Great Neapolitan Earthquake of 1857/Part II. Ch. XVII

by Robert Mallet

Chapter XVII.

Journey Over the Pass and by Lago Maorno.

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As we had ascended, the soft, ill-bedded limestone of the lower roots of the hills, had gradually given way to indurated liassic-looking hard, elastic, cherty beds of yellow limestone, in distinct and highly-inclined layers, probably metamorphic (see Section)—these, after many alternations came back to the same soft, cretaceous-looking stuff, and are finally lost, (as we top the steep and get upon the more level table of the shoulder and elevated valley), under a deep loose deposit of almost perfectly pure and fine white sand, (from which the pass and town derive their name), mixed largely, but unequally, with impalpable chalky particles. This extends to beyond Tardiano, under the southern summit of Monte Vajana; but at various points the rock beneath might be seen, and proved that we had lost the limestone proper, and got upon an endless succession of thin beds, all more or less metamorphic, consisting of ash-yellow limestone, often highly quartzose, hard and elastic, alternating with thick and thin beds, of grey and purple and green clays, and of variegated marls; and in one place some beds of 4 to 6 feet of impure alabaster, all highly inclined. (Geolog. section, Diagram No. 241.)

At the highest point of the pass, near Arena Bianca, The barometer stands at 26.50 inches, thermo. 42° Fahr. at 12:30 P.M., and the elevation above the sea at Naples is 3158.3 feet. At Tardiano, the contrary slope towards the valley basin of the elevated mountain lake, the gloomy Lago Maorno commences. This lake is commonly pronounced Lago Maouri by the peasantry. Many singular beds of metamorphic shales, and indurated clays, with some thin seams as hard as chert or jade, and, lithogically, very like the latter, are passed, and at about 500 feet above the level of the Lago Maorno, where the descent has become again very steep, along by the northern side of a nameless torrent that discharges into it, great beds of intensely hard, flinty, and hæmatitic, dark-blue grey limestone occur, with huge seams of very hard, black and brown hæmatite, 12, 15, 24, and 30 inches thick. All these run here in a general direction, for a considerable way, nearly east and west, but as they extend, are much twisted towards the north. They are very nearly vertical, their dip being only 15° to the south. The torrent separates these beds to the south from the cretaceous limestone which here makes its appearance once more. Sloping up steeply to the north for more than half a mile, with a bare, weathered, and water-furrowed surface, without a blade of vegetation, extend parallel, and nearly vertical beds of green, grey, and purple clays, alternating with beds of yellow, soft, clayey sandstone, many yards in thickness, standing in ridges high above the worn-down clays. (See enlarged plan and section, Diagram No. 242.) My attention was directed, in particular, to these beds of ferruginous rock here, by finding that the compass would no longer work, the needle turning round 90°, within a few paces' change of position. On examining the hard ferruginous beds, of thick, blue grey, flinty limestone at A (plan and section) I found, with some surprise, that they exhibited at the exposed south face, for about 200 feet in length, numerous large fresh-made fractures, running nearly vertically, right through the whole of the strata, up to the dense but soft clay beds to the north. Further to the east the beds are covered up by clays on both sides, and the ends of the strata alone visible. These were much obscured by the torrents of creamy clays that the rain was washing over them; but yet I was enabled to trace similar fractures across the tops of the beds, and extending vertically downwards through them, for a distance of about 250 yards eastward. To the westward, the torrent runs through these indurated beds, and cuts off those to the west of it from any contact visible with the clays.

At the base of the vertical beds below A, in the bed of the torrent, great masses of freshly-fractured and fallen rock were lying, and in several of the fractures, the widest of which was open, on an average, about an inch, on the south face, of the nearly vertical beds, I found fresh fractured splintering fragments in various spots. These had dropped down a few inches only, and I could replace and fit them into the spots from which they had fallen in the jaws of the fissures. There could be no mistake as to the freshness of the fractures, for all the old and weathered portions of the rock, were a deep iron rust, in colour; but the fresh-broken surfaces, a bright blue grey, of a deep tint. Many large fragments from the tops of the upcrop of the beds, had also been detached at weathered fissures, from the south face, and lay thrown into the bottom of the torrent.

It was unmistakably obvious, that the fractures had been produced by the transit of the earthquake, and that the push, of the vast piled-up mass of comparatively soft heavy clay beds, &c. to the north with which they had been forced against the barrier of these hard limestone beds that ramparted them in, had been of such force as to fracture the latter in many places. The hard unyielding rock had broken; the softer clay beds had merely been slightly compressed, and changed insensibly in form; hence they presented no evidence of the force that they had transmitted to the fractured rock. On ascending the slope of the clay beds, northward about 500 yards, however, I found confirmatory evidence of my conclusion, in great masses of fresh fractured and fallen sandstone from the thick beds at C. (Diagram No. 242.)

The N. E. face of the deep gorge beyond C, not shown in diagram, that brings another torrent down in a N. W. to S. E. direction, also showed great falls, of these soft sandstone cliffs in the bottom. I had great difficulty in descending the wet clay beds, which, devoid of a single pebble, presented no foothold whatever upon their unctuous and slippery slope, and a fall produced much the effect, of being dipped into a succession of paint pots.

This fact was to me one of peculiar interest: it was the first example I had found, of actual fracture of beds of hard rock in situ, by the impulse of au earthquake wave—a phenomenon in kind totally distinct, from such breaking off of great masses, as the rock falls of Campostrina, or the Arguilles of Padula. Here were beds of the very hardest and toughest rock, such as, with difficulty, I broke specimens from, with the hammer, fractured for many yards in depth (fully thirty yards was visible) and extending over a great length of bedding, and yet free from any other sign of violence, or any other sensible disturbance of position. It realized forcibly to the mind the enormous power of the impulse of shock with even this moderate velocity when acting at once upon great masses, at free or outlying surfaces; and is suggestive of the much more potent effects, that must be occasionally produced in loftier ranges, subject to still more powerful impulses. Hooker’s account of the rock dislocation witnessed by him in the Himalayan shocks recurred to mind. Strictly interpreted, however, even this, is but an example of dislocation by secondary effects, of the wave, not by the wave itself.

The Lago Maorno now comes into view, a dreary pool of about a mile long by half as much wide, in the midst of a mountain basin, surrounded with deep tenacious clay soil, apparently of great fertility, but swampy and wet. Across this the mules passed with much labour, sinking nearly to the knees. The shallow valley basin, devoid of tree, house, or human being, is surrounded with low barren hills, all apparently of soft limestone, those to the eastward of the lake, coming down close and abrupt to its margin. Above and beyond the hills to the N. E., Monte Spagnoletto rises high, and powdered with snow; and far away to the S. E., I see the lofty summits of Monte Raparo, and Monte Armizzone, deeply covered with it, as well as the intervening ridges.

All attempt to cross these now, and gain access to Castel Saraceno, Chirico Raparo, Carbone, Calvera, Latronico, Episcopio, and many other towns lying deep in the mountain recesses, far to the S. E. and east, I found would be impracticable. All these towns, and many others around them, had suffered severely.

At the margin of the Lago Maorno, the barometer marked 27.10 inches, thermo. 42° Fahr. at 3 P.M., and the surface of the lake I find to be 2526 feet above the sea, at Naples. Crossing the piano, or basin of the lake, we ascend again, cross the Serras, of Cerzuto and Pizzuto, and on the ridge of the latter gain the first view of Moliterno, and dimly discern through the rainy atmosphere, Sarconi, Spinosa, Viggiano, Marsico Vetico, and even Montemurro, all towns, nearly or quite destroyed. We now commence to descend, by the side of the highest fork of the Fiume Sciavra, which falls into the Agri, along the east flank of a wild and grand ravine, with the torrent in the bottom, which, in its now swollen state, seems to be sweeping bodily before it, masses of the beds, of red and yellow clays and marls, and calcareous detritus, that to a depth of 30 to 60 feet form its boundaries, and conceal the formations beneath the sloping plain, the Piano of St. Martine.

At the highest point I passed upon the Serra di Cerzuto, the barometer stood at 26.65 inches, thermo. 45° at 5^h. 5^m. P.M. Naples time. The height above the sea was 2994.4 feet.