﻿258 KEV. W. HOWCHIN ON GLACIAL BEBS OF [May 1908, 



beach-pebbles, and impart striation to them by scraping the bottom 

 before coming to rest. 



It is difficult to determine the exact position of this old Cambrian 

 ice-field. The pre-Cambrian highlands are only indicated, at the 

 present time, by sunken reefs and low peneplains. The largest 

 known area of pre-Cambrian rocks that comes within the range of 

 probability for such an ice-field includes part of the continental 

 shelf, together with the geological axes found in Yorke Peninsula, 

 Kangaroo Island, and Eyre Peninsula. 



Yorke Peninsula is an old pre-Cambrian ridge that remained 

 above sea-level during most of the Cambrian Period, and was finally 

 submerged only at so late a stage in the geosynclinal folding as to 

 receive but a thin deposit of Cambrian material, and that limited 

 to the later deposits. The only circumstance known, calculated to 

 throw light on the direction of the ice-drift, is the discovery in the 

 Petersburg Range of two erratics of graphic granite (one glaciated), 

 that show identical features with a rock of the same character, 

 in situ^ in southern Yorke Peninsula, 122 miles from the spot where 

 the erratics were found. This would indicate a drift from south- 

 west to north-east, radiating from that part of the country where, 

 the greatest known exposure of pre-Cambrian rocks exists and 

 around which the Cambrian beds thin out. Shore-ice would be 

 quite equal to the transport of the two erratics just mentioned. 



(b) ? Interglacial Periods. 



Attention has been directed to the possibility of the glacial con- 

 ditions being arrested at certain stages by intercalated periods of 

 higher temperature and a restricted distribution of morainic 

 material. Some grounds for believing that such was the case is 

 suggested by the interbedded slates and limestones, which are often 

 completely, or approximately, free from coarse material, and exhibit 

 bedding-planes of sedimentation. At present, however, the evidences 

 for one or more distinct interglacial periods, that were general in 

 their effects, cannot be regarded as conclusive. Where well-defined 

 interglacial periods occur, we may expect that such periods will be 

 indicated by zones which possess some stratigraphical resemblance 

 over wide areas. This, however, does not appear to be the case, as 

 the intercalated beds occur at various horizons in the till with a 

 marked irregularity in different districts. Local causes may have 

 led to a temporary variation of sediment in certain areas, at one 

 time closing the avenues to floating ice, leaving them open 

 almost exclusively to fluviatile sediment, and at other times re- 

 versing these conditions. Fuller data must be awaited before any 

 definite conclusions can be reached as to the possible climatic alter- 

 nations which may have occurred during the progress of the 

 Cambrian Ice-Age in South Australia. 



