Cryospheric systems - Fundamentals of the Physical Environment - page 359

The solid character of ice supports debris across the
entire range of particle sizes and, unlike fluids, does not
sort them according to velocity. Subglacial large-scale
'plucking' or joint-block removal has been a harder
process to resolve and complex schemes of pre-glacial or
subglacial frost shattering and pressure release have been
invoked. The principal objection to the latter was outlined
in Chapter 13 and, in general, these processes also founder
on how ice not only dislodges large blocks of underlying
rock (whilst also constraining them) but entrains them in
the ice stream. Freezing-on seems to defy the pressure-
melting effects of very large blocks protruding into the
basal ice.
Quarrying is now thought to be particularly effective
in the presence of meltwater at the high confining pres-
sures which generate pressure-melting at the glacier bed.
Clear traces of high-pressure flow were already known
from plastically sculptured p-forms and water-scoured
sichelwannen . Pre-existing rock mass discontinuities pre-
empt the need for frost or other fracturing and provide
water access. Recalling the Mohr-Coulomb criteria in
Chapter 13, high-pressure water provides uplift ( u ) for
individual blocks like a hydraulic jack. This raises shear
stress considerably behind the block ( v ) and effectively
'firehoses' it into the ice stream - where it is crushed
and/or frozen-on in areas of

Glacier transport and deposition
Glaciers transport debris in sub glacial, en glacial and
supra glacial positions ( Figure 15.10 ). Debris in transit
may remain in one of these positions, from where it is
eventually deposited, or may move to another. Glacigenic
sediments are deposited by ice directly or may be reworked
by meltwater and are not as structureless as was once
thought ( Plate 15.9 , lower). The relative importance of ice
and water is evident in the variety of sedimentary facies
associated with a single glacial event. Modern research
draws further distinctions between terrestrial- and

(a)

lower temperature or

pressure ( Plate 15.8 ).

(b)

Plate 15.8 Recent glacier quarrying in granite by the
Nigardsbreen glacier, south-west Norway. The side view (a)
shows block displacement along vertical and horizontal
fractures and the rear view (b) reveals the effects of high-
pressure water jacking, sufficient to insert water-rounded
boulders into the developing fracture.
Photos: Ken Addison

Plate 15.7 Crescentic chatter marks made by the inter-
mittent contact of a large abrading tool.
Photo: Ken Addison