Beachrocks are rocks that indicate a former shoreline as well as the sedimentary constitution of ancient beaches. In micro-tidal environments, they accurately determine the position of the medium sea level during the lithification process. They are formed by the beach sediment cemented within the intertidal zone. The objective of this paper is to study study the beachrocks of the Cabo Frio Island, Arraial do Cabo-Rio de Janeiro from the geological, geomorphological, petrographic and geochronological point of view. The methods applied in this study consisted in sampling, petrographic analysis and radiocarbon dating. The results suggest that the beachrocks of this island were formed between 11.900 and 3800. These ages correspond to sea level conditions below the current level, at the current level and above the sea level in relation to the current level during the Holocene. The transition between the Pleistocene and the Holocene in the State of Rio de Janeiro was identif i ed by this paper, corroborating the proposals of other authors.