Geology Reference
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Figure 5.9.
Corestones in a matrix of grus, Snowy Mountains, New South Wales.
stage refers to the period of differential fracture-controlled subsurface weathering, the second to
the phase of differential erosion during which grus is evacuated and the corestones are exposed
as boulders. If erosion outpaces weathering, the regolith is eroded, with only a few erstwhile core-
stones remaining as boulders. If, however, weathering proceeds more rapidly than erosion, most
corestones located in the near surface zone are reduced to grus. Those that persist emerge at the
surface as boulders. Once exposed they are no longer in constant contact with moisture, and
become stable, while weathering continues in the regolith, leading to the common situation of
boulders being underlain by considerable thicknesses of grus ( Figs 5.4 a nd 5.9).
The progress of weathering, and hence the shape and size of corestones, depends on the dura-
tion of uninterrupted subsurface weathering, and on the changing physics, chemistry, biology and
circulation of the shallow groundwaters. The characteristics of the latter may, in some measure,
reflect atmospheric climatic conditions, but they affect the rate of activity rather than the eventual
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