« PreviousContinue »
The writer has records, more or less complete, at four points, one on the Ohio and three on the Beaver. The first of these is at Dam No. 6 (Merrill) on the Ohio, near the mouth of Raccoon Creek. This set consists of a series of drill holes in a line across the river, at regular intervals of one hundred feet. Only two holes reach bed rock, one hundred and two hundred feet respectively from the northern bank of the river. All the other holes are deeper, but in no case reach rock. In all cases after passing through the immediate bed of the stream, the alluvium is much finer than shown by the escarpment of the terraces.
The next record is at the mouth of Wallace's run. No accurate records of strata are at hand, if indeed any were kept, but it is well established that in passing through the 60' of alluvium the material grew successively finer, and about twenty feet of it, immediately overlying the rock bottom, consisted of a very fine silt.
The third set of soundings reach rock in all but one hole, and consist of test wells sunk to deterinine the foundation for piers, about one-half mile below the mouth of the Connoquenessing. The records of all these wells agree and it is possible to form from them an accurate knowledge of the stratification of the filling of the buried channel at that point. Three distinct strata are recognized, the top of each horizontal, and conformable to each other, but non-conformable to the rock bottom on which they lie. Immediately overlying the rock bottom is a layer of fine silt, reaching in the middle of the channel a thickness of twenty-five feet, but thinning out to nothing on the sides. The top is horizontal and the thinning at the sides is due entirely to the rising of the rock bottom. Overlying the silt is a fine sand, eight to ten feet in thickness This also abuts against the rock at each side. Overlying this in turn is a fine gravel. The top of this stratum has been slightly eroded by the present stream, but it lies conformably on the sand stratum. Over the gravel is a thin deposit of river detritus.
The last of the records is at the railroad bridge between Wampum and Moravia. This set is incomplete, as the wells do not reach rock, but as far as they go they entirely agree with the others in the successively increasing fineness of the filling material, from the surface downward.
It should also be said that in a number of excavations about Beaver Falls and New Brighton “quicksand” has been frequently reported, and it is recognized by all persons conversant with deep excavations in the buried channel, that the material grows finer as excavation proceeds.
If we examine the structure of the terraces as revealed by excavations in the midst of them, we find them in harmony with the character of the filling of the buried channel. A well at Beaver Falls commenced at the top of the terrace, and excavated over eighty feet to rock, showed first a few feet of coarse material, followed by coarse gravel, which in turn grew finer until it corresponded with the gravel found in the buried channel. At Beaver, after passing through a few feet of the coarser material, the same is true, reaching, as we know from old wells, to the present level of the Ohio. At Georgetown, after passing through eight feet, no more “bowlders” were found to present river level.
It is not intended to convey the impression that the terraces, or rather the alluvial portion of them, are made up bodily of the finer material, for it is believed there are evidences from the escarpment that in the middle of the valleys the upper half of the filling is coarser than the average, but it is evident the stream depositing the alluvium had by no means the transporting power heretofore assigned it. In the refilling of the inner gorge the depositing stream was not a torrential current, but at first was quite sluggish, depositing fine silt only, gradually increasing in carrying power until the whole of the inner alluvium was deposited, but apparently at no time of sufficient eroding power to remove the deposit just previously made.*
Beaver, Pa., Jan. 5, 1895.
Art. XII.— The Glacial Land-Forms of the Margins of
the Alps ; by Hugh ROBERT MILL.
[From the Geographical Journal, January, 1895.] At the close of the sixth meeting of the International Geological Congress, which was held at Zurich during August and September, an excursion occupying a week was arranged in order to afford an opportunity of examining the remains of ancient ice-action far beyond the reach of actual glaciers on both the southern and northern slopes of the Alps. This trip was planned and directed by Professors Penck and Brückner and Dr. Du Pasquier, who, from their previous knowledge of the localities to be visited, had drawn up a guide in the form of a pamphlet of 86 pages, entitled “Le Système glaciaire des Alpes," which is published in vol. xxii of the Bulletin of the Neuchâtel Society of Natural Sciences.
The special object was to exhibit the part played in the formation of the land surfaces at the base of the Alps by the moraines or glacial formations, strictly so called, of the great Ice Ages, and also of the intermediate fluvio-glacial deposits of moraine material which had been rearranged by water on the retreat of the ice. To these was added in voluntarily, rather too much experience of the “pluvio-glacial” conditions which several days of steady rain at the beginning and at the end of the excursion induced on the steep surfaces of the clay slopes over which Professor Penck led his followers.
The main point of interest to the glacial geologist was the proof afforded by the sections of the occurrence of at least three successive periods of great glaciation separated by relatively long intervals, during which the vast volumes of water liberated by the melting ice dispersed and rearranged the moraine material. To a geographer the interest centered rather in seeing how the scenery and structure of great stretches of country were determined by the heaping up upon the plains of extensive systems of low hills-low, that is, when compared with the Alps, for some of them exceed a thousand feet in height-differing entirely from the mountains of elevation lying beyond them. These hills and fluvio-glacial plateaus represent the amount of glacial erosion and transport; they are the rubbish heaps of the mountain sculpture. Their effect on the broad geographical features of the alpine border is very clear in determining the lines of communication. The amount of weathering they have undergone, according to the different ages of the deposits, decides the character of the soil, which in turn reacts on the vegetation and appeals directly to the eye, the general aspect of the landscapes of the first glaciation differing in a marked degree from those of the last. Unfortunately, the weather was throughout unsatisfactory for photography, and the attempts made to take comprehensive views were failures, as had been expected. Some fair results were, however, obtained in detailed sections, which are of geological rather than geographical interest.
The excursionists met at Lugano on Monday, September 17, when thirty-seven members assembled, including representatives of Austria-Hungary, Germany, France, Russia, Norway, Holland, Switzerland, Italy, the United States, England, and Scotland. The weather was hopelessly wet, the one interesting result of which was to throw into the shade the distinguishing peculiarities of the Italian lakes, and reveal the essential similarity of their scenery to that of the English lakes and Scottish lochs. Professor Penck explained, and in some cases subsequently demonstrated, that the lower ends of the North Italian lakes were dammed by glacial accumulations, thus raising their level far above the rim of the rock-basins which contain their deeper water, and accounting for such peculiarities as the “recurved hook” of Lugano
A somewhat exciting boat-trip down the rapid Ticino landed the geologists at a fine section where the river had cut deeply through its moraine bar. Here, so far as sky or soil or vegetation were concerned, one might have been in Scotland instead of Italy. The steep bowlder-clay slope, grown in part with the common coltsfoot, when ascended, led to a level moorland, the poor soil of which was covered with heather, not shrubby as in more northern latitude, but composed of long separate flower-stems with exceptionally large heads of blossom. In the distance sombre pine woods crowned the hillocks, but at a turn of the path maize and sorghum were found as common field-crops, and the similarity to northern lands disappeared.
On Monday evening the party reached Ivrea by steam tramway from Santhia, and the whole of Tuesday was occupied in seeing, as well as the mist would allow, the vast glacial amphi. theater which surrounds the town, and in crossing the steep ridge of the Serra and the ferretto-covered slopes of moorland which succeed it to Biella, whence Milan was reached not long before midnight.
The morainic amphitheater is both the largest and most typical of the southern slopes of the Alps. Two ramparts of moraine material diverge nearly at right angles from the mouth of the narrow valley of the Dora Baltea, gradually diminishing in height, and these are finally united by an arc of moraines convex to the south, so that the whole completely surrounds a central plain, the two little lakes occupying the center of which overflow by the Doire, which cuts across the southern barrier. The eastern side of the amphitheater includes the largest moraine hill of the system, so large that it by no means belies its name of the Serra. It is a ridge more than 12 miles long, and in its highest part more than 1300 feet above the bottom of the depression, towards which the sides slope at an angle of 20°. The accumulation is the result of several glaciations, the moraine esterne, or early bowlder-clay, being covered with a red weathered crust of ferretto, the intercalation of which between the older and newer moraines is one of the proofs of the occurrence of an interglacial period.
A railway journey next morning allowed a fine forenoon to be spent in driving from Lonato to Salo, on the Garda Lake, through moraines and fluvio-glacial formations—some of them compact conglomerates. Here the successive glaciations were very clearly shown in several sections, the lower moraine of the earlier Ice Age having its pebbles much weathered ; in some cases even the granites had crumbled into clay, retaining only their original form. Above this came a layer of conglomerate formed of ice-scratched pebbles stratified by running water, and on the top a fresher bowlder-clay much less weathered than that below.
Coming up the Garda Lake at night, the contrast between physical and political geography was finely shown by the uniform cliff walls and continuous water-surface broken by the long beam of the electric search-light at the Italian frontier station, which swept the lake all night for the prevention of smuggling.
On Thursday, September 20, the excursion left Riva by rail at 6.20, and had an excellent opportunity, on the journey to the Brenner line at Mori, of seeing that ice is not the only agent which is capable of producing scenery by the accumulation of detritus. The landslip-covered plateau of Loppo, with its lake formed in a hollow of the dolomitic detritus, and the still more extensive piles of landslip material about Mori, contrasted and compared in many ways with the glacial phenomena seen farther south. The Austrian Railway Company had provided, free of charge, an observation car at the end of the train, from which a good view was obtained of the deltaic wilderness through which the lateral tributaries entered the Adige, and of the extraordinary fertility of the alluvial flats, where maize and vines in alternate narrow strips covered almost all the available land. Later the porphyrite gorges marked the passage across the center of the range, and beyond the Brenner saddle carriages were provided at Matrei to allow of a more detailed examination of the terraces of the Sill valley than would be possible from the train. The vastness of this accumulation of moraine, fluvio glacial deposit, and moraine