djacent to the bank is a ring of 56 pits, known as the Aubrey Holes, marked by circular concrete spots. The area between the inner edge of the bank and the outermost stone settings includes at least two further settings of pits: the Y and Z holes. On the central area of the site, there are the stone settings, the sophisticated arrangements that set Stonehenge apart from any other prehistoric monument in Europe. In their construction two types of stone were used: sarsen and bluestone. The sarsens used in the central settings are much larger. The bluestone is a mixture of rocks found on the Preseli Mountains in SW Wales. The most widely accepted theory regards the arrival of the bluestones on Salisbury Plain as the result of human effort, with the route being partly overland and partly by water. In its complete form the outermost stone setting consisted of a circle of 30 upright sarsens, of which 17 still stand, each weighing about 25 tons. The tops of these uprights were linked by a continuous ring of horizontal sarsen lintels, only a small part of which is now still in position. The stones in the sarsen circle are carefully shaped and the horizontal lintels joined not only by means of simple mortice-and-tenon joints, but they are also locked using what is effectively a dovetail joint. The edges are smoothed into a gentle curve, which follows the line of the entire circle. The bluestone setting, concentric the outer sarsen circle, consisted originally of about 60 stones, but many have fallen, dissolved or been crushed. Inside these two circles lies the sarsen horseshoe, consisted originally of five sarsen trilithons (a Greek word that means three stones), each comprising two uprights with a horizontal lintel. Although now fragmentary, the arrangement shows the careful grading of the five trilithons, the tallest of which is 6.7m (22ft) high above ground level. Enfolded within this massive horseshoe lies a smaller horseshoe arrang...
