Ponce beaches are the result of mineral aggregate found in them, and this is, in turn, the result of silt carried by the Ponce river system from the mountains of Cordillera Central. In this manner the composition of its beaches are determined by the geological regions through which the nearby rivers flow. Beaches west of Ponce (El Tuque, Las Salinas, Matilde, etc.) exhibit sand that is mostly white, resulting from its calcareous content, and primarily composed of coral fragments and marine shells. Beaches east of Ponce (La Guancha, Club Nautico, Hilton, Cabuyon, Vayas, etc.) exhibit sand that is mostly dark sand with magnetite, and composed of fragments of volcanic rock, quartz, and calcareous detritus. Also, the sand at mainland Ponce beaches tends to be generally dark while in the offshore Ponce islands sand is white.