They are called trincheras (trenches).

Some of them do not appear to be very old, and many present the appearance of tumble-down walls, but the stones of which they are constructed were plainly used in their natural state.

Although many of the boulders are huge and irregular in shape, they were used just as they were found.

The building material always conformed to the surroundings: in places where conglomerate containing water-worn boulders abounded, this was used; where porphyry was prevalent, blocks of that material were employed.

There is no trace of dressing or cutting, but in the mason work considerable skill is evident.