COASTAL WORKS 245 
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reached by water which had dropped nearly all its silt 
slsewhere. 
The filling of an estuary or lake may be aided by silt- 
jetties being built up by rivers, which deposit their silt as a 
bank in the stagnant water on each side of their entrance. 
Banks thus formed are slowly raised above the water-level 
as silt-jetties. The delta of the Mississippi with its finger- 
like projections has been thus extended into the Gulf of 
Mexico. Silt-jetties subdivide where a river Dbifurcates 
against an obstacle, and the junction of adjacent jetties 
breaks up an estuary into separate lakes; the river twists 
and winds between these lakes, though separated from them 
perhaps only by a narrow swampy bank. The Norfolk 
Broads are typical examples of this formation. They were 
originally part of an estuary, which has been silted up owing 
to the narrowing of its mouth by the Yarmouth spit, and 
divided by confluent silt-jetties into shallow lakes. The 
horders extend inward and the lake is reduced to a small 
round pool; and its disappearance completes the conversion 
of the estuary into an alluvial plain. 
CoasTAL ProTECcTION BY PLANTING AND GROYNES— 
Strong masonry has been often used as defence against the 
sea. It however may be undercut and out-flanked, and its 
fall provides ammunition by which the waves more effectively 
hatter the coast. William Smith! the Father of Geology, 
was sent in 1801 to the Norfolk coast to devise means for its 
protection ; he carefully observed wave action and condemned 
masonry as dangerous; he recommended ‘embankments 
as like as possible” and of the same materials as those 
thrown up by the sea. They by their looseness disarm the 
waves, and provide permanent and cheap defence. Sand 
dunes are effective protection but they may be blown away 
by the wind. Their migration can be stopped by such 
olants as Marram grass, Psamma arenaria. Its long under- 
ground stems and roots form a firm mat and the spikes of 
grass stop the movement of sand grains on the surface. A 
growth of Marram grass converts a moving into a fixed dune. 
It may thereby defend a coast from the sea, and save a 
fertile plain from being overridden by sand dunes from an 
adjacent desert. 
-J. Phillips, Memoir Wm. Smith, 1844, pp. 50-3.