C) THE ALLUVIAL THEORY
All the theories supported by the main geologists agree that either the geologic constitution of the island and its location induce to maintain it is constituted from geologically recent alluvial drifts; this would give also some scientific validity to what passed on by tradition.
As a result of deepened bibliographical searches and analysis of the surveys available in that time Gioacchino De Angelis d'Ossat [4] (1865-1957) (that, however, was not able to obtain the authorizations to execute further specific surveys, neither to examine the geognostic data collected for the impressive restoration works of the Fatebenefratelli hospital in 1934 by Bazzani, for whose foundations approximately 800 concrete piles were used), confirmed the alluvial origin of the island; such thesis is supported also by altimetry of the island, equal or slightly lower than the banks, and by the sandy characteristic of the banks themselves (Arenula)
Also the results of the deep drillings known in that time confirm the underground presence of layers of loose materials: fluvial drifts in the upper part and fluvial-lacustrine one in depths.
The most important boring was carried out below the small central barrel-vault of the Fabricio bridge across the foundation pillar up to a depth of 35.90 mt. The examination of the 31 samples, together with those relevant to two smaller boring (10.15 and 15 mt.) executed upstream and downstream the bridge, was carried out by Clerici in 1911 [6].
The survey produced the following results (the elevations are referred to the "zero level" at Ripetta):

  • from mt. 9.50 to 0.5 drilling of the bridge pillar (below the central barrel-vault)
  • from mt. 0.50 to –16.50 "sabbie sciolte ed elementi di varia grossezza" [loose sands and elements of various dimensions] up to small gravel
  • from mt. –16,50 to –26.40 layers of clayey sands, clays and marlspict.C1 - Schematic
                                      geological section across the
                                      Tiber island

Therefore all the found materials are loose and alluvial.
The pict. C1 [3] shows the schematic geologic section from the Capitol to the Janiculum through the Tiber island; the Clerici's survey is indicated in correspondence to the middle of the left branch.