Popular Science Monthly/Volume 46/December 1894/The Ancient Outlet of Lake Michigan
| THE ANCIENT OUTLET OF LAKE MICHIGAN. |
By Prof. W. M. DAVIS,
HARVARD UNIVERSITY, CAMBRIDGE, MASS.
THE reports of several of our State Geological Surveys contain references to former outlets of the Great Lakes, when their waters were for some reason turned from their present lines of discharge. A brief mention of the ancient overflow of Lake Michigan across the flat divide at Chicago and down by the Des plaines and Illinois Rivers to the Mississippi appears as long ago as 1868 in the account of the geology of Cook County, Illinois, by Bannister, in the third volume of the Geological Survey of that State; and of this more below.
A more explicit description of the ancient Maumee-Wabash outlet of Lake Erie was given more than twenty years ago by G. K. Gilbert in the first volume of the Geological Survey of Ohio. The region is very flat, with a faint divide separating the eastern and western slopes; across this divide the old channel is "not less than a mile and a half broad, and has an average depth of twenty feet, with sides and bottom of drift. For twenty-five miles this character continues, and there is no notable fall." To the northeast, the channel opens out on the even floor of an ancient lake, whose shore lines diverge to the outlet. In the southwest the channel touches bed rock at Huntington, and then descends more rapidly. Most of the flat passage from the lake outlet to the sill of rocks is now "occupied by a marsh, over which meanders the Little River, an insignificant stream, whose only claim to the title of river seems to lie in the magnitude of the deserted channel of which it is the sole occupant. At Huntington the Wabash emerges from a narrow cleft of its own carving, and takes possession of the broad trough, to which it was once but a humble tributary." Mr. Gilbert's further account of the peculiar river courses of that district is extremely interesting, and illustrates to perfection how much meaning can be given by intelligent study to a country as flat and apparently monotonous as northwestern Ohio. It is noticeable that the explanation which Mr. Gilbert then suggested for the reversal of the present overflow of Lake Erie was an uplift of the land to the northeast; but Prof. Newberry, director of the State Survey, calls attention in a footnote to the possibility that the overflow resulted from an ice barrier in the valley of the St. Lawrence; and to this conclusion nothing has lent stronger support than Mr. Gilbert's subsequent observations of marked lacustrine shore lines in New York, from which we know that the land was depressed and not raised in the northeast at the time of this and other similar overflows.
The broad channel now followed by the Minnesota River is analogous to that across the Maumee-Wabash divide, although the lake from which the river flowed to cut the channel has shrunk away so far as to withdraw its waters beyond our northern boundary. The former occupation of the Minnesota channel by a large river was first pointed out by General G. K. Warren, in the Annual Report of the Chief of Engineers, 1868, page 307; and a fuller account of it was published in the American Journal of Science for December, 1878. Warren looked to a northward elevation of the land as a reason for the former southward direction of drainage, as Gilbert had done in Ohio; and this view generally obtained until the region was carefully studied out by Warren Upham, whose reports are found in the annual volumes of the Minnesota Geological Survey, and by whom a special account of the valley is given in the Proceedings of the American Association for 1883. Here the name of river Warren is proposed for the ancient stream by which this great trough was excavated: in earlier papers Upham had given the name of Agassiz to the lake from which the river issued. Further account of the ancient lake and river is found in the first two volumes of the final report on the Geology of Minnesota, now in progress.
The same observer has described a southwest overflow from Lake Superior, when the greater part of its basin was presumably occupied by retreating ice, and its waters rose about five hundred feet above their present level. The overflow took place across the pass between the valleys of the Bois Brulé and the St. Croix Rivers in northwestern Wisconsin; the channel across the pass being about a thousand feet wide and nearly a hundred feet deep, although its depth is now somewhat decreased by a marshy filling from which the headwaters of the Bois Brulé run back to the present lake (Geological and Natural History Survey of Minnesota, ii, 1888, p. 642). The lower course of the St. Croix follows a "great valley," whose fuller history is deferred to later volumes of the Minnesota Survey; its lakelike expansion above its confluence with the Mississippi is said to be due to alluvial obstruction by tributaries (ibid., pp. 377, 643). These studies by Upham in the Northwest were only continuations of the work that he had begun in New Hampshire several years before, where he recognized the shore lines of a small glacial lake in the southern part of the north-sloping Contoocook Valley, with an overflow to the southeast at Greenfield, N. H. (Geological Survey of New Hampshire, iii, 1878, pp. 116-119). A few years ago, in company with Mr. C. L. Whittle, I traced out a number of deltas and shore lines on the slopes of this picturesque valley, but have not since then been able to complete the attractive study of mapping and restoring the old lake.
Upham's observations on the shore lines of Lake Agassiz have fully demonstrated that in northern Minnesota and Dakota, and in Manitoba farther north, the land was depressed, not elevated, at the time of the overflow of river Warren from Lake Agassiz; and hence, there as in the St. Lawrence Valley, the cause of overflow must be looked for in the retreating front of the Pleistocene ice sheet. In the Bulletin of the Geological Society of America (vol. ii, 1891, pp. 243-276) Upham has described a number of glacial lakes associated with large river channels, north of our boundary in Canada. The channels are now deserted by the great streams that carved them, and are occupied only by smaller streams, which are frequently "laked" by the alluvial deposits brought in by lateral tributaries, as will be referred to again further on.
In 1885 Gilbert traced out the shore lines of the expanded waters of Lake Ontario, afterward named Lake Iroquois by Spencer, and showed that they converged to the southeast, and at Rome, N. Y., an outlet was found through what is now called the valley of the Mohawk. Only a brief mention of the attractive problem offered by this locality has yet been published. Spencer has called attention to the probable former discharge of Lake Huron and Georgian Bay across the province of Ontario by way of the river Trent (Proceedings of American Association, xxxvii, 1888, p. 198); and Gilbert has suggested that at an earlier stage there was another outlet farther north, by way of Lake Nipissing and the Ottawa River the account of this being found in his excellent History of the Niagara River, published by the Commissioners of the State Reservation at Niagara, in their sixth annual report. The reading of this history will greatly increase the pleasure of an intelligent visit to the great cataract. It was of the outlet by way of the Ottawa River that Wright gave an account in the New York Nation for September 22, 1892.
The abandoned channel of overflow of the ancient Lake Bonneville at Red Rock Pass in northern Utah, and the "old river bed" leading from Sevier Desert to Great Salt Lake, well known from Gilbert's monograph, are analogous to the old channels here considered, although the overflows there were not produced by glacial barriers.
All these abandoned channels have certain features in common. At their upper end, where they trench across a divide of greater or less distinctness, they open out upon lacustrine plains of greater or less extent and distinctness, whose converging shore lines may be traced to the point of discharge. The breadth of the abandoned channel is relatively constant throughout a great part of its length; from which we may infer that the volume of water received from the lake at its head was large in comparison with that received from the tributaries lower down in its course. None of the deserted channels are cut to a great depth; but, whatever their depth, they are inclosed by banks or bluffs that are still distinct and comparatively steep; thus showing that relatively short periods of time elapsed both during and since their occupation by large rivers. The rock-cutting done by Niagara in post-glacial time seems to be a much greater piece of work than that accomplished by any of the temporary lake outlets during the closing phases of the Glacial period; but none of them, as far as I have read, had an opportunity for active work equal to that of Niagara.[1] They are nearly all comparatively shallow; but an exception to this rule has been pointed out to me by Mr. Gilbert, to which a paragraph may be devoted.
Emmons, the geologist of the second district of New York, long ago described what he took to be fissures in the Potsdam sandstone of northern New York, but which to modern interpretation appear to be gorges or chasms cut by rivers, presumably constrained into that position by drift or ice obstruction. The Ausable chasm is a well-known instance of these "fissures," but one of the examples described by Emmons has no river running through it. It lies close to the Canadian boundary in Clinton County, sixteen miles west of Lake Champlain, and is thus described in Emmons's report (Geology of the Second District, New York, 1843, pp. 309, 310): "The fissure or gulf, as it is called, is three hundred feet deep and about sixteen rods wide. Its walls of sandstone or conglomerate are perpendicular at the deepest part. The small lake at the bottom is said to be one hundred and fifty feet deep. The direction of this fracture is north, seventy degrees west, and the rock dips at a small angle from each side of it. . . . At Keeseville and Cadysville large rivers, the Ausable at the former and Saranac at the latter, still occupy these gorges as their channels, and have sufficient force and power to sweep out, especially in the time of high water, all rocks of an ordinary size. At this place there is merely a small rill discharging itself from a small lake of dead water, insufficient in itself to accomplish any perceptible change. To account for the present condition of this rock, we have therefore to go back to a period when some current swept through this gorge with great force and power; for by no other means could the materials which once filled the space between the present walls of the gulf be removed."
Returning to the general features of the abandoned channels. it is next to be noted that the small lateral streams which now enter the old river course find it much too large for their volume. This is especially true near the divide across which the old channel was cut. It is only at some distance down the channel that enough water has entered through lateral streams to form a considerable river; yet all along the channel maintains about the same width. Evidently, therefore, it was not cut out by the existing drainage. In consequence of the small volume of the longitudinal streams now occupying old channels, they are frequently more or less obstructed by the alluvial fans built at the entrance of lateral tributaries; thus swamps or long, narrow, lakelike expansions of the rivers are produced up stream from the fans. This was first noticed by Warren, and since then the list of examples has been greatly increased by Upham and others. In the last of Upham's papers referred to above, he describes a number of lakes of this kind on the Qu'appelle and Pembina Rivers and elsewhere. Long Lake in Assiniboia is about fifty miles in length, but only one or two miles wide. Lac qui Parle and Lake Traverse, in the old channel at the head of the Minnesota River, are of this kind. The sluggishness of the Minnesota and of the Illinois Rivers just above their junctions with the Mississippi has been attributed to the same cause, and this would indicate that at the time when the channels of the Minnesota and the Illinois were occupied by the large rivers which once flowed through them, these held the place of main streams, while the Mississippi came into them with smaller volume as a tributary.
There is on this account a curious contrast to be noted between the excavation of the late glacial channels that were cut out during the closing stages of the Glacial period by the overflow from glacial lakes, and the clogging of the preglacial valleys that were commonly filled with sands and gravels by streams that came directly from the retreating ice front without delay and filtering in lakes, as in southeastern Ohio. Both of these kinds of valleys mark the courses of "constrained" drainage near the end of the Glacial period. At the ice front the water supply in both cases was doubtless surcharged with detritus; but the waters that had to accumulate in lakes marginal to the ice front before flowing away as rivers must have been nicely filtered, so that they issued clear and blue from the lake outlets; while the others had to carry their detritus down stream for many miles, and must have been of gray and turbid color for long distances. The plentiful clear waters of the streams of the first class ran down the valleys that led from the lowest pass in the lake rims and cut down their channels to a moderate grade, oftentimes so moderate that the present river occupants of the valley are unable to keep them clear of the alluvium that is brought in by tributaries; hence they are frequently characterized by swamps or linear lakes, especially near their heads. On the other hand, the streams of the second class, surcharged with detritus from the ice, frequently demanded a steeper grade than they found, and therefore deposited much of their load, filling up their valleys with broad flood plains of sand and gravel, such as are now growing in front of the Greenland and Malaspina glaciers; and sometimes they may have transformed side streams into lakes, such as now characterize the side streams of the Red River of Louisiana.
The record of the latter feature is truly a rarity in the past, but it has been deciphered by Andrews as perceptible in some of the lateral tributaries of the gravel-filled valleys of Ohio. The following is Andrews's account. After describing the terraces of sand and gravel derived from the glacial drift and occurring along those streams whose sources lie in the "area of the general drift," he says: "There is in the second district [southeastern Ohio] another and very distinct system of terraces, found on streams emptying into the larger streams bordered by true drift terraces. They may be called backwater terraces. When in the Ohio, Muskingum, Hocking, etc., rivers, the water in the drift era stood eighty or ninety feet higher than at present, the backwater would set back up all the tributaries. In this still water the sand and sediment brought down these tributaries were deposited; or, in other words, the still-water areas were silted up, as mill ponds often are. When afterward the main streams fell to their present level these affluents cut through the backwater beds and carried away much of the soft materials, but left in many places fringing terraces which tell very plainly how they were formed. In these backwater terraces we find no true drift sand or gravel. The beds are entirely of home origin. Such terraces I have seen in the Little Scioto River, above its junction with the Ohio at Sciotoville, on Duck Creek, and on the Little Muskingum River in Washington County, and on Sunday Creek in Athens County. I have no doubt they are to be found on a large number of streams." (Geology of Ohio, ii, 1874, p. 444.) This record shows a delicacy of observation and a skill in physical interpretation that have impressed me as exceptional and admirable.
Taking up again the comparison of the two classes of constrained streams, it is seen that the channels of the first class were cut down to so gentle a grade by the filtered glacial streams that they are now not infrequently found to be filling up, and lakes are forming in them; but the valleys of the second class were filled so high by the gravels deposited from the surcharged glacial streams that they are now being terraced, and the lakes that were formed on their lateral tributaries are now discharged by the cleaning out of the clogged valleys. A pretty contrast for our geographical lessons!
It is noticeable that the abandoned channels of the glacial lakes are now generally utilized as natural paths of communication. The Indians easily passed from the head of the Minnesota to the Red River of the North; and indeed at times of high water they paddled their canoes over the flat divide. The Chicago outlet of Lake Michigan was naturally chosen for the path of the Illinois and Michigan Canal, and now two railroads follow the same well-graded course. More important still is the broad channel into which the Mohawk flows from the Adirondacks, and which we therefore call the Mohawk Valley. This well-opened passage determined the location of the Erie Canal; and that, taken with the drowning of the Hudson River, by which navigable tidewater is carried up to Albany, has undoubtedly been the determining cause in the development of New York city as our greatest seaport.
As a good number of these abandoned channels have been described, and as it is very probable that others will be found, it seems to me worth while to recognize them as constituting a special group of river-made forms of brief and peculiar history, deserving recognition and representation in our study of physiography. In this connection a particular interest attaches to the former outlet of Lake Michigan, because it is the only one of the old outlets that is now mapped with any approach to accuracy. Twelve sheets prepared by the United States Geological Survey—namely, the Chicago, Riverside, Calumet, Desplaines, Joliet, Wilmington, Morris, Ottawa, Marseilles, Lasalle, Hennepin, and Lacon sheets—already represent a length of over a hundred miles of the former lake outlet, and give an efliective illustration of its peculiar features.
Before speaking of the old channel, I must turn from that theme to give the maps a fuller introduction, for they seem as yet to be very little known to our teachers and scholars. They are constructed to serve as the topographical basis for the geological map which our national survey is charged to prepare. No suitable map existing. Major Powell, director of the survey, organized a topographical corps in 1882, under the charge of Mr. Henry Gannett, to whom the mapping of the country was intrusted. The progress of the work has manifestly been embarrassed by the expense of the survey over so vast a country, by the need of comparatively rapid progress, and by the difficulty of securing experienced topographers; but all considerations of distribution, scale, cost, and time have been duly discussed, and as a result we have already several hundred map sheets of areas distributed over many States, on which the topographic features of our land are represented to us as they never have been before. The scale employed varies from one to four miles to an inch on the maps thus far issued, but in future only the larger scale of a mile to an inch will be used. Relief is indicated by brown contour lines at intervals varying from five to fifty or more feet, according to the scale of the map and the character of the surface. Water is printed in blue; names, roads, boundaries, etc., in black. The accuracy of the work is not so great as many geographers and geologists would like to see. The expense of more accurate work would perhaps prevent its execution; but it may be justly said that a great increase of knowledge is made by the publication of these maps. Some of the sheets are better than others, and some of the poorer ones have been severely criticised by those who know the ground; but, as far as I have been able to observe, the character of the work is rising, and the interest in it is increasing. A sympathetic popular support and an intelligent criticism of this great undertaking will go far toward securing its improvement and extension. The study of geography will be greatly advanced when these maps are properly distributed to schools, as they soon should be, so that every high school at least would have not only its own district represented, but would possess also a collection of typical maps of other districts.[2] At the same time the demand for more and better maps will be increased. In my own teaching, I find these maps simply invaluable. It is not only the younger students who learn lessons from them. Many a problem is effectively introduced and illustrated by the maps that would otherwise remain out of reach to teachers as well as scholars. The case which I am about to describe illustrates this point very clearly.
On noticing, a year or more ago, that the topographic sheets thus far issued for Illinois included a good part of the old Michigan outlet, I resolved to have a look over the ground at the first opportunity. A visit to the World's Fair was therefore arranged to include a trip down the Illinois valley, maps in hand, and in the good company of several friends from the Chicago University. The assistance given by the maps in gaining an appreciation of the form of the country raised the high estimate that I had previously formed of their educational value. They are simply indispensable in geographical study.
Lake Michigan is inclosed at Chicago by the contour line of five hundred and eighty-five feet above sea level; and the lacustrine plain on which Chicago is built lies mostly under the contour of six hundred and ten, the western suburbs rising to six hundred and thirty feet. In the southern part of the city, near the World's Fair grounds, several faint sandy lake ridges may be traced, rising three or four feet above the dark soil of the plain. The Desplaines River comes from the north on the western part of this plain, ten miles from the lake; its narrow channel, with comparatively steep banks to the water's edge, being sunk ten or twenty feet below the plain. Opposite Summit Station, on the Chicago and Alton Railroad, the river enters a broad, swampy trough, which it follows to the southwest. It is this trough that was briefly described as the lake outlet by Bannister twenty-six years ago; and from his rather casual mention of it I infer that its meaning was then generally appreciated by those who were familiar with the ground. Since then it has been frequently mentioned in a general way in geological literature. The trough is about a mile wide and lies just below the contour of five hundred and ninety feet. Its banks become better defined as it enters the rising ground farther west. From the entrance of the Desplaines eastward to the lake there is no perceptible divide. The canal by which the South Branch of the Chicago River is connected with the Desplaines joins the channel in the western part of the city; and it is by deepening the river channel farther down stream (southwest) that a sufficient volume of lake water is to be diverted through Chicago, thus returning, in a measure, to glacial conditions of drainage, and purifying what is now a very turbid stream.
In addition to this chief passage across the flat divide, there is another one, also mentioned by Bannister, about twelve miles farther south at the village of Blue Island, on the southern end of Washington Heights (six hundred and fifty feet). Here is a long, shallow, swampy trough, again at a level of five hundred and ninety feet, running west from the lacustrine plain through the rising land, and joining the Desplaines at Sag Station, Chicago and Alton Railroad, nine miles below Summit. The Calumet River runs toward the trough from the southeast, but turns abruptly eastward near Blue Island and flows to Lake Michigan. The western end of this southern trough is drained into the Desplaines by a little stream called the Feeder. The swampy part of the trough, between the Calumet and the Feeder, is probably inclosed by faint alluvial fans, swept down by brooks from the higher ground on the south; indeed, it is quite possible that the abrupt turn in the course of the Calumet at Blue Island results from the obstruction of a former westward course in this manner.
The plain west of Lake Michigan, for a distance of twelve or fifteen miles, appears to have been under the lake waters during the greater time of the westward overflow. Its level surface, its fine, dark soil, and the occasional sandy ridges that traverse it have already been mentioned. Its western boundary, at the point where the Rock Island Railroad gradually ascends to higher ground, is rather distinctly defined by a low but definite bank, apparently an old shore line of the lake, the base being near the contour of six hundred and twenty feet.
Farther west there is a belt of higher ground, whose contours reach seven hundred or seven hundred and fifty feet. On the topographic maps this belt appears to be a plateau-like swell, well dissected by streams; but on the ground it has the appearance of a faintly marked moraine, and it is so represented on the soil map of Illinois, prepared by Mr. F. Leverett and exhibited in the Illinois State Building in the World's Fair. Its morainic form is indicated by numerous faint mounds and small hollows, and the railroad cuts show it to be composed largely of drift. The spurs and valleys, apparently of simple drainage development as indicated on the map, do not justly represent the expression of the surface at this point. A more appreciative drawing of the contour lines is required to express this faint morainic topography; but I do not think it should escape representation on a scale of 1:62,500. It would be interesting to compare a careful contouring of a small portion of this belt with its generalized portrayal on the survey sheets.
The old channel, now occupied by the Desplaines, crosses this belt of higher ground in a well-marked trough. The breadth of the flat bottom of the trough is almost constant at a measure of a mile; its depth below the immediately adjacent upland is about seventy-five feet. This is partly cut in horizontal Niagara limestone, and the descent into the flat-bottomed trough is accomplished on steep sloping bluffs, somewhat dissected by narrow, short, and steep-sided ravines. This may be called the Lemont channel, from the village of that name at its middle, where quarrying is now going on in the limestone in order to increase the westward discharge from Lake Michigan, as stated above.
The morainic belt has a width of fifteen or twenty miles on the Desplaines and Joliet sheets. Joliet lies near its western base. Farther west there is a second belt of higher country, also represented as a morainic belt on Leverett's map, of which further mention will be made below. Between the two belts there is a strip of lower country, about twenty-five miles wide, whose elevation at and below the junction of the Desplaines and Kankakee Rivers varies from six hundred to five hundred and fifty or less. This I shall call the Morris basin, from a town of that name near its middle. On entering the basin, the old channel is less distinctly defined than before, chiefly because of the smaller depth to which it could be cut in the lower ground. The bluffs are only about fifty feet high at Joliet, while at Morris, eight miles below the entrance of the Kankakee, they are hardly perceptible. Through this district the floor of the old channel is generally rocky, and in the floor the Desplaines River—or the Illinois, as it is called below the entrance of the Kankakee has cut a narrow and shallow trench. Occasionally heavy gravel beds were seen, but their origin could not be determined in our rapid excursion.
A peculiar feature of the northeastern part of this district is the subdivision of the old channel into several courses, as if a number of almost equally good lines of escape had been originally offered to the lake overflow, along all of which the waters ran for a time, but into one of which they were gradually collected, that one being the channel now followed by the Desplaines past Joliet. Two of these temporary branching channels are represented on the Joliet sheet, a short distance above that city, in the form of swampy passages connecting the Desplaines with the Dupage River. A third is seen southwest of Joliet, and is now utilized by the Illinois and Michigan Canal.
The second belt of higher ground again reaches a level of seven hundred and fifty feet north and south of the town of Marseilles, after which it may be named; and through this belt the old channel again appears as a depressed linear plain, now about a mile and a half wide at an altitude near four hundred and ninety feet. It is inclosed by steep bluff's, a hundred or more feet in height. Back from the bluffs, narrow and steep-sided ravines dissect the rolling upland for a distance of from two to four miles. There are flat alluvial fans in front of some of these ravines, and it may be for this reason that several small lateral streams in the neighborhood of Marseilles enter the river by direct courses instead of first running a distance down the valley, as is so often the habit of side streams while traversing the flood plain of their master. The contour lines of the maps do not indicate the occurrence of any fans, the lines being concave toward the mouth of the streams; but I think some of them should be slightly convex in that direction. It is possible that small matters of this kind may have escaped the attention of the topographers, although the scale of the maps is large enough to show them clearly if they really exist. As in the Morris basin, the present river about Marseilles is sunk in a narrow trench, twenty or thirty feet below the broad plain of the old channel bottom.
West of the Marseilles morainic belt—the second belt of higher ground—there is a broad stretch of even country at a height of about six hundred and fifty feet. The city of Ottawa is located in the old channel where it traverses this even upland; the inclosing bluffs here vary from seventy-five to a hundred feet in height, being frequently cut back by ravines, as already described. The level of the Illinois River at this point is a little under four hundred and fifty feet. The floor of the old channel stands at four hundred and seventy feet. Its descent from Summit Station is therefore one hundred and twenty feet in about seventy miles; but it must be remembered that part of this slope may be due to post-glacial elevation of the land to the northward.
The entrance of the Fox River from the north at Ottawa was one of the special features that I wished to see. It runs near the western base of the Marseilles morainic belt, and its trench below the general upland is as deep as the old channel; but it is narrow and steep-sided, like the many side ravines of the old-channel bluffs, although in volume the Fox is at least half as great as the Illinois. It has a flood plain of slight width where its banks are of fire clay, as at Dayton, three miles from Ottawa; but farther up and down stream, where it is inclosed by sandstones, the rocks rise directly from the water's edge, and steep bluffs rise above the rocks to the upland. The descent of the river bed is relatively rapid, amounting to about sixty feet in the first ten miles above its mouth.
The other rivers that enter the old channel present the same peculiarity as the Fox, but as they come in over lower ground their valleys are less deep, and therefore less noticeable. The Desplaines has already been described as flowing in a narrow trench in the plain west of Chicago, until it abruptly enters the swampy bed of the old channel. The Kankakee has a similar narrow valley when it joins the Desplaines, from the southeast, in the Morris basin, the two rivers forming the Illinois. Next is the Fox, and below this is the Vermilion, with a steep-sided, narrow valley like those of its fellows. The contrast between the narrow valleys of the side streams and the broad channel followed by the Illinois is strongly marked.
This is all plain enough on the ground; it is distinctly shown on the maps; but it should also be represented by photographs. If any readers of this article happen to have views illustrative of the district here described, I beg that they will communicate with me. There should, indeed, be a photographic exchange established by a union of our professional and amateur photographers, in which good views might be selected under certain conditions for purchase or exchange. At present it is a very difficult matter to find views of the simpler landscapes of our country, however well represented the greater mountains may be.
When all these features are considered together, there is good warrant in the old belief of the southwestward overflow of Lake Michigan. The considerable breadth of the old channel, in which the Illinois is cutting a narrow trench, must be due to the great volume of the old lake outlet which once filled the channel from bluff to bluff, just as the present tributary rivers fill their valleys. The old outlet river fitted its broad valley as well as the existing rivers fit their narrow valleys. It must have been large compared with its tributaries, because the breadth of its channel increases so slowly in a distance of nearly a hundred miles. Like the Niagara below Buffalo and the St. Lawrence among the Thousand Isles, the old outlet near Joliet split up into a more or less complicated network of channels, and its discharge seems to have continued just about long enough for the selection of one of these as the survivor. But it is manifest that the old outlet, like all its fellows from other expanded lakes of the close of the Glacial period, did not exist long enough to broaden its channel by lateral meanderings. The special phases of climatic periods by which these constrained river courses were determined were too short-lived to allow the development of meandering rivers—far too brief to measure so long a part of a river's history. The old outlet of Michigan endured long enough to clear off the drift from its path, and to make a beginning of cutting its channel down into the underlying rock; but it does not seem to have cut the rock down as deep as it might have done if more time had been allowed, for even the smaller rivers of to-day have trenched the floor of the old channel since the outflow of the lake has been turned over another path. To be sure, something of the ability to do this may be ascribed to the change in the attitude of the land, a presumable elevation to the north since the ice went away; but we have no definite measures of the amount of this elevation in the district here considered.
The good fortune of having maps of this interesting district should bring it clearly before many students who may not see it on the ground. It appears to have so many features common to the other examples of its class that it may well be taken as their type. As other similar channels are mapped, it will be interesting to see how far their essential features are merely repetitions of those so clearly shown on the maps of the old outlet of Lake Michigan.
- ↑ Russell's Geological Reconnaissance in Central Washington gives an account of a temporary displacement of the Columbia River when its valley was obstructed by ice and its waters ran through the Grand Coulée: a basin was then excavated beneath a cataract in the course of the river, and the basin now holds a lake, although the river and the cataract have disappeared (Bulletin 108, United States Geological Survey, pp. 91, 92).
- ↑ It may be noted that the conference on geography held in Chicago in the winter of 1892-'93—one of several conferences held under the auspices of the National Educational Association—appointed a subcommittee to prepare a list of these and other governmental maps of use in teaching. This list has been published, with explanatory notes, under the title of Governmental Maps for Use in Schools, by Messrs. Henry Holt & Co., New York. Many of the maps can be obtained free of cost from the governmental bureaus, and others can be had for a merely nominal cost.