CHAPTER II.

DEVONIAN AND QUATERNARY FORMATIONS.

1. Thickness.

A. Quaternary Deposits.

Except over areas covering but a few square yards, the rock beds of the county are concealed by a mantle of more or less unconsolidated material, varying in thickness from a few inches to about 150 feet. The average depth would probably be from 40 to 50 feet, although there are many square miles within which the depth to the rock is less than 25 feet. The deepest beds of this covering are found in the northwestern corner of the county, where just southwest of Milan, they reach a thickness of 150 feet. In the southeastern part of Erie township a thickness of 80 to 90 feet is attained. Extending in a northeast and southwest direction there are three belts along which this superficial covering is very much less, in some instances being practically absent and thus exposing the bed-rock. The most easterly belt of the three begins in the southern part of Whiteford township, extends northeastward across Bedford, northwestern Erie, the western part of La Salle, through the city of Monroe to Brest and into the eastern part of Berlin township. Throughout this entire extent the deposit rarely exceeds ten feet in thickness and is on an average much less, hence it is accompanied by numerous quarries and natural rock exposures. The middle belt is approximately parallel to the eastern just located and extends from Ottawa Lake sink, through the northeastern corner of Whiteford, across Ida, western Raisinville into the southern part of Exeter township, beyond which the deposit increases to 30 to 40 feet. Upon the map (Plate XV), there is shown by means of the contour lines, the depth of the rock from the general surface of the land. These contours thus show, also, the thickness of these deposits referred to. Just where the sand accumulations of the Forest Beach are added to

those of the clay, an apparent deepening of the rock occurs as shown by the contours on either side of Lambertville and in Sections 19, 20 and 21, of Ida township. The third belt is less well defined, shorter and more interrupted, the thinner portions of the drift being located at Petersburg, Dundee and on the Macon, where it is but two feet thick.

§ 2. General nature.

The great bulk of the deposit above referred to constitutes the so called "drift," an accumulation for which the great continental ice sheet, to be later described, was responsible. This consists, in the main, of a great mass of stiff, blue clay, entirely without stratification and carrying varying proportions of sand, pebbles and boulders. This is technically known as "till." When thoroughly compacted and filled with stones it is popularly called "hard-pan," in which form it greatly retards the operations of ditching and well digging. Upon exposure to the atmosphere, as at the surface or by means of a natural or artificial excavation, the color changes from blue to yellow or a rusty brown, owing to the oxidation of the iron. The alteration in color may extend but a few inches, or it may reach to a depth of fourteen feet or more, as about Dundee. The change in color marks the lower limit of percolating surface water, which depends upon the structure of the bed itself. Embedded in the clay there are frequently encountered lenticular masses of "quick sand," more or less stratified and of varying thickness and extent. Less frequently beds of gravel, from fine to coarse, are encountered, but are probably not continuous under any considerable area. Some of the deposits reported as "gravel" by farmers and drillers are beds of sand simply, or till rather more heavily charged with pebbles than is usual. This is probably true of the beds of "cobble stones" reported as occurring at a depth of 25 feet in Sec. 20 (S. W. 4, S. W. 4) of Ash. At the bottom of wells in Sections 4 and 6, and in Sec. 20, at a depth of 30 feet, Milan township, a bed of gravel is reported. In Section 9, still nearer the surface, a three foot bed is said to occur, overlain by 13 feet of clay and underlain by hardpan. Southward in Dundee township, Section 8 (T. 6 S., R. 6 E.), gravel was encount ered at 28 feet, and in Section 26 was found overlying the rock. Eastward in London township, Sec. 20, a fifty foot well stopped in gravel. In Summerfield township similar reports of gravel strata were obtained from both farmers and drillers. It overlies the rock

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in Sec. 4, with a heavy bed of blue clay above, but is struck in Sec. 7 at a depth of 14 to 16 feet and furnishes water at the place of T. M. Taft. In the same section (N. W. 1, S. W. 1), at John Long's place, coarse gravel was reached at a depth of 53 feet, overlain by a three foot bed of "putty clay." The gravel was penetrated three feet and the well abandoned because the hole could not be kept clean. In Sec. 27 there is on an average 11 feet of a sandy loam, one to six inches of blue clay and beneath a stratum of rounded pebbles varying from the size of a pea to that of the fist. The layer is four to six inches in thickness, but may be represented by a few stones simply. Beneath this is a heavy deposit of yellow, followed by blue, clay and hard pan. In Bedford township, Sec. 4 (N. E. 1, N. W. 4), a 3 to 4 foot bed of gravel is stated to overlie the rock, which is struck at 26 feet. With the exception of the last record all the reputed layers of gravel occur within the limits of Summerfield, Dundee, Milan and London townships. Some of them may be merely inclined gravel filled fissures or cracks.

§ 3. Boulders.

Scattered rather sparingly over the clay areas of the county are rounded masses of crystalline rocks, such as are known in placenearest in Canadian regions to the northeast. In the western and northwestern portions of the county they repose directly upon the surface, but eastward are more and more embedded in the soil and frequently struck only with the plow. They consist in the main of gneisses, schists, diorite and diabase, and, occasionally, limestone, sandstone, and conglomerate. A well known type of the latter is represented by a boulder 6x3x23 feet lying along the roadside at the S. W. 1, N. W. 1, Sec. 3 Milan. It contains rounded pebbles of brown. and bright red jasper, very characteristic and readily recognized. The parent bed is in western Ontario, north of Lake Huron, and its fragments are found in Ohio, Indiana, Illinois, Wisconsin and even so far south as Kentucky and west as Iowa. In size these boulders vary from cobble stones, and less, up to masses weighing many tons. One of the largest in the county, and probably the largest in this section of the state, lies about one and one-half miles east of the village of Ida, upon the farm of Christopher Knapp, claim 521. It is an enormous mass of hornblendic gneiss, contorted, of a light gray color, having a length of 20 feet, maximum breadth of 15 to 16 feet and standing four feet above ground (See Fig. 1). Presumably there

is as much or more below the surface as there is above, and it can scarcely weigh less than two hundred tons. A large fragment of what appears to be identically the same rock lies upon the S. E. 1 of Sec. 27, Whiteford township, farm of F. B. Doty. The boulder is 6x10x3 feet and there is said to be as much rock beneath the surface as in sight. Smaller fragments of the same rock are seen farther south.

A mass of limestone, large enough to be mistaken for an outcrop, lies in the western part of Frenchtown township, upon the property of Alexander Stewart (claim 21, N. River Raisin). It formerly projected above ground but building stone has been removed from it and there is now an opening into it 10 by 18 feet, containing three to four feet of water. It is of a light gray color, has a strong bituminous. odor, is rich in corals and brachiopods and effervesces vigorously in dilute acid. It lies directly over a bed of sandy dolomite, very different in its nature from this material. Mr. Frank G. Strong, of Monroe, at one time did some boring about the mass and informs me that upon one side only was rock struck with the auger. It seems very probable that we have here an unusually large mass of Corniferous limestone, which was pushed, or carried, beneath the ice sheet from a point a few miles to the northeast and finally left embedded in the clay. In 1865 Winchell described numerous masses of exactly this nature, occurring in the northern part of Lenawee and Hillsdale counties, the southern and eastern portions of Jackson and southern and western Washtenaw.* These masses were occasionally so large that they were mistaken for outcrops, lime-kilns were erected and lime burned until the supply of stone was exhausted. From an examination of their fossil contents Winchell identified them as Corniferous and concluded that their state of preservation was such that they could not have been transported from the outcrops of this formation known to occur about Mackinac. All the masses of this nature then known lay to the north of the Corniferous outcrop in southern Michigan and northern Ohio and Indiana. The conclusion was reached that they had been frozen into the shore ice of a lake or sea, detached and floated northward, where they were gently dropped upon the melting of the ice floe. The evidence of a northward transportation of drift was further strengthened by the finding of delicate Hamilton and Marshall fossils as much as thirty miles north of the

*Some Indications of a Northward Transportation of Drift Materials in the Lower Peninsula of Michigan. Am. Jour. of Sci., Second Series, Vol. XL, pp. 331 to 338.