Baetic System
Geologically Cala Arenas is located within the Algeciras-Los Nogales unit and is part of the Baetic System in the Campo de Gibraltar. The local rocks are mainly composed of flysch comprising marl, sandstone and mica, locally known as the Algericas Flysch, that dates from the Eocene–Oligocene period. The rocks include various soft minerals, especially clays, marl, calcarenite and sandstones, that have eroded to leave behind the harder limestones and thus form the chain of alternate coves at Cala Arenas.