Summary Narrative of an Exploratory Expedition to the Sources of the Mississippi River, in 1820 / Resumed and Completed, by the Discovery of its Origin in Itasca Lake, in 1832

by Henry Rowe Schoolcraft

The tract of country separating the southern shores of Lake Michigan from the Illinois River, is a plat of table-land composed of compact limestone, based on floetz or horizontal sandstone. This formation embraces the contiguous parts of Illinois, and spreads through Indiana, Ohio, and the Peninsula of Michigan. It is overspread with a deposit of the drift era, covered with a stratum of alluvial soil, presenting a pleasing surface of prairies, forests, and streams. These features may be considered as peculiarly characteristic of the junction of the Rivers Kankakee and Des Plaines, which constitute the Illinois River. This junction is effected about forty miles south of Chicago.

The fossil in question occurs about forty rods above the junction of the Kankakee. The sandstone embracing it is deposited in perfectly horizontal layers, of a gray color and close grain. It lies in the bed of the Des Plaines. The action of this stream has laid bare the trunk of the tree to the extent of fifty-one feet six inches. The part at the point where it is overlaid in the western bank is two feet six inches in diameter. Its mineralization is complete. The trunk is simple, straight, scabrous, without branches, and has the usual taper observed in the living specimen. It lies nearly at right angles to the course of the river, pointing towards the southeast, and extends about half the width of the stream. Notwithstanding the continual abrasion to which it is exposed by the volume of passing water, it has suffered little apparent diminution, and is still firmly imbedded in the rock, with the exception of two or three places where portions of it have been disengaged and carried away; but no portion of what remains is elevated more than a few inches above the surface of the rock. It is owing, however, to those partial disturbances that we are enabled to perceive the columnar form of the trunk, its cortical layers, the bark by which it is enveloped, and the peculiar cross fracture, which unite to render the evidence of its ligneous origin so striking and complete. From these characters and appearances, little doubt can remain that it is referable to the species juglans nigra, a tree very common to the forest of the Illinois, as well as to most other parts of the immense region drained by the waters of the Mississippi. The woody structure is most obvious in the outer rind of the trunk, extending to a depth of two or three inches, and these appearances become less evident as we approximate the heart. Indeed, the traces of organic structure in its interior, particularly when viewed in the hand specimen, are almost totally obliterated and exchanged, the vegetable matter being replaced by a mixed substance, analogous, in its external character, to some of the silicated and impure calcareous carbonates of the region. Like those carbonates, it is of a brownish-gray color and compact texture, effervesces slightly in the nitric and muriatic acids, yields a white streak under the knife, and presents solitary points, or facets, of crystals resembling calc spar. All parts of the tree are penetrated by pyrites of iron of a brass yellow color, disseminated through the most solid and stony parts of the interior, filling interstices in the outer rind, or investing its capillary pores. There are also the appearances of rents or seams between the fibres of the wood, caused by its own shrinkage, which are now filled with a carbonate of lime, of a white color and crystallized.

From an effect analogous to carbonization, the exterior rind and bark of the tree have acquired a blackish-hue, while the inclosing rock is of a light-gray color, characters which are calculated to arrest attention.

There is reason to conclude that the subject under consideration is the joint result, partly of the infiltration of mineral matter into its pores and crevices, prior to inclosure in the rock, and partly to the chemical action educed by the great catastrophe by which it was translated from its parent forest, and suddenly enveloped in a bed of solidifying sand.

At the time of my visit (August 13, 1821), the depth of water upon the floetz rocks forming the bed of the River Des Plaines, would vary from one to two feet; but it was at a season when these higher tributaries, and the Illinois itself, are generally at their lowest stage. Like most of the confluent rivers of the Mississippi and their tributaries, the Des Plaines is subject to great fluctuations, and during its periodical floods may be estimated to carry a depth of eight or ten feet of water to the junction of the Kankakee. At those periods, the water is also rendered turbid by the quantity of alluvial matter it carries down, and a search for this organic fossil must prove unsuccessful. But during the prevalence of the summer droughts, in an atmosphere of little humidity, when the waters are drained to the lowest point of depression, and acquire the greatest degree of transparency, it forms a very conspicuous trait in the geology of the stream, and no person, seeking the spot, can fail to be directed to it.

The sand-rock containing this petrifaction is found in a horizontal position, differing only with respect to hardness and color. The remains of fossil organized bodies in this stratum are not abundant, or have not been successfully sought. It is probable that future observations will prove that its organic conservata are chiefly referable to the vegetable kingdom. It is certain, that this inference is justified by the facts which are before me, and particularly by the characteristic appearances of the strata in the bed of the River Des Plaines, where the imbedded walnut is the representative of the ancient flora. At a short distance above, where the bed of the Des Plaines approaches nearer the summit level, limestone ensues, and continues from that point northward to the shores of Lake Michigan. In the vicinity of Chicago, where this limestone is quarried for economical purposes, it is characterized by the fossil remains of molluscous species.

Lake Erie lies at an elevation of five hundred and sixty-five feet above the Atlantic.[ [251]

There exists a water communication between the head of Lake Michigan, at Chicago, and the River Des Plaines, during the periodical rises of the latter, but its summer level is about seven feet lower, at the termination of the Chicago portage, than the surface of the lake. From this point to its junction with the Kankakee, a computed distance of fifty miles, the bed of the Des Plaines may be considered as having a mean southern depression of ten inches per mile, so that the floetz rocks at its mouth, lying on a level of forty-eight feet eight inches below the surface of Lake Michigan, have an altitude which cannot vary far from five hundred and fifty feet above the Atlantic. There are no mountains for a vast distance either east or west of this stream. It is a country of plains, in which are occasionally to be seen alluvial hills of moderate elevation; but the most striking inequalities of surface proceed from the streams which have worn their deep-seated channels through it; and an oceanic overflow capable of covering the country, and producing these strata by deposition, would also submerge all the immense tracts of secondary and alluvial country between the Alleghany and the Rocky Mountains, converting into an arm of the sea the great valley of the Mississippi, from the Gulf of Mexico north to the Canadian Lakes. We find in the alluvial soil along the Illinois and Des Plaines blocks of granite, hornblende, and gneiss, of the drift stratum, exhibiting the same appearances of attrition, and of having been transported from their parent beds, which characterize the secondary tablelands along the margin of the great American lakes, the prairies of Illinois, and the western parts of New York.

There is nothing, perhaps, in the progress of modern science, which has tended to facilitate geological research so much as the study and investigation of fossil organic remains. They teach, with unerring lights, how extensively the ancient flora and fauna of this continent have been prostrated, leaving their exact impressions, in all their minuteness, in the newly-formed stratifications. That these impressions, fresh and vivid as we find them, should mark the eras of depositions and crystallization of rocks from the suspension of their elements in water, is the observation of Werner, and it is to him we owe the elements of the Neptunian hypothesis. His general recognition of the epochs of the primitive, transition, and secondary rocks, appears too probable not to commend itself to adoption with regard to all strata which can be conceived to be the products of watery menstrua.

But it remained for Werner, who was the first to perceive an order in strata, also to point out the important application of fossil organic bodies in elucidating their eras, and the natural order of their superposition.