36 



flown, but extraordinary tides do not reach: the estuarine 

 limestone." (D 



The geological section exposed in the present excavation 

 at Dry Creek is of very great interest as showing alternations 

 of the height of the land in relation to the sea that has led to 

 repeated modifications of our coast-line. It has been a com- 

 plex movement in which the sea has twice transgressed upon 

 the land and twice retired during recent geological times* 

 This conclusion is reached by a twofold testimony — (1) the. 

 stratigraphical succession, and (2) the zoological evidence.. 



With regard to the geological succession, there are two 

 fossiliferous horizons, one at or near the surface and the other 

 at a depth of 18 ft. below the surface, and in between these 

 two marine horizons there are some 16 ft. or 18 ft. of allu- 

 vial wash. The upper marine bed was not detected in the 

 sinking now under description, but its prevalence in the 

 neighbourhood is abundantly evident. The bed of triturated 

 shells (No. 4 in section) which immediately overlies the oyster 

 bed, may have accumulated, at least in part, by the action 

 of surface water acting on the fossiliferous material after 

 the retirement of the sea; but if we exclude this doubtful 

 bed, there remains 15 ft. of fresh- water deposits that mark 

 the interregnum between the two encroachments of the sea. 

 The blue clay (No. 6 in section) that underlies the oyster 

 bed is no doubt the tenacious blue clay of the Adelaide plains, 

 probably of Pleistocene Age, which is met with in most sinkings 

 in Adelaide and neighbourhood, and forms the brick-earth 

 of our local potteries and brick-making. It is a fresh-water 

 deposit, and marked the base of the water-level in the present 

 sinking at Dry Creek, as the oyster bed, which is immediately 

 above it, carried a strong runner of water. 



There is a marked contrast in the organic facies of the- 

 two shell-bearing beds. The upper-bed carries just such mol- 

 lusca as live in our estuaries to-day, and in about the same 

 relative proportions. It is essentially a present-day type of 

 deposit. The lower marine bed, in addition to carrying such 

 forms as still live in the Port Creek, contains others that do 

 not exist there at the present day. The large oyster, Ostrea 

 angasi, which is the most striking shell in the lower bed, 

 although plentiful, in places, in Spencer Gulf, no longer 

 occurs, or but rarely, in our local waters; and Area trapezia, 

 which is also a very common form in the Dry Creek lower 

 marine bed, is no longer an inhabitant of South Australian 

 waters. These two shells do not occur in the superficial 



(i) Trans. Philosoph. Soc. of Adelaide [Roy. Soc, S.A.'],. 

 1878-9, p. lxix. 



