material eroded by a glacier, or weathered debris that has fallen onto
a glacier, is transported in one of three different positions.
the base or subglacial zone of the glacier: subglacial debris
the glacier or englacial
zone: englacial debris
the surface or supraglacial
zone: supraglacial debris
occurs when rock debris, previously frozen into, or lying on a glacier,
is laid down on a land surface by the melting of ice. This process
occurs at the front of a glacier during the warm summer months and
melting at the base of the glacier. All glacial deposits are collectively
referred to as drift despite
the fact that this term owes its origin to the early 19th century
that these deposits were primarily derived from the melting of floating
icebergs during the biblical flood. Drift can be subdivided into
types of glacial sediment.
Till:an unsorted mixture of clay, sand, gravel and
boulders that is deposited directly from melting ice.
contact stratified drift: stratified
sand and gravel that is sorted by the action of meltwater streams
and deposited next to the glacier.
deposits: well-sorted sand and gravel
deposited over a wide area of the proglacial area (in front of the
glacier) by meltwater streams emanating from the glacier.
both ice contact stratified drift and outwash deposits are also called
The word ‘till’
was first used by A Gelkin in 1863 to describe stony soil in Scotland,
and is now universally accepted as the name for unsorted debris deposited
directly from a glacier. In Britain, the term boulder clay was often
used to describe unsorted glacial deposits, but is now considered misleading
since till may contain neither, clay or boulders. Till deposits show
considerable variation in sediment structure, degree of compaction,
orientation of large stones or clasts and shape of clasts (Photo).
These variations reflect different processes involved in the formation
of till and are used as the basis for till classification.
There are two basic
types of till, subglacial
and supraglacial and these
can be further subdivided according to how the debris was released from
the glacier and whether the till is modified after initial deposition.
are deposited at the base of glaciers
and consist of compact, clay-rich debris with subrounded
and striated clasts. (Photo)
Compaction is caused by the compressive weight of the
overlying ice, and the high proportion of clay and modified clasts
from glacial crushing and abrasion at the base of the glacier. These
tills often display pronounced orientation of the long axes of
large clasts parallel to the direction of glacier flow, which is caused
by the alignment of stones in response to the forces promoted
by ice movement.
of subglacial tills include the following.
meltout till: formed
by the direct release of debris from the base of a melting glacier.
Meltout can involve the release of a thick debris layer or single
till: formed by the accretion
of subglacial debris against rough
bedrock. These tills are highly compact and directly overlie
till: forms by the deformation of weak rock or pre-existing till
by the pressures exerted by moving ice. Not suprisingly, these tills
exhibit various deformation structures such as folds and shears.
These tills form
at the surface and around the margins of glaciers.
They are composed of loosely compact, coarse blocky debris
with little or no clay and silt. The majority of clasts are angular
in shape since this surface debris (which is mainly frost weathered
rock that has fallen onto a glacier surface) is not modified by abrasion
or crushing during transport (Photo).
However, subrounded glacial debris can be present because basal material
may be moved to the surface along compressive thrusts.
of supraglacial tills include the following.