SrKicnr, Cockaynk, Laixc;. — Mount Arroirtnititli Dixirict. 325 



matheniatical relation between the form of the spit and the circumstances 

 determining the wave-motion in the lake, and an examination of the spit 

 as they occur strongly suggests that their shape is a well-defined geometrical 

 curve. In the initial stages of the formation of the spits it is probable that 

 they are largely built up by a feeble shore-current due to wave-action, but 

 directly a small spit is formed the waves Avould be almost entirely responsible 

 for its prolongation. Both of these spits end in a rounded nose, 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 ofE 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 (see fig. 2). The 

 peculiarity of these spits can thus be put down absolutely to wave-action, in 



contradistinction to those formed on 

 the sea-coast, which are attributed 

 largely to littoral currents. It is evi- 

 dent, however, that wave-action alone 

 can form spits, and this must be a 

 contributory cause in a large propor- 

 tion of marine spits. 



Hooked spits in lakes are specially 

 referred to by G. K. Gilbert in his 

 paper on the "Topographic Features 

 of Lake-shores " (5th Annual Report, 

 United States Geological Survey), but 

 he ascribes them principally to the 

 action of the littoral currents ; in 

 Lake Heron, however, these appear 

 to play an insignificant part in their 

 formation. 



Lake Heron is at such an eleva- 

 tion above the sea that every winter 

 it is heavily coated with ice. In ordi- 

 nary seasons there is a covering of as 

 much as 9 in. in thickness, a remark- 

 able feature for such a large lake in an 

 insular climate like ours in a latitude 

 of only 43° S. The shores exhibit 

 traces of the action of ice in the ridges of gravel which are pushed up by 

 it as it expands after contraction in cold weather. Ice contracts as the 

 temperature is lowered, and in doing so draws away from the shore, leaving 

 a narrow lane of open water ; this freezes immediately, and when the tem- 

 perature rises the ice expands again and is forced up the beach. Ridges 

 formed in this way occur on Lake Heron, as well as on the smaller lakes 

 Tripp and Acland. The stones composing the beaches are rounded on their 

 edges and corners by the movement of the ice, and especially so when the 

 ice breaks up in spring before an early north-wester. The floes are then 

 piled in heaps on the exposed shore of the lake, and the wind keeps on 

 driving others forward, which occasionally shoot up on the inclined planes 

 formed by the masses miderncath, so that they are carried as much as a 

 •chain from the edge of the lake, scoring the ground and ploughing it into 



PiC4. 2. — Showing Relation of "Waves 

 TO Form of Spit. 



