The crucial phase in the creation of the site came during the next six hundred years, when the incomplete bluestone circle was transformed by the construction of a circle of 25 trilithons (two uprights crossed by a lintel) and an inner horseshoe formation of five trilithons. Hewn from Marlborough Downs sandstone, these colossal stones (called sarsens), ranging from 13ft to 21ft in height and weighing up to thirty tons, were carefully dressed and worked – for example, to compensate for perspectival distortion the uprights have a slight swelling in the middle, the same trick as the builders of the Parthenon were to employ hundreds of years later. More bluestones were arranged in various patterns within the outer circle over this period. The purpose of all this work remains baffling, however. The symmetry and location of the site (a slight rise in a flat valley with even views of the horizon in all directions) as well as its alignment towards the points of sunrise and sunset on the summer and winter solstices tend to support the supposition that it was some sort of observatory or time-measuring device. The site ceased to be used at around 1600 BC, and by the Middle Ages it had become a “landmark”. Recent excavations have revealed the existence of a much larger settlement here than had previously been thought – the most substantial Neolithic village of this period to be found on the British mainland in fact – covering a wide area. Nothing is to be seen of the new finds as yet, though there are plans to re-create a part of the ancient complex.