Drift

Drift (from “drive”), a verb or noun used in various connexions with the sense of propelled motion, especially (but not necessarily) of an aimless sort, undirected. Thus it is possible to speak of a snow-drift, an accumulation driven by the wind; of a ship drifting out of its course; of the drift of a speech, i.e. its general tendency. The word is also used in some technical senses, more immediately resulting from the action of driving something in. But the most important technical use of the word is in geology, as introduced by C. Lyell in 1840 in place of “Diluvium.” The earlier geologists had been in the habit of dividing the Quaternary deposits into an older Diluvium and a younger Alluvium; the latter is still employed in England, but the former has dropped out of use, though it is still retained by some continental writers. The Alluvium was distinguished from Diluvium by the fact that its mammalian fossils were representatives of still living forms, but it is a matter of great difficulty to separate these two divisions in practice. “The term drift is now applied generally to the Quaternary deposits, which consist for the most part of gravel, sand, loam or brickearth and clay; it naturally refers to strata laid down at some distance from the rocks to whose destruction they are largely due; but, although applied to river deposits, the word drift is more appropriately used in reference to the accumulations of the Glacial period.

“The occurrence of stones and boulders far removed from their parent source early attracted the attention of geologists, but for a long period the phenomena, now known as of glacial origin, were unexplained, and the drifts were looked upon as little more than ‘extraneous rubbish,’ the product of geological agents, quite distinct from those which helped to form the more ’solid’ rocks that underlie them.” (See H. B. Woodward, The Geology of England and Wales, 2nd ed., 1887.) The conception of an underlying “solid” geological structure covered by a superficial mantle of “drift” is still retained for certain practical purposes; thus, the Geological Survey of Great Britain issues many of the maps in two forms, the “Solid Edition,” showing the “solid geology,” which embraces all igneous rocks and the stratified rocks older than Pleistocene, and the “Drift Edition,” which shows only such older strata as are unobscured by drift.

In writing and in conversation the geological expression “drift” is now usually understood to mean Glacial drift, including boulder clay and all the varieties of sand, gravel and clay deposits formed by the agency of ice sheets, glaciers and icebergs. But in the “Drift” maps many other types of deposit are indicated, such, for instance, as the ordinary modern alluvium of rivers, and the older river terraces (River-drift of various ages), including gravels, brickearth and loam; old raised sea beaches and blown-sand (Aeolian-drift); the “Head” of Cornwall and Devon, an angular detritus consisting of stones with clay or loam; clay-with-flints, rainwash (landwash), scree and talus; the “Warp,” a marine and estuarine silt and clay of the Humber; and also beds of peat and diatomite.

See Glacial Period; Pleistocene; Boulder Clay.

(J. A. H.)

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