El Marco's soil is a chalky composition of earth know as "albariza" (alba means white in Latin). This is a white organic marl, formed by sediments of an inland sea that covered the area in the Oligocene era.
Albariza soil is rich in organic remains (shells, sea urchins, starfish ..) which explains its great fertility. It also has a great capacity to retain moisture, storing the winter rainfall to sustain the vines during the long dry season.
The Jerez growers plant their vines on low ridges of albariza, facing southwest.