Sedimentological evidence shows that the costal Unare Lagoon was formed in Holocene, during the Flamenca Transgression, approximaty 5000 years BP. Testimonies of a former delta of Unare river were found on lagoon surroundings, a formation of rounded stones in reddish lime matriz, that discordanty lies on a silty clay formation from Pliocene, currently forming hills of about 50 m above sea level near Clarines Town. Towards west, close to Boca de Uchire town, formations of Stone Lines were found. Southern side is surrounded by plains, in which paleosoils from Flamenca Transgression that reached 6-7 m above sea level were found. In 2-4 m depth excavation opened on these plains, paleosoils strata of rich in organic matter black mud, with fossils of Crassotrea sp. and gastropod Malogena malogena rest of urchins, coral and foraminifers were observed, evidence of an carly marine invasion with C14 data analysis ages of 3940 40 A.P. and 2170 60 A.P. Heavy mineral deposition carried on exclusively by the sea are covered by mineral deposition of continental origin. Currently sea level rise was observed near La Cerca town, where the litoral bar is moving towards the continent, reducing Lagoon surface and destroying exposed mangrove forests, formerly located in inner side of the sandy bar.