I think, therefor it is schist
«Schist (from the greek σχιστός schistós, “split”) is the generic name of several types of metamorphic rocks which are easily identifiable for being strongly laminated. Metamorphic clay, due to the increase in pressure and temperature (metamorphism), becomes first an argillaceous schist (shale), and next, by continuing the metamorphism, it turns into slate, which then becomes phyllite, and finally schist.»
We work on rocks which are over 500 million years old. They retain the winter water, which is consumed in the summer by the vines.