The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part IV. Engineering geology

Collection:: Economics Books

COASTAL WORKS 245 pe Lil 2S al 1 e Sy al >f h 8] & 01 € J A g 1 y “EY n h 1€ 1 7 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.

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