Manchester Memoirs, Vol. Ixii. (1918), No. 9. 11 



allowed the accumulation of the deposits of mud and silt which 

 underlie the Upper Forest-bed. Whether the pebble-bed at the 

 base of this sand in various places can be correlated with the bed 

 with facetted and wind-worn pebbles at Dove Point on the Wirral 

 coast is uncertain, owing to the entire absence of the Shirdley Hill 

 Sand at this locality. Its relative position, however, with regard to 

 the Drift deposits is suggestive of such a correlation. 



It is of some interest to note that the basal gravel-bed of the 

 Shirdley Hill Sand and the bed with facetted pebbles at Dove Point 

 appear to occupy a somewhat analogous position with regard to the 

 Drift to that of the basement bed, or " Steinsohle," of the Loess of 

 the North German plain, where facetted pebbles are of frequent 

 occurrence. Whether such pebbles occur in association with the 

 Shirdley Hill Sand has not been recorded. 



From the foregoing remarks it seems legitimate to conclude 

 that an intimate connection may exist between the period when the 

 pebbles were eroded and the laying down of the Shirdley Hill Sand 

 in early post-Glacial times, as the blown sand would provide the 

 necessary medium for modifying the pebbles. 



Having discussed the geological horizon of the facetted pebbles 

 dealt with in this paper, we can pass on to the question of the signi- 

 ficance of such pebbles. 



That the various pebbles had been first fractured before being 

 acted on by sand-blast is conclusively proved by the evidence 

 of the pebbles themselves. The probable cause of this splitting 

 might, therefore, be considered first. 



Judging from the generally rounded and perfect condition of the 

 stones embedded in the underlying Glacial Drift, it seems a reason- 

 able assumption that the pebbles concerned must have been in a 

 similar perfect condition when they were left at, or near, the bare 

 surface of the Drift on the retreat of the ice. Such being the case, 

 they would be subject to the varied atmospheric agencies which 

 bring about the disintegration of rocks. Of these perhaps the most 

 important are variations of temperature and alternations of frost 

 and thaw. 



The combined action of frost and thaw seems to provide a suffi- 

 cient explanation for the splitting of pebbles left exposed on the bare 

 surface of the Drift. It is possible that many of these Drift pebbles 

 possessed incipient joints at the time they were carried along by 

 the ice, and in this case continued exposure to alternations of frost 

 and thaw would cause these joints to increase and result in the 

 splitting or complete fracturing of the pebbles. 



It is of some interest to note that at the Pendleton section I found 

 several wind-worn pebbles lying about on the talus which had been 

 split into two or three pieces during the frost of last winter. The 

 pieces were lying together and the splitting was along joint- or 

 bedding-planes (see Fig. 8) . They were all sedimentary rocks ; 

 I saw no granites or igneous rocks so split, though such occur in 

 abundance among the wind-worn pebbles obtained in situ, all of 

 which show modification by sand-blast on their fractured faces. 



