Page 290 - Fundamentals of Geomorphology

GLACIAL AND GLACIOFLUVIAL LANDSCAPES 273

Plate 10.15 Braided outwash plain in front of, and to the side of, the snout of the debris-covered Casement Glacier,
Glacier Bay, Alaska.
(Photograph by Mike Hambrey)

conditioned by glaciation and deglaciation to fashion processes that are directly conditioned by glaciation’,
paraglacial landforms (see Ballantyne 2002). Once ice which includes proglacial processes and processes occur-
disappears, several changes occur in former glacial land- ring ‘around and within the margins of a former glacier
scapes. Rock slopes steepened by valley glaciers become that are the direct result of the former presence of ice’.
unstable and vulnerable to slope failure and rockfall once Moreover, they recognized a ‘paraglacial period’ – the
the ice no longer acts as a buttress. Slopes bearing a man- time during which paraglacial processes operate. Later,
tle of drift but no vegetation become subject to rapid they extended the notion to include all periods of glacier
reworking by debris flows, snow avalanches, and slope retreat, and not just the Late Pleistocene deglaciation
wash. Glacier forelands become exposed to wind erosion (Church and Ryder 1989).
and frost action. Rivers pick up and redistribute large Colin K. Ballantyne (2002) recognized six paraglacial
amounts of unconsolidated sediment of glacial origin, ‘land systems’ – rock slopes, drift-mantled slopes, glacier
later depositing it in a range of terrestrial, lacustrine, and forelands, and alluvial, lacustrine, and coastal systems –
marine environments. Wind entrains finer sediments, each containing a variety of paraglacial landforms and
particularly silts, and may bear it thousands of kilometres sediment facies. Taken together, he regarded these land-
and deposit it as loess deposits (p. 311). This accelerated forms and sediments – talus accumulations, debris
geomorphic activity follows deglaciation and lasts up to cones, alluvial fans, valley fills, deltas, coastal barrier
10,000 years, until the landscape adjusts to non-glacial structures, and so forth – as storage components within
conditions. an interrupted sediment cascade. The cascade has four
June M. Ryder (1971a, b) coined the term ‘paraglacial’ primary sources of material – rockwalls, drift-mantled
to describe alluvial fans in British Columbia, Canada, slopes, valley-floor glacigenic deposits, and coastal
formed through the reworking of glacial sediment by glacigenic deposits. And it has four terminal sediment
rivers and debris flows after the Late Pleistocene deglacia- sinks – alluvial valley-fill deposits, lacustrine deposits,
tion. Michael Church and Ryder (1972, 3059) then coastal and nearshore deposits, and shelf and offshore
formalized the idea by defining ‘paraglacial’ as ‘nonglacial deposits.