Probably a trench, Mirador de las Tres Huegas, from the Zaragoza side
Martha Bridegam writes: Since this photo was taken from the western side of the hill, and the Loyalists held the eastern side of the Sierra de Alcubierre, it's entirely possible this trench was a Fascist/Nationalist position. If anyone has more detailed historical information on this part of the front, please advise.
"... The front line here was not a continuous line of trenches, which would have been impossible in such mountainous country; it was simply a chain of fortified posts, always known as ‘positions’, perched on each hill-top. In the distance you could see our ‘position’ at the crown of the horseshoe; a ragged barricade of sand-bags, a red flag fluttering, the smoke of dug-out fires...."- Homage to Catalonia, Chapter 2
"All my memories of that time are memories of scrambling up and down the almost perpendicular slopes, over the jagged limestone that knocked one’s boots to pieces, pouncing eagerly on tiny twigs of wood. Three people searching for a couple of hours could collect enough fuel to keep the dug-out fire alight for about an hour. The eagerness of our search for firewood turned us all into botanists. We classified according to their burning qualities every plant that grew on the mountain-side; the various heaths and grasses that were good to start a fire with but burnt out in a few minutes, the wild rosemary and the tiny whin bushes that would burn when the fire was well alight, the stunted oak tree, smaller than a gooseberry bush, that was practically unburnable. There was a kind of dried-up reed that was very good for starting fires with, but these grew only on the hill-top to the left of the position, and you had to go under fire to get them. If the Fascist machine-gunners saw you they gave you a drum of ammunition all to yourself. Generally their aim was high and the bullets sang overhead like birds, but sometimes they crackled and chipped the limestone uncomfortably close, whereupon you flung yourself on your face. You went on gathering reeds, however; nothing mattered in comparison with firewood...."- Homage to Catalonia, Chapter 3
