The course of the river has changed since Roman times; it has swerved to the west, so that the western abutment is quite under water, and the eastern one is high and dry, and separated from the river by quite a mountain of silt, overgrown with grass and trees.
The stones of which the abutments are built are very massive, one of them measuring nearly 5 feet in length. They must have been brought from the Black Pasture Quarry. Many of them have lewis-holes in them, for lifting; some of the holes have been filled up with cement. The earlier parts have no lewis-holes in the stones, which were evidently put in position by hand.
A continuous iron cramp follows the outline of the abutment where it faces the river, being anchored inward by iron bars.
That Severus did repair the bridge there is little doubt, for the feather-broaching which is characteristic of his period is to be seen on some of the stones.
One of Trajan's coins shows a bridge with wooden arches. The later bridge may have been like this, or they may both have been flat wooden platform bridges. It is clear that some means of closing each of the four water-openings by a kind of portcullis would be necessary to prevent the passage of an enemy when the stream was low. In times of "spate," these portcullises would have to be raised. A peculiar barrel-shaped stone, 4 feet long, lying amongst the ruins, with holes all round for the insertion of spokes, may have served as a counterpoise in the process of raising; and two round stone pillars, the remains of which also lie there, might have taken a part in the same scheme. In Cumberland such a water-gate is called a "heck."
Fig. 7.—Section of a stone (S) with a lewis-hole, showing the method of lifting by means of a lewis.
The two wedge-shaped pieces of iron, A, A, are first inserted in the hole, and the third piece, B,
is then placed between them. The pin of the lifting-tackle, C, is then passed through all three pieces.