The Valdarno basin is one of the well developed intermontane basins of the Northern Apennines. The basin was filled, from Middle Pliocene to Middle Pleistocene, during three tectono-sedimentary cycles, bounded by unconformities (Sestini, 1936; Merla & Abbate, 1967; Azzaroli & Lazzeri, 1977; Abbata, 1983; Magi, 1989; Sagri, 1991; Albianelli et alii, 1995). The presence of unconformity-bounded stratigraphic units (Ubsu, Salvador, 1987) suggested subdivision of the basin fill into three synthems (Benvenuti, 1992). Particular attention was dedicated to the sedimentary units located at the transition between the first and second stages of the basin evolution and well exposed along the south-western margin (S. Giovanni Valdarno area). Field work allowed two new sedimentary units to be defined (Rena Bianca Sands and Montecarlo Sand and Silts). The Rena Bianca Sand consists of alluvial sediments, deposited by ephemeral streams, locally strongly reworked by eolian processes (Magi & Sagri, 1996; Ghinassi et alii, in press). The deposition of the Rena Bianca Sand occurred under arid climatic conditions, developed in the basin during a global climatic deterioration at the Middle-Upper Pliocene transition (Ghinassi et alii, in press). The overlying Sabbie e Limi di Montecarlo alluvial deposits, record the gradual change toward more wet conditions, followed by the fluvio-palustrine environment of the Limi di Terranuova (Albianelli et alii, 1995). Detailed fieldwork, has shown that there is an angular unconformity at the base of the Rena Bianca unit, and a progressive unconformity at the top of Sabbie e Limi di Montecarlo unit. In this context, we can locate the boundary between the first and the second synthems (Castelnuovo Synthem and Montevarchi Synthem respectively), at the base of the Rena Bianca Sand, including newly described units in the lower portion of the Montevarchi Synthem. In the Valdarno Basin, the occurrence of tectonic-related unconformity surfaces (angular unconformity and progressive unconformity) at the middle-late Pliocene transition, may be associated with a major tectonic phase well documented in other basins of the northern Apennines (Pascucci, 1997; Boccaletti & Sani, 1998; Bossio et alii, 1998; Martini et alii, 2001).