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 241 violence. This effect was discovered by the first Eddystone Lighthouse (1700). The door had been strongly supported to resist the waves, but it was blown outward by the expansion of air compressed within the lighthouse. The wearing back of the coast is aided by subaerial denuda- tion, which generally produces an upward slope inland above the sea-cut part of the cliff. Wave action is aided by animals and plants. Seaweeds growing on a rock enable a wave to move it. Animals bore into rocks and their acid secretions dissolve the cement ; Pholas bores into limestone; the shipworm, the Teredo, burrows through timber; and sea-urchins (Echinoidea), browsing on films of seaweed, wear pits even in granite. TransPorT OF Brace Mareriar—The material obtained by the wearing back of a cliff is rolled to and fro by the waves and reduced to shingle, which drifts along the shore. Ac- cording to one view this movement is due entirely to waves made by the wind; but according to another it is due to tidal currents. Both agencies act in varying degrees; the movement of pebbles is usually by the waves, but guided by the persistent currents. Beach shingle and river shingle may be distinguished by the shape of the cobbles (stones of about 4 to § inches in diameter), which in a river are rolled along with the long axis at right angles to the current, and are typically ovoid. Cobbles on a sea-beach are spun around by the tide, the base is worn flat, and the upper side is rounded by the scour of sand, until they become bun-shaped. Marine abrasion forms a shelf or plain of marine denuda- tion between high and low tides. ‘Stacks’ or pillars of hard rock may rise above it. The * Old Man of Hoy” in the Orkneys is a pillar of Old Red Sandstone about 450 feet high. Some bands of rock with the layers on end and lying in contorted shale were attributed to glacial action, until Murray Macgregor recognized them as stacks that had fallen on a muddy shore. Sea-caves are formed where rocks are eaten into hollows. Raised lines of caves often give evidence of the uplift of a coast. CONTINENTAL SHELF—A broad shelf of mud, sand, and shell beds with occasional exposures of rock, borders the continents and extends from the shore till, at the depth of

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