Macon County, Illinois - Chapter 6: Geology, 1880

Chapter 6: Geology

ERE is little of scientific interest in the geological features of this county. Few older formations than the Drift have been discovered, and even in the Drift the similarity of the deposits is so great that the geolo- ;t finds no incentive to extended investigation. The surface of the county is mostly composed of black prairie soil, from three to ten feet deep.

Underlying this is a subsoil of clay, gravel and sand. Still deeper, occur alternate beds of clay, quick-sand and gravel, intermixed with various shales, slate, limestone, sandstone and several thin strata of coal. As may be seen in another part of this article, ten strata of coal, varying in thickness from a few inches to three and a half feet, have been found, at different depths, in a bore of 875 feet.

Formation Of Prairies

This is a subject that has provoked much scientific discussion in the last half century ; but, after long and careful investigation, a majority of the most eminent scientists, including Profs. A. H. Worthen, State Geologist of Illinois, and Lesquereaux, who has published a most excellent paper on this subject in the first volume of the Illinois Geological Reports, have reached the conclusion that the vast treeless plains, or prairies, of the West, with their peculiar surface soil, owe their origin to the same causes that are at present operating to form prairies, though on a less extensive scale. The black, rich soil, so characteristic of prairies, is doubtless due, as has already been stated in the chapter on Geography, to the growth and decay of successive crops of vegetation, which in the geological ages of the past, under a far higher temperature and more favorable atmospheric conditions than now exist, grew to an extent unknown since the appearance of man upon the earth. These prodigious crops of plants and grasses were from year to year submerged, and becoming decomposed, contributed their annual accumulations to the surface of the country. By the continuation of this process for untold centuries and by the subsequent recession of the waters that once covered the entire Mississippi valley, a black, mucky soil, was

The peaty soil, or muck, thus produced was by the gradual disappearance of the water from the low and level portions of the land, converted into the rich vegetable mold of the prairies, which in course of time, became adapted to the wants of men and animals. The soil in such a formative state as has just been described, would be unfavorable to the growth of trees, a practical demonstration of which may be seen nowadays in the treeless swamps and sloughs found in timbered regions.

Alluvium. - This term is applied to the recent deposits and formations made by rivers, creeks, &c. Alluvial matter is usually composed of earth, sand and gravel that originally belonged to the great Drift Period. The banks of the Sangamon river are from eight to nine feet high, and composed of dark clay, vegetable mold, with sand intermixed. On the gravelly shoals numerous small shells are found, belonging to the genera Paludina, Melania, Cijclas, and Unio. In portions of the river-bed where the current is less rapid, Anodontas are common. In the marshy tracts on the flat prairies the remains of old shells have been found, belonging to the genera Planorbis and Lymnea. By the recent tile-draining of these ponds and marshes, their beds have been rendered arable, and the calcareous matter of these shells doubtless adds much to the productiveness of the soil.

Drift. - Geologically this name is applied to the vast collections of loose earth, rocks, boulders, etc., that are widely distributed over the surface of the land, and that were transported from the north by the agency of glaciers, ancient seas and ocean currents. At Decatur there are from six to ten feet of mostly yellowish-brown clay ; then similar clay with boulders and pebbles extending downwards several feet, and this in turn succeeded by blue and brown clay and pebbles. " On Big Creek, sec. 4, T. 15 N., B, 3 E. the drift appears as follows :

" (1). Thirty feet of drift clays, with loose pebbles, sand and clay; at lower part comminuted sand and clay. On the lower slope of this is an abundant growth of Equisetum, or scouring rush, associated with Gentiana alba and Pedicularis lanceolata.

"(2). Eight feet. The upper part dark brownish- black ; lower part black, and apparently of vegetable origin ; no pebbles seen ; when struck, a hollow sound is produced. Debris from above falling over unites with this, and forms a marshy talus, on which were growing Lobelia syphilitica, Sagittaria variabilis and several other marsh plants." - III. Geol. Pep.

Large boulders, composed of quartz, feldspar, mica, and hornblende, and pebbles of granite, quartzite, sandstone, porphyry, limestone, chert, spar, &c., are found in great quantities along the streams and scattered far and wide over the prairies. Several large rounded masses of granite and other azoic rocks have been observed lying here and there over the country south and east of Decatur, showing conclusively the agency of glaciers in transporting these huge boulders to their present positions.

Three miles north of Oakley lies a boulder of gray granite, irregular in shape and measuring eight feet across in two directions, and extending four feet above the ground. It is surrounded with rich prairie soil, and is entirely isolated, no other boulders appearing in the vicinity.

Wells. - On the flat prairie lands south of Decatur living wells are reached at a depth of from twelve to fifteen feet. In the east part of the county water is found at a depth of from twelve to sixty feet, but it fails in dry seasons. Beneath the blue clay, in beds of sand, from twenty-seven to forty feet deep, abundance of pure cold water is obtained. In the north-east portion of the county water is found at from ten to twenty feet below the surface ; near Forsythe, from sixteen to thirty feet. On the high prairies, in the western part of the county, water is reached at various depths. The region of Harristown appears to be the continuation of a low drift ridge, where a well, dug through ten feet of yellow clay and seventy feet of blue clay, failed to yield any but surface water. Two miles north-west of that place, also in an elevated region, water was struck at thirty feet, and an abundant supply remains throughout the year. There are some excellent springs in the county which have been mentioned in another chapter.

Gas-Wells. - In 1871 Wm. A. Wilson, Esq., living ten miles south of Decatur, struck gas in a well on his farm at a depth of forty-five feet. It issued from a formation just below the hard pan, and was discovered at several different localities on the place. By means of pipes he conducted the gas to his dwelling, and used it for both light and fuel. The bed of quicksand, out of which the gas flows, has been penetrated to a depth of fifteen feet without reaching the bottom. The gas continued to flow uninterruptedly for several years, and the supply was apparently inexhaustible; but latterly the vein became weaker, and at the present time the gas has almost ceased to flow. Another boring was made a quarter of a mile east of this well, and at a depth of fifty feet, passed through a bed of black peaty soil five feet thick, but without striking gas. A second gas well was struck on the north-east side of Decatur, at a depth of forty feet, and flowed copiously for six months, when the well, for some reason, was filled up. Forty rods south of this second well, and at the same depth, another vein of gas was found, but after flowing violently for some time, the supply failed, and the well was closed up. Thegas from these wells isa light carburetted hydrogen, usually called "marsh gas," and burns with a bluish flame, emitting intense heat. It is highly inflammable, and, when pure, is perfectly odorless and usually free from sulphur. It has been found in great abundance at several places in the State, but of its origin little is yet definitely known. The fact that it is generally found in beds of peat and decayed vegetable matter, indicates that it is of vegetable origin. Numerous wells, affording the same quality of gas, have been dug to a depth of from sixty to seventy feet, near Champaign, Illinois, and the gas, at that depth, being apparently inexhaustible, has been utilized for illuminating and heating purposes. Doubtless this gas, as well as petroleum, when it comes to be better understood, will be found to exist regularly in certain geological formations belonging to the Carboniferous Age, and to owe its origin to the same cause or causes that produced the immense coal fields of the world.

Coal. - In 1875, a boring was made for coal at Decatur with a diamond drill, by the Western Coal Mining Company. The work was abandoned at a depth of 507 feet, 1 inch, and probably at from eighty-five to one hundred feet above the level of the Springfield and Howlett coal. The record of this bore is as follows:

The eleven feet, nine inch, blue limestone, found in this bore, at the depth of 345 feet, is supposed to be the same as that forming the Carlinville and Shoal Creek bed, which is usually from 210 to 240 feet above the coal beds at Springfield. Since the foregoing facts appeared in the State Geological Report another boring has been made at Decatur, and, at a depth of 608 feet, a stratum of coal, three and a-half feet thick, believed to be the Springfield vein, has been found.

In 1875-6 this second and deeper boring was made for coal just north of the Rolling Mill, on the east side of Decatur, by Mr. J. Edward Bering, of the manufacturing establishment of Chambers Bering and Quinlan, and to his kindness we are indebted for the

A shaft 7 by 14 feet is now being sunk near the place where this boring was made. A depth of 130 feet has already been reached, and it is intended to prosecute the work to the stratum of coal 3} feet thick (shown above at the depth of 608 feet) which, in connection Avith the adjacent fire-clay beds, it is thought, will pay for mining. The ten distinct strata of coal found in the bore recorded above, with the thick beds of shale, clay, slate, etc., intervening, show that the lands in this region, during the formation of these coal beds, must have undergone a similar number of successive elevations and depressions. Vegetation grew and accumulated during the elevation of the lands above the water, and the vegetable matter thus accumulated was converted into coal during the submergence of the lands. The beds of shales, slates, etc., were also formed under water, by sedimentary deposits. It is estimated that thirty feet of vegetable matter would be required to form one foot of coal. If so, some idea may be gained of the vast growth of vegetation there was upon the earth during the Carboniferous, or Coal-growing, Age, to form the coal beds of the world, some of which are forty and even fifty feet in thickness. The stratum of coal shown above as being 3* feet thick, must according to this estimate have required a layer of vegetable matter one hundred feet deep. That coal is of vegetable origin is shown by the fact that it is mainly carbon, w'hich element forms from one-fourth to one half of all the vegetation on the globe. Also thin slices of coal placed under the microscope show distinctly the vegetable structure which may sometimes also be seen with the naked eye. Our common peat-bogs, as has been well said, are excellent examples of "incipient coal beds." In conclusion wTe may say that in economical geology Macon County is rather unfortunate in having neither a coal mine nor a stone quarry. However, there is an abundance of excellent brick and tile clay and beds of fire-clay, deep in the earth, as shown by the preceding records. As we are closing this article, we are informed by Capt. J. S. Post, of Decatur, that a huge molar of a mastodon was some time ago found in the bed of the Sangamon a little below the Wabash and Pacific bridge : also, that the tusks of a gigantic animal of olden times were exhumed from a bed of quicksand thirteen miles west of Decatur. These tusks are said to measure seven feet in length. They are now in the State Museum.