Art. XL.—On a Striated Rock-surface from Boatman's, near Reefton.
[Read before the Otago Institute, 11th September, 1888.]
There is no source of geological action more frequently appealed to for elucidation of problems in dynamical geology than ice; and the various traces of its former presence, in the form of striated boulders and rock-surfaces, roches moutonnées and blocs perchés, are familiar to all, if not from actual experience, at least from the text-books.
It is very necessary, in ascribing great geological results to this agency, to be certain that the evidence upon which we base our calculations—frequently erecting a vast superstructure of hypothesis upon a very small foundation of evidence—is indisputably true.
As an example of what might, were its origin not known, have led to misconception and possible error, I beg to bring before this Institute an example of a striated rock-surface, in the grooving of which ice-action had no part. (See Plate XXV.) The fragment of rock forming the subject of this paper was removed by me, on the 4th of May, 1888, from the surface of a large mass of débris resulting from a landslip which occurred at Boatman's, near Reefton, during the preceding March. It consists of a piece of indurated arenaceous clay, containing some mica and indistinct carbonaceous impressions, and its
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geological position is in the Cretaceo-tertiary series of the New Zealand Geological Survey. The surface of the specimen, which is one of innumerable equally well-marked samples which might have been chosen, is scored in more than one direction by grooves or striæ, which vary much in depth, the maximum being about 1/16in.; and were it not that some of these are curved they would present no features distinct from true glacial striæ.
The slip occurred on a surface of the coal-measures dipping at 25°, and was caused by the accumulation of surface-water in a deposit of soil and vegetable growth, the cohesion of which had been destroyed by fires. After proceeding for some distance the direction of the moving mass was changed to about 40° from the straight line by an outstanding mass of quartzose grit, and at this point the whole body plunged over a vertical cliff about 20ft. in height, forming a veritable cascade of mud, and stones, and tree-roots. In the lower portion the angle of slope was reduced to 10°, and at this point a cottage was carried away and destroyed, unfortunately not without loss of life, for an infant member of the family was left behind in the confusion, and the mother perished in a heroic but unsuccessful effort to save her child.
The surface bared in the upper portion of the slip is fireclay, rendered very slippery by the presence of water, while the part which came away consists, below the soil, of about 8ft. of fireclay and quartz grit, the hard fragments of the latter causing the striation of the remaining rock, which at the time of my visit was covered with numbers of well-defined grooves.