From the 1916 Stonehenge guidebook - the key question that the Glacial Drift theorists still can't answer.
THE FOREIGN STONES
While the Sarsens usually awake the greatest interest by reason of their bulk, and the problem of how a primitive people was able to deal with them, a far greater problem is presented by the small uprights, or Foreign Stones, the like of which cannot be matched within a hundred miles of Salisbury Plain, while some can only be found upon the continent of Europe. Fragments carefully removed and submitted to mineralogists have made this fact abundantly clear, and consequently it is possible to arrive at the very definite conclusion that Stonehenge is certainly not a "Wiltshire " monument, and probably that it is not even "British" at all.
Where have the stones come from? One school of writers ventures to suggest Kildare in Ireland. Others suggest Wales, Cornwall, Dart moor, Shropshire, or Cumberland, where similar rocks are to be found, though perhaps not absolutely identical in character. Yet another theory advanced is that the Foreign Stones were transported to the plain as boulders of the "glacial drift." It has even been stated that the gravels of the district contain small pebbles composed of rock similar to these mysterious Foreign Stones. The statement has indeed been made, but as yet no Wiltshire geologist has produced one of these pebbles of which so much is written, and so little seen.
These Glacial Drift theorists, further account for the absence of these foreign stones elsewhere than at Stonehenge, by yet another theory, that they, like most of the Sarsens, have all been used up for millstones, gateposts, and road metal.
There are many millstones and gateposts in Wiltshire, but where is there one which corresponds in any way to the upright Foreign Stones at Stonehenge? The production of pebbles from the gravels of Wilts, or of a specimen gatepost or millstone would at once settle this question. Unhappily this tangible evidence is wanting, so, alluring as the Glacial Drift theory may appear, it must reluctantly be set aside for want of convincing evidence.
Finally, there seems every reason to believe that the small upright stones are "naturalised aliens. from abroad, and that is why they have been described at the commencement of this section as "Foreign Stones." It must not be taken for granted that the small upright stones at present standing represent all the foreign rocks employed. Probably they are merely the hardest and most durable of those used in the original structure, the softer and more friable examples having disappeared entirely, owing to the action of the weather, and possibly also to the assaults of the unchecked relic-monger, who until recent years could with his hammer collect souvenirs with impunity.
STONEHENGE TODAY & YESTERDAY BY
Curator of the Salisbury museum with Plans and Illustrations by HEYWOOD SUMNER.F.S.A.
Sampson Low, Marston & Co. Ltd Price 1/3 net