310 G. Siefanini — Outline of the Geological History of 



It is with this moment in the o^eological liistory of the region 

 that we can correlate the rejuvenation of the di-ainage system, 

 w-hose old valleys are met at from 600-650 meters above sea- 

 level. The high limestone plains of Friuli and the epigenetic 

 gorges which abound in the whole prealpine zone are the most 

 striking examples. Indeed, throughout the upper Miocene 

 and the Plaisancian the drainage system, which we have seen 

 developing since the upper Tortonian, had progressed toward 

 maturity with the partial peneplanation of the country. The 

 high limestone plains of the Karst, which border the Alpine 

 chain in this region, in my opinion, are only remnants of this 

 peneplain, still only partially completed when the elevation of 

 which I am going to speak was produced, carrying these 

 remains to heights varying between 800 and 1,200 meters above 

 the present sea-level. 



The erosion of the profound and narrow gorges which open 

 like passage-ways in the Mesozoic limestones at the mouths of 

 most of the valleys in the Yenetian plain, date back to the 

 upper Pliocene. Farther in the interior of the region it is not 

 difficult to hnd similar traces of this old drainage in the very 

 ancient alluvial deposits, lying at 700-800 meters. 



Post Pliocene Continental Formations. 



After this phase of active erosion, which in the extra-alpine 

 region is indicated by a well marked angular unconformity 

 and by very evident lithologic differences, another phase of 

 sedimentation ensued at the beginning of the post-Pliocene, 

 and conglomerates were deposited as enormous talus slopes at 

 the mouths of the valleys on the plain. These are the con- 

 glomerates which the older Italian authors called " preglacial," 

 but which according to the modern view can be considered as 

 corresponding to a glacial period or a very early interglacial 

 period. 



Above these formations w^ere deposited the well known 

 morainic arcs of Tagliamento, Piave, G-arda, etc., while in the 

 interior of the valleys, embedded in these old alluvial deposits, 

 other alluvial deposits were laid down, in turn greatly terraced. 

 But tectonic movements did not completely cease in the Qua- 

 ternary, for the Yillefranchian conglomerates, of which I am 

 going to speak, are themselves bent into an anticlinal dome in 

 the hills of Montello and Conegliano. 



Character of the Tectonic Movenie7its. 



Summarizing — the Yenetian region during the whole of the 

 Xeogene was a gulf opening into the sea, which then occupied 

 the basin of the Po. 



This gulf was already outlined in the upper Oligocene ; it 

 became larger during the lower Miocene, reaching its greatest 



