the surface profiles have been transformed since the region was first deforested in roman times, as settlers worked outwards from strips flanking the roman roads — the via cassia, the via amerina and the via flaminia — which had been struck through the forest. in the deforested slopes, streams with even moderate flow have cut deeply eroded gullies and valleys in the geologically very recent soft tuff and volcanic ash. a sudden increase in organic sediments in strata corresponding to the third century bce records this erosion following agrarian deforestation, which, far downstream, would initiate the tiber's delta. thereafter the palynological record attests many cultivated plants, and, significantly, nettles, the weed of disturbed, untended corners that follows temperate agriculture everywhere. by the third and fourth centuries ce very little of the primeval forest survived.