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GRAVEL, or PEBBLE BEDS

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Originally appearing in Volume V12, Page 382 of the 1911 Encyclopedia Britannica.
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GRAVEL, or PEBBLE BEDS , the name given to deposits of rounded, subangular, See also:water-worn stones, mingled with finer material such as See also:sand and See also:clay. The word " gravel " is adapted from the O. Fr. gravele, mod. gravelle, dim. of See also:grave, coarse sand, See also:sea-See also:shore, Mod. Fr. grime. The deposits are produced by the See also:attrition of See also:rock fragments by moving water, the waves and tides of the sea and the flow of See also:rivers. Extensive beds of gravel are forming at the See also:present See also:time on many parts of the See also:British coasts where suitable rocks are exposed to the attack of the See also:atmosphere and of the sea waves during storms. The See also:flint gravels of the See also:coast of the Channel, See also:Norfolk, &c., are excellent examples. When the sea is rough the lesser stones are washed up and down the See also:beach by each See also:wave, and in this way are rounded, worn down and finally reduced to sand. These gravels are constantly in See also:movement, being urged forward by the shore currents especially during storms. Large See also:banks of gravel may be swept away in a single See also:night, and in this way the coast is laid See also:bare to the erosive See also:action of the sea. Moreover, the movement of the gravel itself wears down the subjacent rocks. Hence in many places barriers have been erected to prevent the See also:drift of the pebbles and preserve the See also:land, while often it has been found necessary to protect the shores by See also:masonry or See also:cement See also:work.

Where the pebbles are swept along to a projecting cape they may be carried onwards and See also:

form a See also:long See also:spit or submarine See also:bank, which is constantly reduced in See also:size by the currents and tides which flow across it (e.g. Spurn See also:Head at the mouth of the See also:Humber). The Chesil Bank is the best instance in See also:Britain of a See also:great See also:accumulation of pebbles constantly urged forward by storms in a definite direction. In the shallower parts of the See also:North Sea considerable areas are covered with coarse sand and pebbles. In deeper water, however, as in the See also:Atlantic, beyond the See also:loo See also:fathom See also:line pebbles are very rare, and those which are found are mostly erratics carried southward by floating icebergs, or volcanic rocks ejected by submarine volcanoes. In many parts of Britain, Scandinavia and North See also:America there are marine gravels, in every essential resembling those ofthe sea-shore, at levels considerably above high See also:tide. These gravels often See also:lie in See also:flat-topped terraces which may be traced for great distances along the coast. They are indications that the sea at one time stood higher than it does at present, and are known to geologists as " raised beaches." In See also:Scotland such beaches are known 25, 50 and roo ft. above the present shores. In exposed situations they have old shore cliffs behind them; although their deposits are mainly gravelly there is much See also:fine sand and silt in the raised beaches of sheltered estuaries and near See also:river mouths. River gravels occur most commonly in the See also:middle and upper parts of streams where the currents in times of See also:flood are strong enough to transport fairly large stones. In deltas and the See also:lower portions of large rivers gravel deposits are comparatively rare and indicate periods when the See also:volume of the stream was temporarily greatly increased. In the higher torrents also, gravels are rare because transport is so effective that no considerable accumulations can form.

In most countries where the drainage is of a mature type, river gravels occur in the lower parts of the courses of the rivers as banks or terraces which lie some distance above the stream level. Individual terraces usually do not persist for a long space but are represented by a See also:

series of benches at about the same See also:altitude. These were once continuous, and have been separated by the stream cutting away the intervening portions as it deepened and broadened its channel. Terraces of this See also:kind often occur in successive series at different heights, and the highest are the See also:oldest because they were laid down at a time when the stream flowed at their level and See also:mark the various stages by which the valley has been eroded. While marine terraces are nearly always See also:horizontal, stream terraces slope downwards along the course of the river. The extensive deposits of river gravels in many parts of See also:England, See also:France, See also:Switzerland, North America, &c., would indicate that at some former time the rivers flowed in greater volume than at the present See also:day. This is believed to be connected with the glacial See also:epoch and the See also:augmentation of the streams during those periods when the See also:ice was melting away. Many changes in drainage have taken See also:place since then; consequently wide sheets of glacial and fluvio-glacial gravel lie spread out where at present there is no stream. Often they are commingled with sand, and where there were temporary See also:post-glacial lakes deposits of silt, See also:brick clay and mud have been formed. These may be compared to the similar deposits now forming in See also:Green-land, Spitzbergen and other countries which are at present in a glacial See also:condition. As a See also:rule gravels consist mainly of the harder kinds of See also:stone because these alone can resist attrition. Thus the gravels formed from See also:chalk consist almost entirely of flint, which is so hard that the chalk is ground to See also:powder and washed away, while the flint remains little affected.

Other hard rocks such as chert, See also:

quartzite, See also:felsite, See also:granite, See also:sandstone and volcanic rocks very frequently are largely represented in gravels, while See also:coal, See also:limestone and shale are far less See also:common. The size of the pebbles varies from a fraction of an See also:inch to several feet; it depends partly on the fissility of the See also:original rocks and partly on the strength of the currents of water; coarse gravels indicate the action of powerful eroding agents. In the See also:Tertiary systems gravels occur on many horizons, e.g. the See also:Woolwich and See also:Reading beds, Oldhaven beds and Bagshot beds of the See also:Eocene of the See also:London See also:basin. They do not essentially differ from See also:recent gravel deposits. But in course of time the action of percolating water assisted by pressure tends to convert gravels into See also:firm masses of See also:conglomerate by depositing carbonate of See also:lime, See also:silica and other substances in their interstices. Gravels are not usually so fossiliferous as finer deposits of the same See also:age, partly because their porous texture enables organic remains to be dissolved away by water, and partly because shells and other fossils are comparatively fragile and would be broken up during the accumulation of the pebbles. The rock fragments in conglomerates, however, sometimes contain fossils which have not been found elsewhere. (J. S.

End of Article: GRAVEL, or PEBBLE BEDS

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