77 
The Shingle Beach as a Plant Habitat. 
The question of the determining cause in hook-formation is not 
without interest, and as it has been recently raised by Mr. R. 
Speight in a joint paper with Messrs. Cockayne and Laing in con¬ 
nection with the two shingle spits occurring in Lake Heron, New 
Zealand, some reference may be made to the matter. 1 
The causation of hooks, according to G. K. Gilbert, depends 
upon a temporary change in direction of current, combined with 
adequate wave-action, or to the spit becoming “ at a certain stage 
of its growth especially subject to some conflicting current, so that 
its normal growth ceases, and all the shore-drift transported along 
it goes to the construction of the branch.” 
“ The currents efficient in the formation of a hook do not 
co-operate simultaneously, but exercise their functions in alternation. 
The one.brings the shore-drift to the angle and accumulates 
it there ; the other .... demolishes the new structure and redeposits 
the material upon the other limb of the hook.” 2 
In accounting for the Lake Heron hooks Speight emphasizes 
the importance of wave-action, relegating the action of currents to 
an insignificant place. He conceives the hook to be formed by the 
swinging round of the waves in consequence of differential retard¬ 
ation due to the friction of the shelving bottom. “ Both these 
spits end in a rounded nose” (i.e., hook) “ whose position is determined 
by the amount of retardation of the wave in the shallow water. 
The wave will tend to swing round completely, so that it actually 
reverses its direction, and this will maintain a blunt-nosed spit in a 
fixed position as long as the conditions of the bottom of the lake 
in the vicinity are the same. If the floor of the lake keeps on 
shallowing off the spit so that it makes the depth of the lake more 
uniform, then the wave will not swing so quickly, and the spit will 
thus be lengthened.” 
As a contribution to a discussion on a subject which from the 
nature of the case probably comes but rarely under direct observation 
(i.e., the formation of a hook), the following unchronicled incident 
may be allowed a place. Had it been a fall of chalk of equal 
volume at Dover cliffs the occurrence would have been recorded in 
a hundred newspapers! 
At Blakeney Point, as already noticed, the hook-systems reach 
1 R. Speight, L. Cockayne, and R. M. Laing. The Mount Arrow- 
smith District, a Study in Physiography and Plant Ecology. 
Trans. New Zealand Inst., Vol. XXIII., 1910. 
2 G. K. Gilbert. Lake Bonneville, U.S. Geol. Survey Monograph, 
1890, p. 52. Also Rep. U.S. Geol. Survey, 1883-4, p. 95. 
