barrier/weirs: These structures are undisturbed ridges of natural
chalk, occurring only in the sloping southern quadrant, that were
left in place by the original builders of Stonehenge. They extend
fully across the ditch and provide the necessary dams to control
the flow of water along the south by creating the several pools
and low waterfalls. The westernmost weir determines the water
level in the northwest reservoir. Other barrier/weirs stand about
3 feet or less above the floor of the moat and determine the
depths of the water in the basins of the southern quadrant. Since
the chalk is subject to rather rapid erosion by the flow of water,
the weirs are protected by a layer of clay on the tops and sides of
them.