The soil of this vast depression is entirely argillaceous, and therefore impermeable, so that the waters remain there and make of it a region very difficult to cross during the hot season.
Sakonune handi horren lurzorua buztinezkoa da erabat, iragazgaitza beraz, eta, hori dela-eta, ura bertan geratu eta lurraldea gurutzatzea guztiz nekoso bihurtzen da urtaro beroan.