﻿^48 EET. W. HO WCHIN ON GLACIAL BEDS OF [May IQOS, 



Movements on a larger scale are sometimes indicated by the beds 

 being scarred with quartz, as in the Sturt Valley, where at several 

 places there is a local development of quartz in horizontal veins and 

 lenticles, or in reclining folds, arising probably from some measure 

 of thrust. In rare instances these quartz-veins pass into pegma- 

 tites. Passage-forms occur in which the felspathic elements are 

 present only in sporadic crystals, and at one place (in the Sturt 

 Valley) a true pegmatite attains, for a short distance, a thickness of 

 at least a foot. 



V. Illustrative Sections. 



The beds of glacial origin are, as a rule, clearly defined in their 

 stratigraphical limits, both at their upper and at their lower boun- 

 daries. At their upper limits an abrupt change takes place, from 

 indurated boulder-clay to an overlying thick deposit of fine-grained 

 slate (Tapley's-Hill Slate), which is sometimes calcareous and 

 typically quite free from grit or sand. This bed exhibits very 

 distinctive and persistent features as a banded or ' ribbon '-slate,^ 

 and diff'ers locally, mainly, in being fissile by cleavage in the 

 south, and fissile along bedding-planes in the northern districts. 

 Underlying the till are fine-grained laminated quartzites (sometimes 

 argillaceous), of wavy structure, passing down into a thick series 

 of quartzites and slates which form the hilly ridges of the Mount- 

 Lofty and other ranges, as in the Mitcham and Glen-Osmond Beds. 



The following examples of cross-sections will illustrate the 

 stratigraphical features of the glacial beds. 



(fl) Onkaparinga- River Section. (Fig. 7, p. 249.) 



This locality is about 20 miles south of Adelaide. The river here 

 has cut deeply into the Lower Cambrian strata, which exhibit an 

 order of succession similar to that observed near Adelaide, as illus- 

 trated in fig. 1 (p. 236). The glacial beds are exposed in the river- 

 valley, at a distance of 6 miles from the coast, in an outcrop of 1| 

 miles, in one direction, and three-quarters of a mile in the other. 

 On the north and w^est sides, the continuity of the beds is broken 

 by faults ; on the south, they pass under the Tapley's-Hill banded 

 slates, which follow in superior position ; and on the east, they are 

 determined by the outcrop of their base and the underlying 

 (Mitcham) quartzites coming to the surface. 



The river intersects the beds on the eastern side of the glacial 

 area, and then, by taking a sudden curve to the west, flows for fully 

 a mile along the strike of the beds, near their upper limits. In the 

 latter case, several clifi's of the till rise almost perpendicularly to a 

 height of about 250 feet (see fig. 7). 



^ See specimen No. 14,489, preserved in the Collection of the Geological 

 Society of London, 



