Geology Reference
In-Depth Information
Plate 15.1 Brimham Rocks, eroded remnants of Millstone Grit sandstone, Nidderdale, North Yorkshire, England.
( Photograph by Tony Waltham Geophotos )
As we gaze on this wonderful group of insular wrecks, varying in form from the solemn to the grotesque, and presenting now the
same general outlines with which they rose above the sea, we can scarcely resist contrasting the permanence of the 'everlasting hills'
with the evanescence of man. Generation after generation of the inhabitants of the valleys within sight of the eminence on which we
stand, have sunk beneath the sod, and their descendants can still behold in these rocky pillars emblems of eternity compared with
their own fleeting career; but fragile, and transient, compared with the great cycle of geological events. Though the Brimham Rocks
may continue invulnerable to the elements for thousands of years, their time will come, and that time will be when, through another
submergence of the land, the sea shall regain ascendancy of these monuments of its ancient sway, completing the work of denudation
it has left half-finished.
(Mackintosh 1869, 119-24)
OLD PLAINS
preferable since it has no genetic undertones and sim-
ply means 'old plain'. It is worth bearing in mind when
discussing the classic theories of landscape evolution
that palaeoplain formation takes hundreds of millions
of years to accomplish, so that during the Proterozoic
aeon enough time elapsed for but a few erosion surfaces
to form. In south-eastern Australia, the palaeoplain first
described by Edwin Sherbon Hills is still preserved along
much of the Great Divide and is probably of Mesozoic
age. In South America, where uplift has been faster,
there are three or more erosion surfaces. Old erosion sur-
faces are commonly preserved in the geological record as
unconformities.
Some geomorphologists, mainly the 'big names' in the
field, have turned their attention to the long-term change
of landscapes. Starting with William Morris Davis's 'geo-
graphical cycle' (p. 6), several theories to explain the
prolonged decay of regional landscapes have been pro-
mulgated. Common to all these theories is the assump-
tion that, however the land surface may appear at the
outset, it will gradually be reduced to a low-lying plain
that cuts across geological structures and rock types.
These planation surfaces or erosion surfaces are var-
iously styled peneplains, panplains, and etchplains. Cliff
Ollier (1991, 78) suggested that the term palaeoplain is
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