Everything about Glacial Till totally explained
Till is unsorted
glacial sediment. Glacial drift is a general term for the coarsely graded and extremely heterogeneous
sediments of glacial origin. Glacial
till is that part of glacial drift which was deposited directly by the glacier. It may vary from
clays to mixtures of clay,
sand,
gravel and
boulders. Clay in till may form in
spherical shapes called
till balls. If a till ball rolls around in a stream, it may pick up rocks from the streambed and become covered by rocks; thence it's known as an armored till ball.
Till is deposited at the
terminal moraine, along the
lateral and
medial moraines and in the
ground moraine of a glacier. As a glacier melts, especially a
continental glacier, large amounts of till are washed away and deposited as outwash in
sandurs by the
rivers flowing from the glacier and as
varves in any
proglacial lakes which may form. Till may contain
alluvial deposits of
gems or other valuable
ore minerals picked up by the glacier during its advance, for example the
diamonds found in
Wisconsin,
Indiana, and
Canada.
Prospectors use trace minerals in tills as clues to
follow the glacier
upstream to find
kimberlite diamond deposits and other types of ore deposits.
Tillite
In cases where till has been indurated or
lithified by subsequent burial into solid rock, it's known as the
sedimentary rock tillite. Matching beds of ancient tillites on opposite sides of the south
Atlantic Ocean provided early evidence for
continental drift. The same tillites also provided the key evidence for the
Precambrian Snowball Earth glaciation event.
Types of till
There are various types of classifying tills:
- primary deposits – these were laid down directly by glacier action
- secondary deposits – these have undergone reworking (for example fluvial transport, erosion, etc)
Traditionally (for example
Dreimanis, 1988), a further set of divisions has been made to primary deposits, based upon the method of deposition.
Lodgement tills – sediment which has been deposited by plastering of glacial debris from a sliding glacier bed.
Deformation tills – Sediment which has been disaggregated and (usually) homogenised by shearing in the sub glacial deformed layer.
Melt out tills – Released by melting of stagnant or slowly moving debris-rich glacier ice and deposited without subsequent transport or deformation. Split up into sub glacial melt out till (melting of debris rich ice at the bottom of the glacier) and supraglacial melt-out till (melting of ice on the glacier surface).
Sublimation till – similar to melt out till, except the ice is lost through sublimation rather than melt. Often occurs only in extremely cold and arid conditions, mainly in Antarctica.
Some (for example Van der Meer et al. 2003) have suggested that these till classifications are outdated and should instead be replaced with only one classification, that of deformation till. The reasons behind this are largely down to the difficulties in accurately classifying different tills, which are often classified based on inferences of the physical setting of the till rather than till fabric or particle size analysis data.
Further Information
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