26 Brookville Society of Natural History.

ALLUVIAL TERRACES.


BY E. R. QUICK.


There is no more interesting study in our local geology than that of the drift deposits and alluvial formation of the Valley of the Whitewater. The form of the valley is that of a V-shaped trough, one-third of its depth being filled with various terraces of different heights. When the Cincinnati island rose above the Silurian Ocean, erosion began and in all the ages after nature continued her work of cutting the almost level plain with valleys and canons of various depths and breadths. The Valley of the Whitewater we find is narrow, being seldom more than one and a half miles wide, and, to low water, about three hundred and fifty feet deep. Time was, however, previous to the glacial period, when out country was seamed and furrowed by canons as wild and rugged as those of the Colorado country, and what was then the canon of the Whitewater was, as I conjectured in a former paper on this subject, more than four hundred feet deep, or one hundred feet deeper than at present. Boring for gas at a point four miles south of Brookville proves that there are over one hundred and fifty feet of gravel below the bed of the river, deposited upon the lower Silurian formation. The valley of the Ohio at Cincinnati was one three hundred feet deeper than at present; thus at the narrow part of the valley west of the city the river once swept through a canon less than a mile wide and nearly a thousand feet deep.

Examination of the terrace formation along the sides of the Whitewater will show that four times our valley has been filled to a certain depth with the debris of the glacial period, and following each time a great portion of that deposit has been carried away by the action of the river. Each time, after the process of filling up, the water in excavating a new channel left portions of what had been the bottom of the valley and the river sank to a lower depth. This process is still going on, as


Read before the Brookville Society of Natural History, by E. R. Quick.


Brookville Society of Natural History. 27


there are trees standing to day the roots of which are five feet above low water but twenty years ago were below the lowest stage. These terraces, as they are called, are composed of gravel and clay, with an intermixture of larger stones, all variously stratified, each terrace in its own peculiar manner. The terraces are four in number and are numbered from the highest or oldest, in the order of their age. The first, large areas of which may be found on the east side of the river, south of Brookville, but nowhere on the west bank, is, of course, the highest, being, in this vicinity, a bout one hundred feet above the bed of the river. It lies always next the hills on sides of the valley, and affords the best of farming land. In studying a section of it we find, first, the soil, which is generally deep and rich; next, ten or twenty feet of tough yellow clay, mixed with gravel, below which is found a stratum of boulder clay of a bluish-gray color. This layer of clay is found from ten to twenty feet thick in this entire terrace, is impervious to water and causes all the springs found issuing from the terrace formation, and only where it is present can water be obtained in the alluvial deposits above the bed of the river. Loose gravel and boulders underlie it to an unknown depth.

The top of the second terrace lies about twenty-five feet lower than the first, or about seventy-five feet above the river, and is composed of much finer material, affording the best of gravel for roads and railways. But little clay enters into its composition, and that is in lumps or large clods rather than in regular strata. The material composing the entire terrace may easily be recognized as the finer debris from the washings of the first. It lies against the sides of the valley and the older terrace, and no water is found in it until the level of the river is reached. One peculiarity that I have noticed is that it is the oldest in which the remains of the mastodon (elepahs primigenus) have been found, the teeth and tusks of the animal having been unearthed in this drift in several cases. These bones have also been found in bogs on the surface of the older terrace, and in creek valleys of uncertain age, but nowhere else have I seen them deposited in the gravel by the same agency that made the terrace. It should be added that the soil on this terrace is light and sandy, being the poorest for agricultural purposes of any in the valley.

The third terrace, or next to the lowest, is the most extensive of any and having been encroached upon by the river a less number of


28 Brookville Society of Natural History.


years than any preceding it. The lowest stratum, explored at low water of the river, is composed of large stones mixed with gravel almost to the top, which is generally between thirty and forty feet above. Excellent soil is found on this terrace, and as it is unlike the next, above the flood line of the river, it is valuable farming land. No mounds built by past races are found on this or the next in order, while they are quite numerous in some localities on the older terraces.

The fourth and lowest terrace is what are popularly known as the river "bottoms," and lies about twenty feet above the height of low water. It is composed mostly of sand and gravel, covered in most places by a rich sedimentary soil. Its formation is many places antedates the settlement of the country, but in some cases I have known the building of the entire height of the terrace in the lapse of the last twenty years. Nearly its whole extent, in the early settlement of the country, was subject to overflows from the river, but some of the highest portions are now above the flood line. At its lowest exposure are found the shells of existing land and fresh water mollusks, such as are found throughout the later deposits in the Mississippi Valley. In what is known as "the bottom" in the eastern part of Brookville, shells are found where they were deposited by the flowing water, which protected the living mollusk not less than a century ago. The bones of the beaver and the antlers of the deer and elk are also found buried to a considerable depth in this formation.

Adjacent to this and made up from the washings of it and the older terraces are what are known as river bars, lying in what is termed the river bed. They are areas of shifting sand and gravel, but in many cases within the memory of man have been built upon by the floods until they have become a part of the fourth terrace. Thus it will be seen that the latter is still in process of formation. Each terrace seems to be formed of the debris of the older formations. The stratification of the two older formation extends below these later ones, but the third bears some evidences of being a portion of the second.

In many places are found large masses of conglomeration, or as it is locally known, "cement rock," composed of the terrace gravel, cemented by lime where water containing that substance in solution has percolated through it. These rocks are found in large quantities at mouths of small creeks or spring runs when water from the limestone hills has found its way through the soil above.


Brookville Society of Natural History. 29


Enormous accumulations of the boulder clay are found in some of the adjacent creek valleys, one of the largest of which is on the Little Cedar Creek, one mile east of Brookville, where it is known to be not less than one hundred and twenty feet in thickness. Throughout the entire mass are found the limbs and trunks of trees, the bark and growth of which much resemble cedar. (Identified by Coulter as all species of larch, Laryx, I believe) It may be noted as interesting that these pieces of wood are twisted and broken, apparently when the timber was sound. The drilling of a single well in search of natural gas at Cooley's station has demonstrated that from the level of low water to the lower Silurian formation there intervenes one hundred and fifty seven feet of gravel with no indications of boulder clay or other material.

If man lived previous to and left any evidence of his existence in these river gravels they have yet to be noted, as no authentic finds of either the remains of man, or his work, have been reported from the undisturbed strata. The writer has watched excavation of the terraces for many years and knows of no case when such remains have been found deposited with the drift except in the fourth or lowest terrace.


ERRATA.

Page 27, for "Elepahs" read "Elephas."


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