476 Transactions.—Geology. 
higher and wider as more shingle is thrown up it and over it by the waves. 
An island lying off the coast on which there is a travelling beach also 
causes the latter to detach itself from the shore. "The island forms a break- 
water, and the shingle cannot pass across the still water behind it. It 
therefore collects on the windward side until it reaches out to the island, 
from the leeward side of which it continues its course as a detached beach. 
Strong littoral currents and rivers entering the sea cause the waves to 
break before they reach the shore. If there is any travelling shingle it will 
be collected where the waves break, and will therefore form a detached 
beach. 
In short, given a source of supply of shingle or sand, such as a large 
river or a cliff against which the sea beats, there will be a travelling beach 
wherever the average direction of the waves is oblique to the shore on which 
they break, and there will be a detached beach whenever, from any cause, 
the waves, under similar conditions, break before they reach the shore. 
The principal causes of this latter case are those above enumerated. 
_ There are in New Zealand many examples of detached beaches. The 
spit of sand which ends Cape Farewell is an example on a very large scale. 
The source of supply is in this case the whole west coast of the South 
Island with its many rivers. The débris formed by the denudation of the 
land is driven northwards by the prevailing south-westerly seas which 
break obliquely on the shore. At Cape Farewell the breaking of the waves 
against one another instead of on the shore caused the sand to be thrown 
down before reaching the shore on the east side of the Cape. The spit 
began to form and is still forming. Eventually, Golden Bay will become a 
lagoon if a sufficient supply of sand is kept up by the continued denudation 
of the west slope of the island. 
Nelson Harbour is another case of a detached beach. The cliffs to the 
north of the harbour are being cut down by the sea, and the material, 
driven by the oblique north-westerly seas, which are here the most power- 
ful, crosses the indentation of the coast on which Nelson is situated; the 
earthy matter contained in the cliff is washed away leaving only the mass of 
heavy boulders, which are all of the same material as those embedded in 
the cliff from which the boulder bank is derived. 
Lake Ellesmere is another fine example. The coast of Canterbury 
consists of a travelling shingle beach on a scale of magnitude hardly 
equalled in the world. The great Canterbury rivers are nothing but 
mountain torrents on a magnified scale. They have their rise in ranges 
- formed of clay-slates of a very friable nature, and they convey to the sea 
|. great quantities of shingle. 
