Within the horseshoe, and near its apex, lies the famous "Altar Stone" (A), a block measuring about 16 feet by 4. Between the horseshoe and the outer circle another circle of diabase stones is sometimes said to have existed, but very little of it now remains.
The whole building is surrounded by a rampart of earth several feet high, forming a circle about 300 feet in diameter. An avenue still 1200 feet in length, bordered by two walls of earth, leads up to the rampart from the north-east. On the axis of this avenue and nearly at its extremity stands the upright stone known as the Friar's Heel.
In 1901, in the course of repairing the central trilithon, careful excavations were carried out over a small area at Stonehenge. More than a hundred stone implements were found, of which the majority were flint axes, probably used for dressing the softer of the sandstone blocks, and also for excavating the chalk into which the uprights were set. About thirty hammer-stones suitable for holding in the hand were found. These were doubtless used for dressing the surface of the blocks. Most remarkable of all were the 'mauls,' large boulders weighing from 36 to 64 pounds, used for smashing blocks and also for removing large chips from the surfaces. Several antlers of deer were found, one of which had been worn down by use as a pickaxe.
These excavations made it clear that the blue-stones had been shaped on the spot, whereas the sarsens had been roughly prepared at the place where they were found, and only finished off on the spot where they were erected.
What is the date of the erection of Stonehenge? The finding of so many implements of flint in the excavations of 1901 shows that the structure belongs to a period when flint was still largely used. The occurrence of a stain of oxide of copper on a worked block of stone at a depth of 7 feet does not necessarily prove that the stones were erected in the bronze age, for the stain may have been caused by the disintegration of malachite and not of metallic copper. At the same time, we must not infer from the frequency of the flint implements that metal was unknown, for flint continued to be used far on into the early metal age. Moreover, flint tools when worn out were simply thrown aside on the spot, while those of metal were carefully set apart for sharpening or re-casting, and are thus seldom found in large numbers in an excavation. We have, therefore, no means of accurately determining the date of Stonehenge; all that can be said is that the occurrence of flint in such large quantities points either to the neolithic age or to a comparatively early date in the copper or bronze period. It is unlikely that stone tools would play such a considerable rôle in the late bronze or the iron age.
At the same time it must not be forgotten that Sir Arthur Evans has spoken in favour of a date in the first half of the third century B.C. He believes that the great circles are religious monuments which in form developed out of the round barrows, and that Stonehenge is therefore much later than some at least of the round barrows around it. That it is earlier than others is clear from the occurrence in some of them of chips from the sarsen stones. He therefore places its building late in the round barrow period, and sees confirmation of this in the fact that the round barrows which surround the monument are not grouped in regular fashion around it, as they should have been had they been later in date.
Many attempts have been made to date the monuments by means of astronomy. All these start from the assumption that it was erected in connection with the worship of the sun, or at least in order to take certain observations with regard to the sun. Sir Norman Lockyer noticed that the avenue at Stonehenge pointed approximately to the spot where the sun rises at the midsummer solstice, and therefore thought that Stonehenge was erected to observe this midsummer rising. If he could find the exact direction of the avenue he would know where the sun rose at midsummer in the year when the circle was built. From this he could easily fix the date, for, owing to the precession of the equinoxes, the point of the midsummer rising is continually altering, and the position for any year being known the date of that year can be found astronomically. But how was the precise direction of this very irregular avenue to be fixed? The line from the altar stone to the Friar's Heel, which is popularly supposed to point to the midsummer rising, has certainly never done so in the last ten thousand years, and therefore could not be used as the direction of the avenue. Eventually Sir Norman decided to use a line from the centre of the circle to a modern benchmark on Sidbury Hill, eight miles north-east of Stonehenge. On this line the sun rose in 1680 B.C. with a possible error of two hundred years each way: this Sir Norman takes to be the date of Stonehenge.
Sir Norman's reasoning has been severely handled by his fellow-astronomer Mr. Hinks, who points out that the direction chosen for the avenue is purely arbitrary, since Sidbury Hill has no connection with Stonehenge at all. Moreover, Sir Norman determines sunrise for Stonehenge as being the instant when the edge of the sun's disk first appears, while in his attempts to date the Egyptian temple of Karnak he defined it as the moment when the sun's centre reached the horizon. We cannot say which alternative the builders would have chosen, and therefore we cannot determine the date of building.
Sir Norman Lockyer has since modified his views. He now argues that the trilithons and outer circle are later additions to an earlier temple to which the blue-stones belong. This earlier temple was made to observe "primarily but not exclusively the May year," while the later temple "represented a change of cult, and was dedicated primarily to the solstitial year." This view seems to be disproved by the excavations of 1901, which made it clear that the trilithons were erected before and not after the blue-stones.