Page 120 - Sedimentology and Stratigraphy

Continental Glacial Deposition 107

ice body to mix and hence no sorting of material consists of clay minerals and fine-grained quartz. Unlike
carried by the glacier will take place. Glacially trans- clay minerals the fine particles in rock flour do not
ported debris is therefore typically very poorly sorted. flocculate (2.4.5) and tend to remain in suspension
Fragments plucked by the ice will be angular and for much longer periods of time. This high proportion
debris carried within ice will not undergo any further of suspended sediment gives the characteristic green
abrasion, and only material on the top of an ice body to white colour to lakes fed by glacial melt waters.
will be subject to weathering processes. In addition to Material carried by a glacier is not necessarily all
the poor sorting, debris carried by glaciers is very the result of glacial erosion. Valley sides in cold
angular and the overall texture is therefore very regions are subject to extensive freeze–thaw weath-
immature. The constituents of tills and tillites are the ering (6.4.1), the products of which fall down the
products of weathering in cold environments, where valley sides onto the top surface of the glacier. In
physical weathering processes break up the rock but more temperate regions detritus may also be washed
chemical weathering does not play an important role. down the valley sides by overland flow and by
For this reason, the mineral composition of the deposit streams, which are active during the summer thaw.
may be very similar to that of the bedrock and unal- Streams may also form on the surface of a glacier or
tered lithic fragments are common. Clay minerals are ice sheet during warmer periods and their action may
often rather uncommon even in the fine-grained contribute to the transport of debris.
fraction of a till because clays form principally by
the chemical weathering of minerals and in glacial 7.4 CONTINENTAL GLACIAL
environments this breakdown process is suppressed. DEPOSITION
The fine-grained rockflour formed byglacial abrasion
is different in composition to similar grade sediment Modern landscapes formerly covered by Quaternary
produced by other mechanisms of weathering and ice sheets display a wide variety of depositional land-
erosion. Rock flour consists of very small fragments of forms (Fig. 7.7), which have been extensively studied
many different minerals. In contrast the same sized and described by glacial geomorphologists (e.g. Ham-
material produced by chemical weathering typically brey 1994; Benn & Evans 1998). The depositional

Fig. 7.7 Glacial landforms and glacial deposits in continental glaciated areas.