tributaries of the ancient Embarras possibly because some of them were kept relatively clear of debris by glacial melt water but probably because of the occurrence of thin deposits in the more shallow tributary valleys. whatever the exact cause for this development, the Embarras, below a point nearly due east of Charleston, and its major tributaries such as Polecat, Whetstone, Kickapoo and Indian creeks were developed as pre-Wisconsin or post-Illinoian stream pattern. Those streams developed their valleys during the Sangamon interglacial period and served as drainage lines for melt water during the Wisconsin glacial stage.
The third and latest invasion of ice into this area occurred in very recent times. Only some 25,000 years have elapsed since the latest ice sheet retreated from this part of the state of Illinois. The Wisconsin ice sheet moved into Illinois from the northeast and moved southward and westward to a position along a line from Paris through Charleston, Shelbyville, Decatur and Peoria. The glacial drift deposited at this position of the ice front formed a series of ridges known as a terminal moraine and is called the Shelbyville moraine. This deep deposit, built to an elevation of 700 to 750 feet in this vicinity, accumulated as the glacial ice melted and the material carried in and on the ice was dropped. It was in this region that rate of melting equaled the rate of movement of this latest ice sheet and as this continuous flow of ice from the north was checked by melting, tremendous quantities of debris were brought into the region and deposited.
The Shelbyville moraine immediately to the south of Charleston is composed of a complex of three ridges extending in a general east-west direction interlaced by several small ridges trending north-south between the major formations. The complexity of the morainal deposit would indicate a series of minor retreats and advances during the thousands of years the ice front was present in this area. After the extended period of nearly stationary location an increase in temperature or a decrease in ice supply, or both, caused the ice front to retreat. The retreat was rather rapid to a position approximately 10 miles north of Charleston. Here the ice front again halted for a period of time sufficiently long to build a ridge which stands 20 to 30 feet above the surrounding plain. Following this short period, in which the recessional Cerro Gordo moraine was formed, further climatic change produced a retreat to a position near the present location of Champaign.
In the area immediately north of the city of Charleston we find a relatively thin uniform layer of glacial debris referred to as the ground moraine, which lies at an elevation 75 to 100 feet below that of the terminal moraine south of the city. In the rather rapid retreat of the ice front there was not sufficient time for the accumulation of thick deposits. These thinner ground moraine deposits represent the material which was rather uniformly dispersed throughout and on top of the glacial ice. The ice front was not a sheer precipitous wall hundreds or thousands of foot in height but was a relatively thin-edged tongue of ice which increased in elevation and thickness toward the north. Melting of the surface had reduced the depth of the ice near the southern edge and debris which had been frozen into the upper portion of this great mass had accumulated on the top as the upper ice melted. Water from this melting ice produced streams and ponds on the top of the ice, the water eventually finding its way to the ice front or falling into crevices and escaping underneath the ice front. This movement of water on top of the glacier concentrated some of the debris in stream channel or in ponds. The form of those accumulations was of course destroyed as they were forced to the ice front and became part of the terminal moraine. However, in the ground moraine many of the ice surface deposits are preserved.
Deposits in pond basins were dropped as the ice retreated to form small hills, called kames, Charleston cemetery hill is composed of material which was once a deposit in the bottom of a glacier surface pond. On the uplands just east of the Embarras River and north of route16 is located a ridge which stands l5 to 25 feet above the surrounding land and winds across the surface for several miles. This ridge, and esker was formerly the deposit of sand and gravel in the channel of a stream which flowed on top of the glacial ice in this area. These minor variations in ground moraine deposition amid the major morainal deposits of the Shelbyville and Cerro Gordo moraines interrupted the flow of melt water coming from the ice. This interruption of drainage produced, in certain areas of the ground moraine, large lakes and ponds which remained for extended periods of time before natural drainage was established and the water removed.
The development of the present stream pattern might be considered a process of adding post-glacial drainage lines to those streams which were able to perpetuate their existence throughout the Wisonsin stage of glaciation. The major streams of the area, excluding those north of the Shelbyville moraine were well established on the Pre-Wisconsin landscape. The Embarras north of Route 16 and the tributaries of this portion of the river represent the major post-glacial stream developments. Within the terminal moraine, post-Wisconsin stream action has been limited to headward extension of the streams and modification of the valleys and drainage basins through the development of small tributaries.
The surface exposed as the ice retreated to positions north of the present location Charleston was 100 foot or more above the level of interglacial channel of the Embarras. This difference in elevation and the excessive supply of water, resulting from the melting ice, caused rapid cutting of a deep drainage channel. Today this portion of the Embarras flows in a steep-walled valley which averages 75 to 100 foot deep. The intensity with which the valley was formed was checked as the glacial ice retreated farther northward and the melt water began to flow into other drainage basins, the Vermillion and Sangamon principally. The reduction in available water accounts for the abrupt change in the Embarras in the vicinity of Hindsboro. Here the river changes from a deeply entrenched stream to a meandering upland stream which is just beginning to cut into the glacial drift.
The significance of the deep entrenchment of the Embarras in northeastern Coles County can be seen in the tributaries to this portion of the river. Only one major tributary is found, the Little Embarras River. Most of the streams which flow into the Embarras are less than two miles from headwaters to mouth. These streams flow rapidly down extreme gradients (many 50 to 70 feet per mile) and carry rather large volumes of water. The volume of water is often greater than normally expected because those channels are frequently used as outlets in artificial drainage patterns and receive water from large acreages of upland. The volume of water coupled with the speed of flow has caused rapid erosion of these small valleys and has produced aggradation in the main stream. Normally these processes will continue until all of the surface is reduced to a peneplain compatible with base level established in the pre-glacial channel of the lower Embarras River.
In conclusion may I consider briefly two of the major problems of this area and indicate the relationship between those problems and the process which are developing the physical landscape. Actually we might consider these as two phases of the problem of water management. As a result of the physiographic history of this area farmers north of Charleston are faced with the necessity of providing artificial drainage of land. The lack of sloping land and the character of the glacial debris are in part responsible for those problems. Rather large areas were lakes and ponds originally and the concentration of clay particles on those surfaces made them less permeable than they might otherwise have been (inadequate management of the soil has also been a major contributing factor, but this paper is confined to physical processes).
Artificial removal of excess water has been attempted by two methods, the construction of open ditches and the laying of tile. Both have been successful in part and both in numerous instances aggravated the problem of land management.
Many open ditches and tile lines were constructed leading into the short, very active tributaries of the Embarras River. This increase in the supply of water in those ravines has intensified the erosive power and frequently active streams have developed along those artificial drainage lines establishing deep channels in land which just a few years ago was flat upland and fertile farmland. Many acres of formerly productive farmland have been forced out of cultivation in northern Coles County in less than a century largely through the development of those over-active ravines. Since the spring of 1949 I have watched one such cut change from a gully l5 feet in length and with a maximum depth of 18 inches to a ravine 150 yards long and 11 foot deep near the mouth. Adequate protection of the outlets of those drainage lines could have been provided which would have checked the further extension of those tributary streams.
A second problem which is directly associated with the development of the landscape is that of providing a water supply through the creation of reservoirs. Since the Embarras River originates in a region of loose glacial debris it can be expected to carry large quantities of sand and silt at all stages of flow except extremely low water. However, the fact that most of this unconsolidated material lies at an elevation nearly 100 feet above the level of the river tremendously increases the load potential of the stream. Under natural conditions this river flows rapidly through gorges nearly 100 feet deep gathering from innumerable tributaries large quantities of gravel, sand and silt to suddenly open onto a broad valley having a bedrock base which cannot be rapidly eroded.
This broad valley provides less gradient and the velocity produces overloading of the stream and valley filling develops. The head of aggradation of the Embarras Valley is approximately the head of Lake Charleston. Prior to the creation of the lake much of the finer material could be carried at normal water stages to the lower reaches of the river. The development of the lake at this point ensures that a very minimum of material will be carried beyond this point and that maximum aggradation will be accomplished. It is little wonder that Lake Charleston, situated where the powerful youthful Embarras first pours its load of sediment into the mature valley, now after only five years is so filled with silt that motor boats may operate in just a few restricted channels. Charleston may soon face the problem not of taking the silt out of the water but of getting water out of the silt.