Engineering Geology

Chapter 1
Figure 1.20
Thin section of Fell Sandstone (a quartz arenite), Lower Carboniferous, Northumberland, England.
siliceous cement. Frequently, siltstones are interbedded with shales or fine-grained sandstones,
the siltstones occurring as thin ribs.
Loess is a wind-blown deposit that is mainly of silt size and consists mostly of quartz particles,
with lesser amounts of feldspar and clay minerals. It is characterized by a lack of stratification
and uniform sorting, and occurs as blanket deposits in western Europe, the United
States, Russia and China (Fig. 1.21). Deposits of loess are of Pleistocene age and, because
they show a close resemblance to fine-grained glacial debris, their origin has customarily
been assigned a glacial association. For instance, in the case of those regions mentioned,
it is presumed that winds blowing from the arid interiors of the northern continents during
glacial times picked up fine glacial outwash material and carried it for hundreds or thousands
of kilometres before deposition took place. Deposition is assumed to have occurred
over steppe lands, and the grasses left behind fossil root holes, which typify loess. These
account for its crude columnar structure. The lengthy transport explains the uniform sorting
of loess.
Deposits of clay are composed principally of fine quartz and clay minerals. The latter represent
the commonest breakdown products of most of the chief rock-forming silicate minerals.
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