The choice of the building site for the Basilica of St Peter in Rome was a long, laborious process. The pre-existent basilica, dating from the reign of Constantine, had suffered from severe problems of stability due to the fact that a fair portion of it rested on sand and gravel of recent origin, having been deposited there by the periodic flooding of the Tiber – hardly the sort of terrain with geo-technical characteristics one looks for in a construction site. The new Christian basilica was wisely located on Pliocene era compacted clays, the best footings available in the Eternal City. Other attempts made on alluvial terrain ran afoul of enormous problems related to stability; even Bernini’s campanile was afflicted by cracks and partial collapse, and finally razed in 1629. The problems, then, are ones of statics which have nothing to do with the damage suffered by the monuments of Imperial Rome ranging from the column known as the Colonna Antonina to Flavius’ Amphitheatre caused by earthquakes unleashed in the Apennines – which damage for that matter was increased by the flooding mentioned above.
Equally difficult was the choice of the ornamental materials for Rome’s new basilica, due to geological conditions as well as to the particular historical context when the mother of all churches was being built. Here we must go back to the XVI Century with Rome just having been sacked by German mercenaries, the Lanzichenecchi, when the will to renewal was stronger than ever. It is for this reason the Michelangelo was called upon at St Peter’s and that Bernini had been granted full license to express his markedly naturalistic bent.
Still, valuable building materials plundered from the monuments of Classical Rome soon were exhausted so that it was necessary to hunt up a fresh supply of stone nearby (so beautiful as to deserve the name “marble”). The times were no longer suited for doing as the Caesars of Ancient Rome had done and have unsullied white marble brought in from Carrara or Paros, or other sumptuous building stone brought in from all over the Mediterranean. It is no mere happenstance that for the most part ornamental materials used in Rome Baroque construction came from no more than 60-70 kilometres away.
Thus it was that between 1627 and 1700 many old quarries were sought out and reactivated, so that even today it is possible to see traces of these operations in old sites in the Sabine Hills region just north of Rome. Still lying in a quarry near the town of Cottanello is a weathered column which engineers of the time did not manage to transport to Rome. Cottanello “marble” was just the answer to changed demands, being beautiful in appearance – deep red fully veined in white –, close at hand and cheap, even though in reality it was not a true marble (at least in a geological sense).
Cottanello “marble” is in reality a marl limestone (which in Italy locally takes the name Scaglia) and was deposited over a vast sea basin some tens of millions of years ago in the Umbro-Sabine region of central Italy. It a beautiful red in colour continuously veined in ever changing patterns of pure white calcite, making its appearance is anything but homogenous.
The rock’s particular complexion is not, however, original in the sense that it is due not to sedimentary but to tectonic processes, meaning the deformations it has undergone as being a part of the Apennines, a mountain chain formed as a result of enormous compressive stress that caused the European and African plates to collide for millions of years in the Mediterranean. The calcite veins and such are the result of complex deformational processes which have substantially changed the size and “form” of the original rock. In particular, a major strike-slip fault running north-south the length of central Italy (the faglia sabina) is responsible for the further increase in complexity of the pattern formed by the white of the calcite and the red of the limestone. Whereas this fault may yet be in motion today, it is certain that its most active period was around 5 million years ago when man had yet to make his appearance and the landscape was quite different from today.
The Cottanello quarry is located precisely within the limited area where the faglia sabina runs, with the material quarried there faithfully recording its greater or lesser proximity to the principal sheer zone. The columns of Sant’Andrea at the Quirinale together with those of St Peter’s Basilica were the first to be fashioned using marble from the quarry and therefore have especially rich, complex veining – which makes the marble more valuable – having been quarried in the heart of the most deformed fault location. The columns of Sant’Agnese in Agone and the facing of Pope Alexander VII’s tomb in St Peter’s were instead fashioned in later times and were therefore quarried in areas farther from the fault, being less deformed and poorer in calcite veins. As one leaves the fault area the geological features change. Anyone can ‘visit’ the Cottanello quarry at a distance simply by taking a tour through the extraordinary places just mentioned. The variegated appearance of the columns at St Peter’s thus reflect not just the difficult times when the Basilica was under construction, but the tormented geological history of the entire central Apennine region as well, with the “marble” of Cottanello being an exemplary summation.