
~ 
= 1502 0 OQUTCROP. d11 
downward pressure of sliding detritus or “soil-cap ’suffices to bend 
over the edges of fissile strata, which, though 
really dipping into the hill, are thus made SSS 
to appear superficially to dip away from it SOY) yy 
(Fig. 229). Similar effects, with even — yy DY 
proofs of contortion, may be noticed under M LL 
boulder clay, or in other situations where Fic. 229—Dzceptive SupEr- 
the rocks have been bent over and crushed HICIAL Dre. 
by a mass of ice. 
When the dip is outward in every direction from a central point, 
it is said to be qud-qua-versal (A in Fig. 231). Strata thus affected 
are thrown into a dome-shaped structure, while when the dip is 
towards a central point, they have a basin-shaped structure. 
Outerop.—The edges of strata which appear at the surface of 
the ground are termed their Outcrop or Basset. If the strata 
are quite horizontal, the direction of outcrop depends on inequalities 
of the ground and variations in amount of denudation. Perfectly 
level ground lying upon horizontal beds shows of course no outcrop, 
for the surface coincides with the plane of stratification. But 
occasional water-courses have usually been eroded below the general 
level, so as to reveal along their sides outcrops of the strata. The 
remarkable sinuosities of outcrop produced by the unequal erosion of 
horizontal strata are illustrated in Fig. 230, where A is a map of 
a piece of ground deeply trenched by valleys, and B that of an area 
comparatively little denuded. In both cases the outcrops are seen 
to wind round the sides of the slopes. 
Where strata are inclined the course of their outcrop is regulated 
partly by the direction and amount of inclination, and partly by the 
form of the ground. When with low angles of dip they crop out, 
that is, rise to the surface, along a perfectly level piece of ground, 
the outcrop runs at a right angle to the dip. But any inequalities 
of the surface, such as valleys, ravines, hills, and ridges, will, as in the 
ease of horizontal beds, cause the outcrop to describe a circuitous 
course, even though the dip should remain perfectly steady all the 
while. If a line of precipitous gorge should run directly with the 
dip, the outcrop will there be coincident with the dip. The 
occurrence of a gently shelving valley in that position will cause the 
outcrop to descend on one side and to mount in a corresponding way 
on the other, so as to form a V-shaped indentation in its course. A 
ridge, on the other hand, will produce a deflection in the opposite 
direction. Hence a series of parallel ridges and valleys running in 
the same direction as the dip of the strata underneath causes the 
outcrop to describe a widely serpentinous course. 
The breadth of the outcrop depends on the thickness of the 
stratum and on the angle of dip. A bed one foot thick inclined at 
an angle of 1°, on a perfectly level piece of ground would have an 
outcrop about 60 feet broad. At a dip of 5° the breadth of the 
outcrop would be a little over 11 feet. At 30° it would be reduced 



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