Landscape in History, and Other Essays

by Archibald Geikie

Such a varied form of ground was eminently favourable for human settlement. The earliest races could find or make rock-shelters almost anywhere. The fertility of the soil afforded to their successors good pasturage and fields for tillage, while the hillocks, girt round with cliffs, and the flat-topped ridges, shelving precipitously to lower ground, offered excellent sites for fortification and defence. Owing to the porous nature of the ground, much of the rain sinks at once beneath the surface, instead of flowing off in brooks. Hence many of the valleys are usually dry, unless in wet seasons. But water can be obtained all over the district by sinking wells, and that this source of supply has been in use from a remote period and to an almost incredible extent, has been strikingly shown by the recent excavations beneath the pavements of the Roman Forum. It would be difficult to find anywhere a form of ground which shows better the influence of geological structure upon the early fortunes of a people.

With some portion of what has been written by Italian and other observers on this district, I have made myself acquainted, and having had the advantage of tracing on the ground the records of the successive stages through which the Campagna has come to be what it is, I propose in the following pages to give an outline of this prehistoric chronicle. I should like to attempt to present to the reader such a picture of the whole sequence of events as has vividly impressed itself on my own mind, avoiding, as far as may be practicable, technicalities and details. Three distinct successive phases can be recognised in this sequence. First came a time when the waves of the Mediterranean broke against the base of the steep front of the Apennines, and when all the low grounds around Rome, and for leagues to the north and south, lay sunk many fathoms deep. Next followed the chief period in the building up of the Campagna. A host of volcanoes rose along the sea-floor on the west side of Central Italy, when ashes, dust and stones were thrown out in such quantity and for so prolonged a time as to strew over the sea-bottom a mass of material several hundred feet thick. Partly from this accumulation and partly by an upheaval of the whole region of Italy, the sea-bottom with its volcanic cones was raised up as a strip of low land bordering the high grounds of the interior, and a few huge volcanoes were subsequently piled up to a height of several thousand feet. Lastly succeeded the epoch in which the volcanic platform, no longer increased by fresh eruptions, was carved by subaërial agencies into the topography which it presents to-day. Each of these three phases has had its history legibly graven in the rocky framework of the Campagna, and some of its memorials may be recognised even within the walls of Rome.

I. The records of the first period lie beneath the Seven Hills on the left bank of the Tiber, but rise high above the plain on the right bank, where they form the chain of heights that culminates in Monte Mario, 455 feet above the level of the Mediterranean. These records, forming the series known to geologists by the name of Pliocene, consist of a lower bluish-grey clay and an upper group of yellow sands and gravels, the whole being probably a good deal more than 450 feet thick. The clay (Plaisancian) has been found to extend, with a remarkable persistence of aspect and contents, from the north to the south of Italy. It has yielded several hundred species of mollusks and other organisms, which show it to be a thoroughly marine silt, deposited on the bottom of a sea of some little depth.

At the time of the deposition of this clay the mountainous backbone of the country had already undergone the greater part of that prolonged series of terrestrial disturbances whereby solid sheets of limestone were folded, crushed, ruptured and driven together into a series of parallel ridges, having a general trend from northwest to southeast, and forming the nucleus of what is now the chain of the Apennines. At the epoch when our story begins, however, this chain was still incomplete, and probably a good deal lower, as well as narrower, than subsequent upheaval has made it. Instead of forming, as it now does, the lofty axis of a broad peninsula, it then consisted of a series of parallel islands and islets, separated from each other by long and often narrow sounds or channels. In general appearance it must have resembled parts of the coast of Dalmatia on the opposite side of the Adriatic. Many of the more prominent mountains of the region stood then entirely surrounded by the sea. The Sabine Hills, for example, rose as an island, while Soracte formed another island farther west. A long strait ran northwards by Rocca Sinibalda and Rieti to Terni; another of narrower width stretched towards Perugia and formed then the estuary of the Tiber. All the Roman Campagna, together with the low grounds on both sides of the Apennines, was at that time submerged under the sea. The great band of volcanic heights and cones that extends from Aquapendente to the Bay of Naples had not yet come into existence, but over their site the waters of the Mediterranean lay many fathoms deep.

The climate of Europe had for ages been of so genial a character that sub-tropical types of life had long flourished both in the sea and on the land of this quarter of the globe. But in the period of geological history with which we are now concerned, a remarkable diminution of temperature was in progress all over the Northern hemisphere. As the warmth grew less, the distribution of plants and animals came to be seriously affected. Many southern forms were extirpated from districts which they had long inhabited, while in their place came migrations of northern species. This modification made itself felt both on terrestrial and marine life. Thus in the Atlantic Ocean a number of northern shells, which had pushed their way southward even as far as the coasts of the Spanish peninsula, were able to enter the Mediterranean when a connection was opened between that inland sea and the main ocean outside. It is interesting to note that among the shells introduced into the Mediterranean basin at this time were Astarte borealis, Buccinum groenlandicum, Cyprina islandica, Panopaea norvegica and others whose appellations sufficiently indicate the latitudes where they now find their chief home. On the land, too, such quadrupeds as the reindeer and the now extinct mammoth wandered from the plains of Lapland and Russia to the shores of Italy. Eventually when the refrigeration gave way to the return of more genial conditions, the northern invaders died out. They have no living descendants now in the south of Europe.

The grey clay which forms the lower division of the Roman Pliocene series is best seen on the right side of the Tiber where it forms the lower half of the ridge of the Vatican and Monte Mario, and where for more than five-and-twenty centuries it has supplied material for making the bricks of which ancient and modern Rome has been so largely constructed. The same clay has been found at lower levels on the opposite side of the river. On the flanks of the Pincian Hill, at the Piazza di Spagna, it was exposed about twelve years ago in some excavations connected with the adjustment of the aqueduct of the Aqua Vergine. Only a few feet below the crowded pavements of that busy thoroughfare lies the old sea-bottom with its abundant relics of marine life. In borings for water which have been made around Rome the same deposit has been ascertained to extend below the later volcanic formations of the Campagna. Thus at the Appia Antica fort, near the tomb of Cecilia Metella, the clay was entered at a depth of about 300 feet from the surface or eighty feet below the level of the sea. As the upper limit of the clay at Monte Mario lies about 200 feet above sea level and the distance from that outcrop to the fort in question is about six miles, it might be inferred that there is here evidence of a southeasterly dip of the deposit amounting to forty-six feet in a mile. But before any inference of this kind can be accepted, some considerations should be taken into account, of the highest interest and importance in relation to the early history both of the Campagna and of the Apennine chain.

Before dealing with these questions, however, let us complete the examination of the marine deposits of Monte Mario. In the valuable section disclosed on the slopes of that hill, the grey clay is seen to become sandy towards the top and to include seams of sand which rapidly increase in thickness, until, with their included layers of gravel, they form nearly the whole of the upper part of the ridge. These yellow sands, generally distinguished by the name of 'Astian,' have been traced, like the clay below them, along nearly the whole length of the Italian peninsula. The striking contrast which, in the nature of their material, they present to the clay, plainly points to a great alteration of the geography of the coasts at the time when they were deposited. The sea must have become rapidly shallower. Not improbably one of the uplifts now took place, whereby the land has been raised at intervals to its present height. The steepness of the descent of the mountains into the sea might not lead at once to much gain of land along the western coast; but instead of the grey mud that had previously accumulated in the deeper water, coarser sediment, brought down by numerous torrents from the hills, now spread out over the sea-bottom. Such a transition from the finest silt to gravel and sand could not fail to affect the distribution of the animals living along the coast-line. Accordingly, on comparing the fossils in the sands with those of the clay, we see that while some of the shells, especially the larger and more massive kinds, continued to flourish in abundance; others, which found their most congenial haunts in tranquil waters, were driven further out to sea.

As the Pliocene deposits so well displayed at Rome are known to preserve throughout Italy the same twofold character, with the same types of sediment and of organic remains, the observer who tries to follow their development in the Campagna is soon puzzled by the way in which they there suddenly disappear and allow their place to be taken by later deposits of volcanic origin, which are known by the general designation of Tuff. The most astonishing example of this local peculiarity is to be found at the Monte Verde, south of the Janiculan ridge. The clays and sands which rise in horizontal layers almost to the top of Monte Mario are there entirely cut out, and the tuff, which lies as a mere thin capping on the crest of that hill, suddenly descends across the truncated edges of the Pliocene strata to the alluvial plain of the Tiber. On the opposite side of the river, the upper sands have been almost entirely removed and the tuff is found lying almost immediately on the lower clay. It is clear that there must have been an extensive, though no doubt local, erosion of these marine strata, before the main body of the tuff was laid down. By what agency this erosion was effected is not quite clear. Not improbably a gap occurs here in the record, representing an interval of considerable duration of which no chronicle has survived.

Passing over this hiatus, we still encounter marine deposits, but these are of volcanic origin, and leave us meanwhile to speculate in the dark as to whether the denudation was the work of the sea or of terrestrial waters. Owing to the thick covering of tuff which has overspread the Campagna and concealed all that lies below, it has become difficult to obtain adequate data for the discussion of this question, which is of considerable interest in the history of the Campagna. The most reliable evidence would be supplied by a series of borings across the district in different directions. Such a series may perhaps hereafter be undertaken for the purpose of obtaining water for the farms and homesteads, which sanguine patriots foresee taking in the future the place of the present solitude, and in that event, the geologists of Rome will no doubt be on the watch for all the information that can be gathered from this source as to the nature of the rocks underneath, and their relations to each other.

In the meantime, much might be done in this attractive department of local geology by a far more detailed study than has yet been attempted of the surroundings of the Campagna. In particular the recognisable stratigraphical horizons among the Pliocene strata should be definitely traced and mapped in detail, where they emerge from under the volcanic tuff. It would then be possible to measure the amount of erosion in various places, and to determine how far the spread of the volcanic sheet across older formations is due to actual unconformability and how far to simple overlap. At the same time, the precise height could be ascertained of the upper limit of the Pliocene deposits, and data would probably be obtained for determining not only the minimum amount of the uplift of the land since the Pliocene period, but also how far and in what directions there may have been any warping of the peninsula in the course of the elevation. We know from the observations of De Angelis that the Plaisancian clays, which at Monte Mario do not rise more than 200 feet above the surface of the Mediterranean, reach a height of as much as 1,050 metres (3,445 feet) in the upper part of the valley of the Arno, near Subiaco, only about thirty miles east from Rome, or an upheaval of as much as 108 feet in a mile. It remains still to discover how far this amount may fall short of the total extent of the post-Pliocene uplift of the Apennine chain.