The alluvial theory has never been confirmed, neither by dedicated surveys nor by stratigraphies coming out from previous boring, generally carried out for building scopes.
This does not allow, therefore, to absolutely refute the theory of Besnier [11] that in 1902, based on verbal information collected from the technicians of the Genio Civile of Rome [Civil Engineers Office], maintained than under the recent sand and gravels layers, one can see “lambeaux du tuf volcanique” [edges of volcanic tufa].
Therefore, according to Besnier, the base structure of the island would be tufaceous, the same nature of the close Capitol and the hills on the left bank; the hardness of the tufa would have forced the river to divide itself in two streams and the tufa rock would have constituted the base for the following alluvial drifts.
It has to be noted that, during the excavations carried out under the S.Giovanni Calibita Hospital [12], some tufa elements have been found (1982, nearby the church) while other excavations carried out upstream of the previous ones and under the most ancient structures of island (1990), brought to light a sandy sedimentary structure.
However not even such assertion have ever been geologically acknowledged and confirmed with specific surveys and the main geologists deem that the tufaceous elements are not indication of a lithoid-lionated tufa layer, but probably remainders of defensive constructions, foundations or moorings as also indicated in the historical sources (Livius).