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

by Robert Mallet

CHAPTER VIII.

THE CALORE AND TANAGRO—ST. MICHAEL'S CAVE—GEOLOGY OF THE VALLEY—CAMPOSTRINA—GREAT ROCK FALLS.

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Accompanied still by the Padre, I then crossed the river, and scrambled up the opposite bank, composed of deep clay and boulders, with gravel, to the mouth of St. Michael's Cave, from out of which issues that portion of the whole waters of the Calore, (as the Tanagro above this is called); that entering the limestone at Polla, at the upper end of the gorge of Campostrina, and finding its way by subterraneous channels, here debouches, and turning some primitive water-wheels at the Catalan forge, falls in a pretty succession of cascades to join the Tanagro again in its open bed. The mouth of this large cavern is probably from 50 to 80 feet above the open bed of the river opposite, and its jaws present evidence of powerful erosion. There are large stalactites within, and at about 300 feet from the mouth further entrance is barred by the water, which issues from the cavern almost clear and pellucid, while that of the river below is as white as milk, with impalpable cretaceous matter in suspension. As the water of the Calore thus divided between the subterraneous and the open channels at Polla is very muddy, and contains much calcareous sediment, it is obvious that that portion which passes through the subterraneous channel is filtered, or at least deposits much of its solid material on the passage, and is no doubt now forming new and strangely situated beds of clayey limestone, within the cavernous heart of the mountain range through which it passes, and which here separates, as by a huge wall, the valley of the Tanagro, from the Piano di Diano.

A short way within the cavern is a shrine of wood, with a rude plaster figure of St. Michael, of about three feet in height, in the interior. I found this 'genius loci' had been overthrown by the shock, and as the shrine is fastened up like a sort of cage, the figure was still leaning supine, against the back of the box at a slope of about 30° with the vertical. The saint had fallen, in a direction from W. towards the E. The base of the image projected widely in front, but less at the rear, and the figure being of a very upright character, it had been thrown over by the first movement, to an angle beyond the range of recovery, by the return stroke of the wave, and so remained out of the perpendicular.

The velocity of the shock had been sufficient to upset the image, but had not been sufficient to overturn the square wood shrine or cage in which it was placed, and which was about 8 feet high and 31/2 feet wide in the E. and W. direction. A sufficient corroborative proof, of the steepness of emergence of the wave here, is afforded by the stability of this shrine, near the top of which, was the stage on which the figure stood. An extremely small velocity, if horizontal, or nearly so, in direction, would have sufficed to overthrow the whole affair.

The volume of water delivered by the stream from the cavern, doe not seem to be above one-twentieth that of the main open stream of the river below, which receives no tributary of importance between this and the other end of the subterranous duct, at Polla.

Close to this cavern, the northern end of the gorge commences, through which the open stream forces its way—a jagged, wall-sided cleft, betwixt precipices, of rather soft, ill-bedded, and cretaceous looking limestone, nearly white in fresh fracture, and whose mean height is probably about 800 feet.

Standing upon the left bank of the river, and 50 feet or so below the level of St. Michael's Cavern, one is enabled to see With some clearness, the geological relations of the main valley and of its lateral ranges, which in transverse section here (looking southward) are approximately shown in Fig. 154. The Apennine limestone of Monte Alburno and its associated range, dips to the south-west and south, but with a constantly varying angle of dip. What is the connection of the beds, of the opposite or eastern chain, with those at the bottom of the valley, is not traceable; it may be one of disunion and dislocation, as the existence of the gorge of Campostrina would suggest; but the beds look, upon the whole, to be parallel continuations at the east side, of those deep under Monte Alburno upon the west.

Above these in the valley, lies the coarse calcareous breceia, in beds approaching conformability at the east side; but where visible through the telescope, at the summits of the underlying ranges, on the west side, appear to be wholly unconformable to the escarpments of Alburno. Small but irregular valley bottoms, are formed at both sides, between Dia.^m Pl. 154

SEction of the Valley at Pertosa.

A. Limestone.
B. Calcareous Breccia.
C. Detritus.

the underlying summits of the breccia mountains, and the flanking chains beyond; and in these, as well as in the bottom of the main valley, a great depth of loose material is deposited, chiefly heavy calcareous clays and boulders. These are deeply cut into by the lateral torrents, and still more deeply by the Tanagro itself, which here rolls over a bed wholly of rounded boulders, the skeleton of the washed-away detritus.

It is not easy here, or indeed anywhere else in the Southern Apennines, to imagine the train of causation (upon any of the usually accepted views of elevation) that led to this formation. It seems probable, however, that before this upper part of the valley assumed its present character, the surface of the breccia occupied something of the line $dd$, &c., if not one still higher, and that enormous masses have been removed by denudation, between those which now form the opposite ranges of "Collines." As respects our immediate subject, it will be obvious that any earthquake shock, emergent from the eastward at a steep angle, must arrive, through an immense thickness of beds of limestone first, and of breccia afterwards, before reaching the surface; and hence with vast loss of vis vivâ, and buffed, as to much of its destructive power.

I was unable to attempt determining, whether the breccia beds lie directly upon the limestone at both sides of the valley, or may have some other thin beds interposed. But I think the first is the fact.

The military road of Campostrina, over the rampart that separates the valleys of the Tanagro and of the Calore (as its higher stream now is called), is led over the mountain at the eastern side of the river gorge, winding round several lateral valley-gorges, and crossing the principal one by an imposing viaduct of considerable altitude, built of ashlar limestone, and carrying a narrow road, over a double range of semicircular arches, upon piers overloaded with material and buttressed out, transversely to the width of the road, to more than twice its breadth at their deepest bases. This viaduct must have received the shock very nearly transversely to its length, but emergent at a high angle. It has sustained no damage whatever, though a top-heavy mass, a sufficient proof of the value of good masonry in an earthquake country. Padre Mancini politely accompanied me on foot to the summit of the pass, about 21/2 Italian miles, to point out the site, of an enormous fall of limestone rock, which had been produced by the shock.

The general direction of the deep narrow gorge, in the bottom of which the foaming torrent of the Tanagro rolls for several miles along, is nearly N. and S. The limestone beds at either side, as well as the jagged serratures of the cliffs, in many places vertical or overhanging, correspond to each other, and prove it to have been torn by separation of the opposite mountain masses.

The strike of the beds is nearly N. and S., and they dip at various angles, but all very steep towards the S. and S. W. The bedding is not very clearly defined, and the rock is lithologically, softer and more of a cretaceous character, than that of Monte Alburno, and may probably belong to a different member of the limestone formation. No fossils anywhere met my eye, and other occupation precluded my looking for them.

The first great fall of rock I found had occurred about

Photo Pl. 155

Vincent Brooks, lith. London.

(illegible text)

(illegible text)

a quarter of a mile beyond the viaduct: its position is shown in Photog. No. 155, the view in which is taken, looking southwards, some of the revetment walls at the turn of the zigzags of the military road, mounting the hill, beyond the viaduct, being visible at the left of the picture. Near the centre of the view, where the white face of rock is visible, a mass of limestone had been thrown off in a S. W. direction, 69° E. of N., from the face of the cliff, and, shattering in its fall, had carried some thousands of tons of rock, small stone, and fine whitish debris, down into the bed of the river. The talus was heaped up at the base of the cliff, and stretched nearly across the whole breadth of the bed, forming a serious obstruction to the current, which was dammed back a good deal, notwithstanding the rapidity of its slope, and the torrent had already washed away large portions of the fallen mass, and as I watched it, was sorting out the finer material, and as it was removed slight falls of stuff continually occurred at the toe of the slope of debris, into the water rushing past its base.

The Padre informed me, that for four or five days after the 16th December, the volume of water discharged at Pertosa, both by the open channel of the river, and by the cavern of St. Michael, was visibly diminished, he thought by as much as one fourth the former delivery, and both currents ran turbid and foul. The delivery of the cavern, he thought, was even now, less than what it had been before, but the open river had returned to its usual regimen, except in so far as it was much whiter in colour, from the suspended chalky limestone, than he ever remembered it, though always more or less, so discoloured.

I remarked that many other smaller falls of rock had taken place, at various points of the gorge, and from the N. E. side of the lateral ravine, which is crossed by the viaduct, whose steeply inclined sides are in many places, covered with loose material and large angular boulder blocks. Several of these had been dislodged, and projected into the bottom, leaving in some cases the torn traces of their headlong descent, in furrows whose direction I found to be, about N. 15° E., the stones falling to the southward.

On gaining the summit of the ridge, next above the viaduct, and looking to the S. W. across the gorge to the opposite mountain, I observed a very singular cavity in the slope of the flank, and at such a distance back from the edge of the cliff, as would render it probable it may be vertically over, the subterraneous duct in the rock, carrying the water from Polla to St. Michael's Cavern.

This is sketched in Fig. 156. It appears as if produced by the falling in of the roof, of a cavernous enlargement of the subterraneous duct at this point; and the mass standing up in the middle of the crater-like cavity, is probably part of the roof, tilted over in the fall, and sustained by other fragments beneath. The Padre said there was no water at the bottom, nor any entrance from it to a subterranean chamber, and it had received no alteration, that he was aware of, since the earthquake. The cavity is probably a quarter of a mile long, from right to left in the sketch.

Amongst the many lying wonders that were narrated about the earthquake, I afterwards heard it circumstantially affirmed that this, was a crater, had been formed at the time of the shock, and that fire had been seen to issue from it.

At nearly the highest point of the road, I found the

Sketch Pl. (illegible text)

Campostrina.

Sketch Pl. 178

Interior Court. Palazzo Palmieri. Polia.

Sketch Pl. 187

Vincent Brooks, lith. London.

Atena.

barom. 28.09 in., thermo. 42° (14th Feb.), which, when reduced, gives the elevation=1913.4 feet above the sea. This proved soon after, however, not to be the very highest point, which I reached at about 150 feet higher. The total elevation, therefore, at the road is 2063.4 feet above the sea, and about 1420 feet above the piano of the valley bottom at Auletta.

At the former point nearly, the road is formed upon a side cutting and small embankment, on limestone covered with 3 or 4 feet average, of arable clay land. It is sustained by an ill-built revetment wall of dry stone, with mortared top courses, in all about 12 feet high. The general direction of the centre line of the road is 30° W. of N., and nearly level. For a length of about 300 yards, an irregular longitudinal fissure was open, in the surface of the roadway, at about a quarter of its breadth from the revetment wall, of about 3 to 4 inches in width, and in some spots still, 10 or 12 inches deep, though much obliterated and filled by rain washings.

The form of the fissure and section of the road, as sketched, are shown in Fig. 157. Three portions of the revetment had been thrown towards the west, and from the position of the fallen material, the direction of the wave-path proved to have been N. 140° W. The portion of the road to the west of the fissure, had slipped and descended about 4 to 6 inches below the former level, as shown in section $b$. It was impossible to tell, whether the revetment wall had been founded upon the rock or not, but from the appearance of the ground at its base to the westward, I believed it had not: in any case, it had gone out at the base, towards the west, and with the mass of earth behind, had been severed from the remainder of the road filling, and slipped at the same moment. It is a case very analogous to the Auletta fissures, with this difference, that here, from the unsupported position of the road embankment and revetment, and the direction of the shock, the separation had much more nearly approached a "throw off" at the instant of shock, mixed with the movement of slip or descent.

About a mile further on, just before the rapid descent commences into the Valley of Diano, another set of road fissures had been formed, where the road is also in side cutting, but slopes off at the western side without any revetment. Here the fissures which are shown in Photog. No. 158 are clearly produced by the slippage off to the westward, of an enormous breadth of the clay land, reposing upon a surface of limestone—sloping westward at about 20° to the horizon—and with beds not much more inclined, and dipping in the same direction, circumstances all favourable to a large slip. The direction of wave-path shown, is about the same as the preceding. The telegraph poles all along this portion of the road, I remarked, had been loosened in the ground, and thrown out of plumb,

Photo Pl. 158	⁠	Photo Pl. 110
Vincent Brooks, lith. London.	⁠	Vincent Brooks, lith. London.
Fissures on the Road, near Polla.	⁠	Polla.

and had not yet been again secured. Being restrained by the wires from rotating at the top, i.e., confined to vibrate in a plane not very widely departing from transverse to the length of the wires, they had not formed conical cavities at their butts, but such as would have been produced by the pole, working forward and back, in a line not quite transverse to the length of wire, but, so far as its restraint would permit, also towards the south, so that on the whole their movement, coincided with the evidence of wave-path here given by everything else.

It was doubtless this swaying drag upon the wire (it is but a single one) produced by the poles that broke the former, and so cut off all telegraphic intelligence, between the great earthquake district and Naples, for above forty-eight hours, during which the most intense anxiety was felt in the capital, as to the fate that had probably overwhelmed the provinces.

Upon the highest summit of the pass is erected a little roadside shrine—the Capella della Madonna della Pieta, which was riven and fissured in a very remarkable manner, and only stood, by help of some pious props, that since the earthquake had been strutted against its tottering back and ends. It is shown in Sketch No. 159, made on the spot, and in Photog. No. 160 (Coll. Roy. Soc.), taken some weeks afterwards.

The plane of the front face of the "Tenementa," is north 45° W., and the fractures clearly indicated a wave-path having an azimuth direction of north 157° 30′ W., or from the N. N. E., and having a very steep angle of emergence. The little structure was built, of coarse limestone rubble, plastered all over, and the cohesion of the mortar joints but small. Applying to it the equation of overturning

${\mbox{V}}^{2}={\tfrac {4}{3}}\ g\ {\sqrt {a^{2}+b^{2}}}\times {\frac {1-\cos \theta }{\cos ^{2}\theta }}$

with the value for ${\mbox{V}}$, (ascertained next day) for the emergent shock at Polla, it is certain that the split-off portions above $b$ and $c$, would not have rocked and returned to their places as found, but have been completely overturned, had the wave-path been at a less angle to the horizon than 61° nearly. The angle of emergence here, therefore, must have been as steep as that, at least, and may have been steeper.

Looking back from this point, and sweeping the mountain side to the westward of the gorge with the telescope, I see a large Casale—upon the slope of the hill distant about four miles—almost in ruins, and can plainly discern by a favourable light, that it has been overthrown by a shock, which there, had the same general direction as here.

The direction of the slope of the hills at both sides towards the southern entrance to the gorge, begins to change and to trend round to the S. W.; and a little further on I catch the first view down upon the grand Vallone di Diano, its plateau level as a sea, stretching away twenty-six miles to the south, and four or five miles wide, the Calore, folded along upon its central surface like a silver cord, losing itself in distance, and the mountains rising almost abruptly from the piano at either side, the further end closed in and surrounded, by pile over pile, of dark grey mountains and snow-clad sierras, at last shutting out the horizon.

I can now perceive that Campostrina gorge, is the hinge, between the two valley systems, and that the valley I have left, and that I am about to enter, have their respective axes almost at right angles to each other, the pivot round which they wheel being the mountain mass behind the town of Polla, and to the S. W. and W. of it. The descent now becomes rapid, and after another mile or so, Polla becomes completely visible, the dominant town of the north end, of the wealthy plain, along whose east and west sides I begin to discern many others.

Polla was an important place; originally, as its name imports, one of the ancient foundations of Magna Græcia. Nothing older than middle-age architecture remained, however, before the earthquake, and of this the Castello, near the summit of the town, was the most prominent. Its position in the rich country around, had produced its rapid modern growth to nearly seven thousand inhabitants, and most of its buildings were comparatively modern and pretty well built. Its streets and houses, churches and belfries, with olive yards and gardens between, spread themselves over the crown and slopes, to the north, south, and east of the large, low, short and well-buttressed spur of solid limestone rock, which juts out from the mountain range at the east side of the Vallone di Diano. The lengthway of this spur, is rather transverse to the general line of the valley, and its steepest side is towards the south. The city looked down upon the Calore, slowly and deeply sweeping past its eminence, and upon its own suburb of St. Pietro, at the opposite or right bank of the river, connected with the city by a fine old bridge of Roman style, and to the southward it gazed for miles over the glorious and unbroken hill-girt plain.

Its position and appearance are seen in Photog. No. 161. (Vide Frontispiece.) As I descended towards it, huge yawning gaps began to show themselves, upon the northern and southern slopes, where for acres in extent, everything had been levelled, all traces of streets annihilated, and where they had been immense mounds and sloping avalanches, of white and dusty stones and rubbish, filled up and encumbered the ground. Between these, shattered and bowing fragments of walls, and torn remnants of once lofty buildings, stood in mighty confusion; beams and rafters, tossed up like the arms of the despairing, stood out hard and black against the pallid heaps. The words of the Hebrew bard, referring to a still more eastern scene of earthquake energy, recurred to memory with a strange reality—"How is the city become an heap, the defenced city a ruin." Months of bombardment would not have produced the destruction, that the awful shudder of five seconds involved, when thirteen hundred houses fell together with deafening crash, and overwhelmed above two thousand of their sleeping inmates, and with clouds of suffocating dust, choked the cries of horror and anguish, that rose from the startled and often wounded survivors. In three different directions, conflagration soon added its terrors to the scene, and beamed up, a flickering and ominous light, into that dreadful night of cold and wailing, throughout the lingering hours of which, in helpless agony, they listened to the passionate entreaties for relief, the dying sobs, of relatives and friends entombed around them, and dreaded for them, more than for themselves, the recurrence of other shocks. The cold gray light of winter's dawn, obscure with smoke and dust, revealed hundreds bruised, or with broken limbs, without a roof to shelter them, many without a garment to cover them.

It required some hours' familiarity with such scenes, before the mind assumed sufficient composure and capability of abstracting the attention, to pursue the immediate objects of my inquiry.