rom 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.)

DRILL. (1) A tool for boring or making holes in hard substances, such as
stone, metal, &c. (an adaptation in the 17th century from the Dutch
_dril_ or _drille_, from _drillen_, to turn, bore a hole; according to
the New English Dictionary the word is not to be connected with the
English "thrill"). The word _drillen_ was used in Dutch, German and
Danish, from the 17th century for training in military exercises and was
adopted into English in the same sense. The origin of the application
seems to be in the primary sense of "to turn round," from the turning of
the troops in their evolutions and from the turning of the weapons in
the soldiers' hands. Drill is, formally, the preparation of soldiers for
their dutie