In his landmark analytical study of this ancient megalithic site entitled "Stonehenge Decoded", astronomer/author Gerald Hawkins describes his discovery of solar and lunar alignments with the standing megaliths and mentions several unsolved mysteries of the place:
(1) A ditch encircling the site has no clearly known purpose or function. Some archaeologists believe this structure served a mystical or spiritual purpose of blocking "evil spirits" which in primitive societies are believed to be unable to cross over water. But the ditch was never made to completely encircle the site - there is a gap. Why was this ditch excavated at great expense of labor?
(2) A large slab of stone located near the end of the ditch has numerous cut marks along the edge. Popularly known as the "Slaughter Stone", it was first erected by the builders of Stonehenge and then intentionally toppled over by them. Archaeologists say that, contrary to popular superstition, it was never used for human or animal sacrifice. What was the purpose of the Slaughter Stone?
(3) The ground on which the stones were erected is not level. One side of the site is a few feet (one or two meters) lower than the other. Yet the tops of the group known as the trilithon horseshoe and other groups of stones have been accurately leveled! How were these megaliths accurately leveled by the builders at the time (about 2 millenia before the time of Christ) without the use of modern surveying equipment?
The builders of Stonehenge flooded the encircling ditch with water. The water's surface established a level reference plane around the site, but it was below ground level. In order to transfer this level reference plane to the tops of the stones they used a very common tool in their culture which has survived to modern times. A "curragh" or "corwgl" is a boat made of animal skins sewn together and stretched over a wooden frame. These boats are made with or without a mast. Such a boat with a mast is very light and displaces very little water.
The Slaughter Stone was erected at the end of the ditch and marked with cuts along the edge to function as another very stable reference of height or elevation. Bringing the boat next to the Slaughter Stone, they transferred an elevation from a cut mark on the stone to a moveable marker on the mast of the boat and then they moved the boat along the ditch to a point on the opposite side of the site. Next, they placed a second boat in the ditch near the Slaughter Stone and repeated the elevation marking process used with the first boat. Then, moving the second boat away along the ditch, they had two markers on the boat masts at the same elevation. From a wooden platform they could visually line up the mast markers. Their line of sight was always a highly accurate level line across the tops of the stones which could be moved to check any position on the construction site. Any stone surface appearing above the markers could be hammered and chiseled until it was made level.
Finally, when the construction of all the megalithic groups was finished and the builders wanted to test the measurement accuracy of their solar and lunar observatory, they saw that the Slaughter Stone which had been used to set up the site was no longer needed and it was blocking their view of the horizen, so they toppled it over to get it out of their line of sight. And so it was left lying on the ground where it had formerly stood tall.
The accuracy of this hypothesis can be checked by comparing the heights of the cut marks on the Slaughter Stone to the heights of the various stone groups on the site.
For more information on ancient megalithic sites, read "The Stars and the Stones" by Martin Brennan - published by Thames and Hudson. To see a real-life mystery of modern megalithic construction, visit the Coral Castle in Homestead, Florida where a secretive Latvian immigrant named Ed Leedskalnin, a man of less-than-average stature, erected structures of tremendous weight without the aid of construction machinery!