A Manual of Elementary Geology: or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological MonumentsLyell, Charles, Sir
Science
A Manual of Elementary Geology: or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments
Lyell, Charles, Sir
Geology
Secondly, if the beds be inclined and intersected by a valley sloping in
the same direction, and the dip of the beds be less steep than the slope
of the valley, then the V's, as they are often termed by miners, will
point upwards (see fig. 74.), those formed by the newer beds appearing
in a superior position, and extending highest up the valley, as A
is seen above B.
[Illustration: Fig. 74. Slope of valley 40 deg., dip of strata 20 deg..]
Thirdly, if the dip of the beds be steeper than the slope of the valley,
then the V's will point downwards (see fig. 75.), and those formed of the
older beds will now appear uppermost, as B appears above A.
[Illustration: Fig. 75. Slope of valley 20 deg., dip of strata 50 deg..]
Fourthly, in every case where the strata dip in a contrary direction to
the slope of the valley, whatever be the angle of inclination, the newer
beds will appear the highest, as in the first and second cases. This is
shown by the drawing (fig. 76.), which exhibits strata rising at an
angle of 20 deg., and crossed by a valley, which declines in an opposite
direction at 20 deg..[57-A]
[Illustration: Fig. 76. Slope of valley 20 deg., dip of strata 20 deg.,
in opposite directions.]
These rules may often be of great practical utility; for the different
degrees of dip occurring in the two cases represented in figures 74 and 75.
may occasionally be encountered in following the same line of flexure at
points a few miles distant from each other. A miner unacquainted with the
rule, who had first explored the valley (fig. 74.), may have sunk a
vertical shaft below the coal seam A, until he reached the inferior bed B.
He might then pass to the valley fig. 75., and discovering there also the
outcrop of two coal seams, might begin his workings in the uppermost in the
expectation of coming down to the other bed A, which would be observed
cropping out lower down the valley. But a glance at the section will
demonstrate the futility of such hopes.
In the majority of cases, an anticlinal axis forms a ridge, and a synclinal
axis a valley, as in A, B, fig. 62. p. 48.; but there are exceptions to
this rule, the beds sometimes sloping inwards from either side of a
mountain, as in fig. 77.
[Illustration: Fig. 77. Cross section.]
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