Glossary - Fundamentals of the Physical Environment - page 883

Dilatancy A property of fine-grained sediments which, unusually, causes expansion and
stiffening when compressed through the rearrangement of grains into a larger volume
and consequent intake of water into the voids.
Dilation Strictly speaking, a deformation involving an increase in the volume of an Earth
material without a change in shape but more often understood mistakenly as 'pressure
release' in rock mass.
Dip The angle of inclination of a rock structure from the horizontal.
Direct deposition The sublimation of ice directly on to a cold surface from water vapour.
Discharge A volume of river flow per unit of time expressed in cubic metres per second
or litres per second.
Disclimax A climax community maintained by human activities; the US term for
'plagioclimax'.
Discontinuous rockmass strength The lower resistance to shear offered by fractured
rock mass, compared with the intact strength of rock between the fractures; in essence,
the continuity of cohesion and friction is destroyed or reduced by the fractures, which
also permit water to enter the rock mass, with further destabilizing effects.
Dispersion tectonics Tectonic processes which lead to the large-scale separation and
spatial dispersion of crustal fragments.
Displaced terrane A terrane or crustal fragment with a distinct suite of rocks which has
been displaced tectonically away from where it formed.
Diversity The variety and relative abundance of species in a defined area.
Downwelling A convergence and subsidence of ocean surface water.
Draa A large aeolian sand dune complex composed of megadunes on which smaller
dunes may be superimposed.
Drainage (1) The process by which water moves over a landscape in rivers, or (2) the
ease with which water moves out of a soil profile by percolation into underlying rocks,
and hence the duration of periods when the soil is free from saturation with water.
Drainage basin A specific geographical area, bounded by a watershed and drained by a
discrete drainage network.
Drainage density The total stream channel length per unit land surface area, normally
calculated for an entire drainage basin .
Drainage network More or less synonymous with the channel network but may also
include rills, gullies and large underground pipes not considered part of a permanent
surface channel network.
Drainage pattern The geometric configuration or plan of a drainage network which
usually reflects catchment geology, tectonic and denudation history.
Draw - down The process and extent of gravitational or artificial withdrawal of water
from drainage basin stores.
Drumlin A large, subglacial bed form composed mostly of till and streamlined in the
iceflow direction; it is indicative of active ice flow and probable deformation in the
sediment body.
Dry valley A surface valley showing evidence of erosion by fluvial processes but rarely
or never occupied by a modern stream.
Dune A mobile sand-wave bed form shaped by fluid motion, found in a wide range of
wavelengths (from centimetres to kilometers ) and environments, ranging from stream
channel beds to coasts and deserts.