GRAVEL RIDGES DEPOSITED BY STREAMS
12. The Origin of Pebbles and Gravel
In determining the origin of the long parallel ridges south of Dayton the most obvious fact is that all are composed of gravel, in other words, of rounded stones or pebble. Now, every geologist knows that pebbles are only the rounded fragments of larger rock masses. In their original state rocks may occur as a series of successive layers, more or less interbedded with clay, ora s large irregular masses of rock, often continuous over many square miles or territory, but never in the form of rounded pebbles. Any one visiting the large quarry northwest of the railroad station at Centerville, or the much smaller quarried in the vicinity of Dayton, can not fail to be impressed by the occurrence of these rocks in layers, often differing greatly in thickness, but extending laterally over wide areas. Similar facts may be noticed in the big railroad cut south of the water-works at Miamis- [p.43] burg, and along the beds of many streams, where the stone has no commercial value. In fact, limestone, sandstone, clay, and shale always occur in layers of different degrees of thickness.
Wherever fragments of limestone occur in smaller masses, these have been broken off from the much more extensive layers, of which originally they formed a part, either by the violence of some spring or summer freshet or by the wedging action of freezing water, where it has penetrated into some crevice of the rock, in winter. Massive rocks may be split by the growth of entering roots of trees. Even alternations of heat and cold may play a part, causing the exposed surfaces of rocks to expand or contract more than the less accessible inner parts. In fact, many agencies tend ot reduce rocks to fragments, but these fragments at first are irregular in form and angular in outline.
All fragments remain angular until rounded by the action of running water. The farther rock fragments are traced down stream, from their parent layers or ledges, the more rounded they become. The original angular corners evidently have been ground off. The sand carried along by the stream is the chief grinding material. It is not pushed against the rock by the running water as firmly as sandpaper in the hand might be forced against a piece of wood, but the final results may be the same. The angular corners of the rock fragments are gradually rounded off. Every little freshet disturbs the position of the fragments enough to cause them to present other angles to the action of the sand. Sooner or later the more or less rounded forms which we expect among the pebbles of gravel are produced. That such a haphazard method of rounding should result in spherical forms is scarcely to be expected, and spherical forms are very rare. However, pebbles with all of their angles more or less rounded off are very common, and rounded pebbles, no matter where found, always are evidence of the former presence of running water.
The rolling action of pebbles along the bed of a stream, during high water, often may be heard by putting the ears below water level. The noise is evidence of the violence of the impact of the pebbles against each other. The quickness with which bricks along the seashore are reduced to rounded form is evidence of the effectiveness of a rolling motion in producing [p.44] rounded forms. Along the Ohio river, entire barges of coal frequently are wrecked and sink to the bottom. In dry summer months, parts of the river bottom sometimes are so widely exposed that the farmers for miles around camp for days on the dry parts of the river bed and collect, from among the sands and gravel, the rounded pebbles of coal, often almost spherical in shape. Many a pile of rounded pebbles of coal may be noticed by the observant eye in the coal buckets of the natives during the early winter months, as evidence of their thrift.
[Photo: Gravel pit on the hillsides northeast of the Ohmer Park area, showing the arrangements of the sands and gravels in layers. The inclined layers were produced by waters flowing toward the east (left), the successive layers representing successively later deposits of a violently flowing glacial stream.]
13. The Arrangement of Gravel and Sand in Alternating Layers
The sorting of pebbles of various sizes into layers is accomplished in nature only by the action of moving water. Any slight increase in the rate of flow of a stream causes the current to roll along fragments of slightly large size, in addition to the smaller pebbles already set in motion. Any slight diminuition in the rate of flow allows a large part of the heavier pebbles to remain, while many of the lighter pebbles continue their journey. The stationary heavier pebbles may be buried by the lighter pebbles which [p.45] are continuing their travels from farther up stream. Later, the finer gravel may be covered by the lighter sands, which continue to move as long as the river is not stagnant.
Then a sudden rise of the stream may roll enough pebbles over the sand to serve as a cover before much of the sand can be removed by the rising waters, and thus the circle of deposition may be repeated over and over again,--coarser pebbles, finer pebbles, sand, finer pebbles, coarser pebbles, finer pebbles, sand, --resulting in the distribution of these materials in more or less horizontal alternating layers.
This arrangement of gravel and sand into more or less horizontal layers, differing in the coarseness of the material involved, is one of the most constant features of deposits made under the influence of running waters. It may be recognized even in deposits of the finest sand. It gives rise to the thin layers often noticed in sandstones. Occasionally these sandstones are stained beautifully with brownish and reddish tints by the presence of iron compounds. The church at the corner of Forest and Grand avenues is an excellent example of the use of such sandstones for building purposes. Also in limestones, the presence of thin layers often may be noticed, especially where their existence has been made more evident by the processes of weathering. In proof of the deposition of sand beds by running water, occasionally even the finest ripple marks are retained. Formerly an excellent example of these ripple marks was exposed in the sand beds beneath the till deposit immediately north of the railroad at the eastern extension of Irving avenue, southeast of the St. Mary’s institute, in the woods long known as the ‘Brothers’ Woods.”
14. Gravel Ridges Formed Under the Influence of Running Water
We are familiar with the presence of sand and gravel in the beds of creeks and rivers. We notice their arrangement in layers, along the banks of streams, where the latter have been undermined more or less by some change in the course of the channel. When these layers of sand and gravel occur only a moderate distance above the present course of the streams, it [p.46]
[Photo: Ripple marks in the sand beds occurring beneath the till in the “Brothers’ Woods” southeast of the St. Mary’s Institute, showing the deposition of these sands in currents of water. Photographed in 1895.]
requires no great effort of the imagination to realize that their existence here indicates the former presence of running water at a higher level. Either the streams formerly were larger and filled up a larger part of the valley, or the present channel has been cut down to a lower level. However, when the layers of sand and gravel occur at the tops of high hills, the former existence of running streams at these high altitudes seems preposterous, at least at first sight, and the presence of sands and gravels at these higher levels requires more detailed explanation.
The long narrow gravel ridges south of Dayton, between Carrmonte and Delco Dell, rise far above the valley bottoms. At many points the crests [p.47] of these ridges rise 80 to 100 feet above the immediately adjacent lowlands, valley bottoms, or hollows. Estimating the eastern margin of the Miami valley bottoms, between the foot of the bluffs at the Calvary cemetery and the Moraine farm at 780 feet above sea level, many of the ridges in the Calvary cemetery attain a height of 140 feet above this level. The highest point of the ridge occupied by the Calvary monument rises fully 160 feet above this level. The Pike ridge, west of the Cincinnati pike, at the O’Neil road, rises 120 feet above the eastern margin of the flat bottom lands. This elevation is attained also by the Sunset ridge, where crossed by the Big Hill road, north of the Moraine farm. Tip Top knob, at the southern end of Old Orchard hollow, equals the elevation of the highest part of the Calvary ridge, namely 160 feet above the eastern margin of the Miami valley bottom lands. The highest part of the Delco Dell ridges rises 140 feet above these bottoms. Similar high points occur along the ridges in the Hills and Dales area.
When it is realized that the tops of these gravel ridges reach elevations of 160 feet above even the most elevated part of the flat bottom lands of the adjacent part of the Miami valley, and that they rise more than 220 feet above the waters in the present channel of the river, the difficulty in accounting for the presence of these gravel ridges by the action of running water is fully appreciated. We are familiar with the lowering or cutting action of water. We know that water has produced the valleys which intersect the hills. However, we are not accustomed to think of water as building gravel ridges up to heights far above the level of the valleys. Such an idea is too suggestive of water running up hill, and appears unreasonable. Nevertheless, all the evidence is in favor of the production of the long gravel ridges, south of Dayton, by means of running water. This evidence is revealed in the numerous gravel pits and road cuts exposing the structure of the interior of the ridges.
15. Gravel Pits and Road Cuts Exposing the Structure of the Interior of the Gravel Ridges
A magnificent exposure of the structure of the interior of one of the gravel ridges is presented by the gravel pit immediately north of the Cal- [p.48]
[Photo: Gravel pit of the Dayton Washed Sand and Gravel Company, west of the northern end of the Chapel ridge, at the edge of the Bluffs north of the Calvary cemetery.]
vary cemetery grounds, where the Washed Gravel and Sand Company is cutting away the northern end of the ridge immediately west of the Calvary ridge. Here the gravel pit has a vertical face of 145 feet. Coarser gravels occur near the top. Finer sands are more abundant near the bottom. The arrangement of the gravel and sand in numerous horizontal layers is clearly in evidence. Layers of coarser gravels alternate with finer gravels, and both are interbedded at various levels with sand. The structure unquestionably is that produced by running water.
How running water could exist at these high levels and build up thick deposits of gravel and sand, apparently without the assistance of lateral confining walls, is another question, which will be taken up later. For the present, it is sufficient to emphasize the fact that the stratification of the sand and gravel, forming these ridges, into interbedded layers of different [p.49]
[Photo: Nearer view of gravel pit at northern end of Chapel ridge, in Calvary cemetery, showing arrangement of pebbles in layers. See page 43.]
degrees of coarseness, is evidence that these gravel ridges were formed under the influence of running water.
Another excellent exposure of the structure of the interior of the gravel ridges is presented by the gravel pit at the northern end of the Chapel ridge, southeast of the chapel in the Calvary cemetery. Here the gravel pit exposes a vertical face 30 feet in height. The coarsest pebbles again occur within 15 feet from the top of the ridge, but coarse and fine gravels are interbedded in more of less horizontal layers, and evidently were deposited under the influence of running water.
At the western end of Mayo avenue, the cut through the Chapel ridge is only 12 feet deep, but is sufficient to indicate that the coarser pebbles occur near the top of the ridge.
The arrangement of the gravels in alternating layers of coarser and finer material is well shown also by the Pike ridge, at the O’Neil road cut, west of the Cincinnati pike. Here the vertical face of the cut is fully 50 feet in height, counting from the lowest part of the cut, but the basal part [p.50] of the cut is covered more or less by loose gravel, brought down by slumping.
Southwest of the sub-power station on the Ohio Electric line, south of the O’Neil road, the large gravel pit exposes a vertical face 80 feet high, and southeastward, on the eastern side of the Cincinnati pike, there is a gravel pit at least 50 feet deep. Still farther southeast, at the northern end of Sunset ridge, on the Governor Cox estate, another gravel pit has a vertical face of 60 feet, and at the southern end of the Sunset ridge, on the Moraine farm, north of the Stroop road, there is still another gravel pit, with a vertical face of 65 feet.
Surely these exposures are sufficient to reveal the general character and arrangement of the materials forming the gravel ridges. Since all of them agree in showing the deposition of the sand and gravel in layers, under the influence of running water, the evidence of the origin of these high gravel ridges, south of Dayton, as stream deposits, may be regarded as conclusive.
16. Gravel Ridges Deposited by Irregular and Often Turbulent Currents
The most obvious fact about the layers of pebbles and sand in the gravel ridges is their irregularity. Usually these layers are not strictly horizontal for any great distance. If any layer be traced along the face of a cut transverse to the ridge, it often will be seen to rise or fall, sometimes abutting against other layers consisting of a different degree of coarseness. Thus, the edge of a coarse gravel layer may abut against a more or less inclined layer of sand, or a layer of sand may wedge out suddenly between two layers of gravel. Frequently it is evident that one of the layers of sand and gravel formerly had a greater lateral extent, but that some change in the direction of the current caused a part of the previously deposited layers to be cut away locally, thus altering the shape of the base of the channel over which the stream flowed. Then the process of filling up the channel was begun again. Similar features are shown also in those gravel pits which expose the structure parallel to the length of the ridge. [p.51]
It is evident that the base of the channels of the streams which formerly deposited these sands and gravels must have risen and fallen in a somewhat similar manner and with about the same degree of irregularity as the present crests along parts of the ridges. Such abrupt, irregular changes in the deposition of sand and gravel suggest corresponding irregular changes in the force and direction of the currents then existing, so that the conclusion can not be avoided that the gravel ridges south of Dayton were deposited by currents which often were very irregular and sometimes were very turbulent.
The coarseness of some of the gravel layers, especially in the upper 25 feet of the ridges, indicates that the currents sometimes were very swift. Pebbles 6 to 8 inches in length are abundant. Pebbles a foot long are not rare, and boulders 15, 18, and even 24 inches in length are present, but are not abundant. However, most of the material forming the gravel ridges is much finer, and implies a less rapid flow of water. By far the greater part of the material consists of coarse sand and of the smaller pebbles, usually not exceeding 3 inches in diameter. The larger pebbles may be very numerous in some of the upper layers, but they form only a comparatively small part of the total mass of the ridges. They suggest rapid currents near the closing stages of stream flow. However, some of the larger boulders may have been washed along only short distances by stream action. It will be shown later that the streams depositing the gravel ridges flowed along channels beneath a great thickness and large expanse of glacial ice, and some of the large boulders may have dropped from the roof of these subglacial stream channels, having been released by the melting of the ice. [p.52]
[Photo: Brassfield limestone in abandoned quarry east of fork of Brandt and New Carlisle pikes, six miles northeast of Dayton. Deposited by irregular currents.]