The Facetted Pebbles of India
Oldham, R. D., (1897) Natural Science: a monthly review of progress, 11, 197-199
It is now nearly forty years since the first account (1) of evidence of ice action in Palaeozoic times and within the tropics was published, and though the concept of a Permian glacial period is now one of the accepted results of geological research, the opposition to its acceptance is by no means dead. Some ten years ago this opposition received an access of strength by the arrival and exhibition in England of certain peculiar fragments of rock, first discovered by Dr Warth (2) in the Permian boulder beds of the Salt Range, which did not merely show a striation like that produced by glaciers, but bore several surfaces or facets which met in obtuse angles, and sometimes completely surrounded the stone. A number of these were sent home, unaccompanied by stones of other types, and an idea seems, perhaps not unnaturally, to have sprung up that these were the normal type of boulder, and not, as was the case, curiosities which were strange to geologists in India, and sent by them to their colleagues in Europe, with a view to enlightenment as to the mode of origin of a feature with which they were not acquainted as a result of ice action.
The basal Permian Boulder bed as exposed in the western Salt Range. A matrix of black shale hosts facetted pebbles, large boulders and small pebbles. Most of the pebbles a re rounded and sub-rounded.
Specimens were exhibited at the Geological Society (3), the British Association (4), and elsewhere, and the general opinion may be expressed in the words of a letter by Dr W. T. Blanford to the Geological Magazine (5), that "the great difficulty in accounting for the origin of these facetted blocks is that whilst the smoothed surfaces are in every respect similar to those on stones worn by glacial action, no fragments from moraines, from boulder-clay, or from other glacial deposits, are known to exhibit the peculiar facetting characteristic of the present specimens."
Such was the general opinion held by most, if not all, of those who saw the specimens, and in the museum at Zurich one of these very facetted stones may be seen, with an endorsement on the label, by Professor Heim, to the effect that he had seen nothing like it in recent glacial deposits.
In these circumstances, the facetted stones being supposed to be the evidence on which was based the claim for a glacial origin of the beds in which they were found, it was natural that the opposition to the claim should be strengthened. In reality, however, the supposition that these facetted pebbles were in some way the result of ice action was based on the fact that they were found in beds which, on quite independent grounds, were believed to be of glacial origin, and this belief would have been in no way affected if the facetted stones had been shown to owe their peculiar form to any other agency than ice.
All this while, however, there was on record the description of boulders of precisely similar character in glacial boulder clays of Post-Tertiary age. In 1879 Professor Credner published an account of the scratched stones found in the neighbourhood of Leipzig, (6) in which he mentions three types; the first being those on which a flat surface had been ground away on one side; the second comprising those on which two or more such surfaces are found meeting in obtuse angles; the third those which show two facets, but are of a rounded or sub-angular form, and bear grooves and scratches scattered over their surface. It would be impossible to give a better classification of the stones found in the boulder beds of the Salt Range, and the closeness of resemblance is only enhanced when Professor Credner's detailed description is read.
This account appears to have been overlooked by all those who saw the Salt Range specimens, for which small blame can he laid, as the volume of glacial literature is so vast that the greater part must remain unread - even by those who devote themselves specially to this subject - and the paper might have remained unnoticed in this connection had it not been accidentally stumbled on while a very different line of research was being pursued. Struck with the light it threw on the origin of these curious pebbles I wrote to Professor Credner asking for further particulars, and in reply was informed that in the collection of the Saxon Geological Survey there are a large number of ice·worn stones showing two or more facets, meeting at an angle, and that in some these facets were distributed round the whole circumference of the stone. He also informs me that after a comparison of the specimens in Leipzig with the figures that after a comparison of the specimens in Leipzig with the figures and descriptions of Drs Warth (2) and Noetling (7), he considers that their nature as glaciated fragments of the same character as those of the gründ-moräne of the northern ice-sheet is beyond doubt.
From this it is evident that·we have, in Post-Tertiary glacial deposits, ice-worn fragments showing all the peculiarities of those found in the Permian boulder beds of the Salt Range, and with this the last objection to accepting their glacial origin should disappear.
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