Page 121 - Sedimentology and Stratigraphy

108 Glacial Environments

characteristics of these features provide information graphic record in the long term. This is because they
about glacial processes in the past few tens of thou- mainly occur in areas that are only regions of deposi-
sands of years of Earth history and provide a basis tion as a consequence of the glacial processes: many
for understanding the origins of the landscapes of the modern glacial landscapes are undergoing ero-
around us. However, most continental glacial deposits sion and over time the continental glacial deposits will
(Fig 7.8) are unlikely to be preserved in the strati- be reworked and removed. Glacial deposits recognized
in pre-Quaternary strata are mostly marine in origin.
Continental glacial facies 7.4.1 Moraines

Scale Lithology MUD SAND GRAVEL Structures etc Notes Accumulations of till formed directly at the margins of

clay silt vf m vc gran pebb cobb boul a glacier are known as moraine. Several different
f
c
types of moraine can be recognised (Benn & Evans
1998). Terminal or end moraines mark the limit of
glacial advance and are typically ridges that lie across
the valley. Push moraines are formed where a glacier
Loess
front acts as a bulldozer scraping sediment from the
valley floor and piling it up at the glacier front. Dump
moraines form at the snout of the glacier where the
melting of the ice keeps pace with glacial advance. If a
glacier retreats the melting releases the detritus that
Glacio-lacustrine
deposits with varves has accumulated at the sides of the glacier where it is
deposited as a lateral moraine (Figs 7.7 & 7.9).
Lateral moraines form ridges along the sides of gla-
ciated valleys, parallel to the valley walls. Where two
glaciers in tributary valleys converge detritus from
the sides of each is trapped in the centre of the amal-
gamated glacier (Fig. 7.10) and the resulting deposit
upon ice retreat is a medial moraine along the cen-
m - 10s m Outwash braided tre of a glaciated valley. When a cold glacier retreats,
stream deposits
the snout of the glacier is often left with a carapace
of detritus left behind as the glacier front has been

Lodgement till.
Thickest deposits
occur close to the
ice front

Substrate

Fig. 7.8 Graphic sedimentary log illustrating some of the Fig. 7.9 A lateral moraine left by the retreat of a valley
deposits of continental glaciers. glacier.