Pedra or Mark Stone

Although aesthetically it looks like a big boulder devoid of any appeal when you to read the sign explaining this stone it's most curious. This large stone was found in the waters of the river Ave during drainage and spring return, and was used as a guide for boaters, so they didn't hit sandbars or rocks on the river bottom. Once retrieved, it was placed on Praca da Ribeira with an explanatory plaque.