Bartrum. — High-water Bock-platforms. 



133 



This note deals with the origin of the rock-platform constituting the 

 rim of the " Old Hat." Similar platforms are greatly in evidence in 

 many parts of the coast around Auckland and in the more northerly 

 harbours ; they are barely covered by mean high tides, and vary in width 

 from a few feet to 30 yards or more. From their seaward margins there 

 is a steep descent for a few feet. Their surface is essentially horizontal 

 but for very minor irregularities, so that they disturb the normal shore 

 profile, which, according to Fenneman, both in building and cutting coasts 

 " is a compound curve, which is concave near the shore, passing through 

 a line of little or no curvature to a convex front."* They are not slightly 

 tilted and uplifted subaqueous shore-terraces, for they are covered at high 

 water. 



Dr. von Hochstetterf did not fail to observe these characteristic benches 

 on his visit to Auckland, but neither he nor Professor Dana appears to have 

 realized that they are not normal submarine platforms. Dana's idea of 





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^ ^ < ^ '°C ^^>.^ High-worer Level 



W/M///7/7//7//^////f^ ^ck' -'filo r F6/^'/ /////i'TT^r-. WoWusiS h ore -o..r'| 



Section Diagram illustrating the Development of High-tide 



rock-platfoems. 



their nature and origin is interesting, particularly as he adduces the " Old 

 Hat " in support of it. He says, " There is, therefore, a level of greatest 

 wear, which is a little above half-tide, and another of no wear, which is 

 just above low tide. "J 



It seems essential to the formation of these high-water rock-platforms 

 that wave-attack be not over-vigorous ; that the rock in which they originate 

 be moderately resistant to erosion, uniform in texture, and subject to 

 comparatively ready decomposition ; and that the coasts have not reached 

 maturity of outline. Given these conditions, they will originate on un- 

 destroyed headlands and stat;ks quite irrespective of the structure of the 

 constituent rocks, for they are particularly well developed at Waiheke 

 Island and at the Bay of Islands, although the shores there are formed 

 by highly disordered sediments. 



* N. M. Fenneman, " Development of the Profile of Equilibrium of the Subaqueous 

 Shore-terrace," " Journal of Geology," vol. 10, pp. 1-32, 1902. 



t F. von Hochstetter, Reise d. o. F. Novara u. d. Erde, Geolog., vol. 1, p. 5, 

 1864. 



X J. D. Dana, " Manual of Geology," 3rd ed., p. 677, 1880. 



