They had their surprise ready, however, as well. From well-hidden positions behind the woods on the top of the plateau they opened a violent bombardment of the British lines with their huge 8-inch and 11-inch howitzers, throwing the enormous shells, which fell with such terrific force as to bury themselves in the ground. Giving off in exploding dense clouds of black smoke, these shells blew away the earth on all sides of them in a rain of fragments of rock, masses of soil and stones, leaving the surface filled with holes wide and deep enough to be the burial place of several horses. This heavy ordnance was kept well beyond the range of the British guns, and employed for high-angle fire. So far as life was concerned, the shells caused relatively little loss. Their flight being visible--they looked not unlike tree-trunks hurled from across the hills--they could be dodged. On realising how little they were to be feared, the British troops nicknamed them "Black Marias," "Coalboxes," and "Jack Johnsons," and shouted jocular warnings. The idea of using these shells was to knock the British defence works to pieces. Some of these works, hastily thrown up, proved to be too slight, and had to be replaced by diggings, which became regular underground barracks. At this time the British lines were in general more than a mile distant, on the average, from those of the enemy. They followed no symmetrical plan, but, adapted to the defensive features of the ground, were cut where there were at once the best shelters from attack and the best jumping-off places for offence. Describing them, the British military correspondent wrote:-- A striking feature of our line--to use the conventional term which so seldom expresses accurately the position taken up by an army--is that it consists really of a series of trenches not all placed alongside each other, but some more advanced than others, and many facing in different directions. At one place they run east and west, along one side of a valley; another, almost north and south, up some subsidiary valley; here they line the edge of wood, and there they are on the reverse slope of a hill, or possibly along a sunken road. And at different points both the German and British trenches jut out like promontories into what might be regarded as the opponent's territory.