EXTINCT GLACIERS OF THE ITALIAN SIDE OF THE ALPS.

Figure 43. Map of the Moraines Of Extinct Glaciers

  (FIGURE 43. MAP OF THE MORAINES OF EXTINCT GLACIERS
   EXTENDING FROM THE ALPS INTO THE PLAINS OF THE PO NEAR TURIN.
   From Map of the ancient Glaciers of the Italian side of the Alps
     by Signor Gabriel de Mortillet.

  A. Crest or watershed of the Alps.
  B. Snow-covered Alpine summits which fed the ancient glaciers.
  C. Moraines of ancient or extinct glaciers.)

To select another example from the opposite or southern side of the Alps. It will be seen in the elaborate map recently executed by Signor Gabriel de Mortillet of the ancient glaciers of the Italian flank of the Alps that the old moraines descend in narrow strips from the snow-covered ridges through the principal valleys to the great basin of the Po, on reaching which they expand and cover large circular or oval areas. Each of these groups of detritus is observed (see map, Figure 43) to contain exclusively the wreck of such rocks as occur in situ on the Alpine heights of the hydrographical basins to which the moraines respectively belong.

I had an opportunity of verifying this fact, in company with Signor Gastaldi as my guide, by examining the erratics and boulder formation between Susa and Turin, on the banks of the Dora Riparia, which brings down the waters from Mont Cenis and from the Alps south-west of it. I there observed striated fragments of dolomite and gypsum, which had come down from Mont Cenis and had travelled as far as Avigliana; also masses of serpentine brought from less remote points, some of them apparently exceeding in dimensions the largest erratics of Switzerland. I afterwards visited, in company with Signori Gastaldi and Michelotti, a still grander display of the work of a colossal glacier of the olden time, 20 miles north-east of Turin, the moraine of which descended from the two highest of the Alps, Mont Blanc and Monte Rosa, and after passing through the valley of Aosta, issued from a narrow defile above Ivrea (see map, Figure 43). From this vomitory the old glacier poured into the plains of the Po that wonderful accumulation of mud, gravel, boulders, and large erratics, which extend for 15 miles from above Ivrea to below Caluso and which when seen in profile from Turin have the aspect of a chain of hills. In many countries, indeed, they might rank as an important range of hills, for where they join the mountains they are more than 1500 feet high, and retain more than half that height for a great part of their course, rising very abruptly from the plain, often with a slope of from 20 to 30 degrees. This glacial drift reposes near the mountains on ancient metamorphic rocks and farther from them on marine Pliocene strata. Portions of the ridges of till and stratified matter have been cut up into mounds and hillocks by the action of the river, the Dora Baltea, and there are numerous lakes, so that the entire moraine much resembles, except in its greater height and width, the line of glacial drift of Perthshire and Forfarshire before described. Its complicated structure can only be explained by supposing that the ancient glacier advanced and retreated several times and left large lateral moraines, the more modern mounds within the limits of the older ones, and masses of till thrown down upon the rearranged and stratified materials of the first set of moraines. Such appearances accord well with the hypothesis of the successive phases of glacial action in Switzerland, to which I shall presently advert.