Prof. J. Park — Pleistocene Glaciation, JSfew Zealand. 395 



In 1915/ near Mano;aweka, on the west side of the liangitikei 

 River, a few miles below the junction of the Hautapii, and nine 

 miles farther down than the lowest preyiously knoM'n pile of 

 andesitic blocks, I discovered the solitary conspicuously striated 

 andesitic boulder to which Mr. Trechmann refers. This block lies 

 on the Rangitikei terrace at the foot of a ridge of Pliocene marine 

 clays, 1,070 feet above sea-level. It measures about 14 X 6 X 55 

 feet, and weighs over 37 tons. The underside and one end for a 

 height of two feet ai'e smoothed, rounded, and scored with innumer- 

 able fine stride and hundreds of deep grooves, most of which run 

 parallel with the longer axis of the block (see PI. XIV, Fig. 2). The 

 smoothed and striated surface has an area of some 90 square feet. 



Mr. Trechmann always refers to the markings as ."scratches". 

 He makes no reference to tlie deeper grooves which occur so plenti- 

 fully. By this omission and the constant reference to scratches, 

 he unconsciously conveys an erroneous impression as to the extent 

 and nature of the markings. And this minatory impression is not 

 diminished when lie states that the surface of the boulder is much 

 decomposed and weathered. As a matter of fact all andesites are 

 prone to weather rapidly ; and if the striae and grooves are Pleistocene, 

 as I contend, the wonder is that weathei'^fng has permitted any trace 

 of the striae to remain. In my opinion, the preservation of the 

 markings is due to the protection afforded by the clays and soil on 

 which the boulder rested, and in which the underside is still 

 partially embedded. 



Mr. Trechmann says that the decomposed surface can almost be 

 scratched with the finger-nail, and that scratches can easily be made 

 on it with a knife blade. The "scratches" could, in liis belief, 

 easily have been made by the boulder moving downhill over 

 gravelly soil or o^er other stones. According to this view the 

 decomposition of the surface had alieady taken place before the 

 boulder began its downhill movement. As this seems to be 

 the essence of his contention, I have again examined the boulder and 

 find that in all the deeper grooves the skin of decomposed rock 

 conforms to the contour of the groove. Clearly the Aveathering 

 took place after the grooves were formed, and not before, as 

 Mr. Trechmann's suggestion would seem to imply. 



The smoothing, scoring, and grooving of a rock mass by its down- 

 ward movement under the influence of gravity is perhaps not 

 accomplished with the ease assumed by Mr. Trechmann. There is 

 no evidence that the Mangaweka erratic does not now lie in the 

 place where it was left by the agent which carried it from Euapehu. 

 The old flood-plain of the Rangitikei is about 250 feet above the river 

 tei'race on whicli the boulder lies. Even if we assume that the 

 boulder did at one time lie on the surface of that old plain it will, 

 I tiiink, be difficult to prove that its downhill movement could 

 produce the smoothing and grooving we now see on the under 

 surface of the block. It seems to me that when the excavation of 

 the old flood-plain reached the boulder one of two things would 



' J. Park, Trans. N.Z. Inst., vol. xlviii, p. 136. 



