Topographically, a very wide expanse of wilderness country had been seen. The entire length of route computed to have been traversed, exceeds four thousand miles, in the course of which we had crossed nineteen portages, over which all the baggage and canoes were conveyed on the shoulders of men. We encountered actual resistance from the Indians at only one point.[ [211] I kept my journals continually before me, and had my pencil in hand every morning as soon as it was light enough to discern objects. I began my geological observations at Detroit.
This ancient city, founded by the French in 1701, stands upon an argillaceous stratum, which is divided, topographically, into an upper and lower bank. Wherever this clay has been examined by digging, it discloses pebbles of various species of rock, denoting it, as far as these extend at least, to be a part of the great drift stratum.
In digging a well near the old Council House, in the northeast part of the city, the top soil appeared to be less than two feet. The workmen then passed through a stratum of blue clay, of eight or ten feet, when they struck a vein of coarse sand, six or eight inches in thickness, through which the water entered profusely. The digging was carried through another bed of blue clay, twenty or twenty-two feet in depth, when the men reached a stratum of fine yellow sand, into which they dug three feet and stopped, having found sufficient water. The whole depth of the well is thirty-three feet. The water is clear and rapid. No vegetable or other remains were found, and but few primitive pebbles.
In another well, situated near the centre of the town, the depth of which is twelve feet, the top soil was found to be two feet and a half; then a bed of gravel, seven feet; a vein of blue clay, eight inches, and the residue a whitish-blue clay, very compact and hard; a copious supply of water having been found. The water is, however, slightly colored, and is of a quality called hard.
In some places, this clay drift yields balls of iron pyrites, which renders the water unpalatable. At what depth the rock would be struck, if the excavation were continued, can only be conjectured. A well has been dug, a short distance below the city, upwards of sixty feet, chiefly through clay and gravel, without reaching the rock; but abraded fragments of granite and hornblende rocks were thrown from the greatest depths.
The bed of the river opposite the city has been stated to consist of limestone rock, but without any proof or much probability. From the fact of its affording a good anchorage to vessels, I am inclined to think that it is wholly composed of clay and gravel.
Detroit Fluviatile Clay.—The argillaceous stratum of Detroit extends along both banks of the river to its head; passes around the shores of Lake St. Clair, and up the River St. Clair to Fort Gratiot—a distance of seventy miles. In this distance there are some moderate elevations and depressions in the surfaces of the soil, but no very striking changes in its general character and composition. The boulder stratum is prominent at Gros Point, at the foot of Lake St. Clair, where the shore exhibited some heavy blocks of granite, and other foreign rock.
St. Clair Flats of Plastic Clay.—At the mouth of the River St. Clair, the current is divided into several channels, and spread over a considerable tract of low ground, which is covered with grasses and aquatic plants. These channels have worn their way through beds of tough blue clay, called the flats, over which there is sometimes not over seven feet eight inches of water in the ship channel. They consequently form an impediment to commerce. The depth is, however, always increased in the spring season, when twelve inches more may be generally relied on. Frequently, during the droughts of summer, a change of wind, and its steady continuance for some time, will allow ships to pass without lighters. The permanent removal of this bar is, however, an object of national importance, which cannot but be felt, as the tonnage of the lakes increases.
Ancient Dune; A Buried Forest.—The principal spot where the lands, in the immediate vicinity of the water, assume any considerable or abrupt elevation, is included between Black River of the St. Clair and Lake Huron. Here the outlet of the lake, which is rapid, washes the base of a ridge, or ancient dune, elevated fifty or sixty feet above the water. Fort Gratiot occupies the upper part of this elevation. The lower part consists of the blue clay stratum, corresponding in character with that found in the wells of Detroit. It is overlaid by a deposit of sand, forming two-thirds of the entire height. This elevation is crowned with a light forest of oak and other species. At the line of junction between the sand and clay, a number of trees are seen to be horizontally imbedded, projecting their roots and trunks in a striking manner above the water. These trees, on inspection, are merely preserved, not petrified. They appear to have been exposed to view, in modern times, by the wearing away of the bank. Certainly, none of the old travellers mention them.
The mode of this formation may be clearly seen. Winds, at some ancient period, have been the agent of blowing the sands, as they were washed up by the lake, and redepositing them on part of a prostrated forest, resting directly on the clay stratum. The trees, thus buried in dry sand, have been preserved. In process of time, the river encroached upon these antique beds, exposing them to view. There are also antique fresh-water shells found in similar positions near this spot. No rock is, thus far, found in sitû in ascending the lakes. The old surface of the country is wholly of diluvial formation, except where it shows lake action.