66 



OUTCROP. 



tion in which the plane of the stratum extends will therefore change 

 only with the direction of upheaval of the beds, or the lines along which 

 they are folded, so as to be brought to the denuded surface. Thus in 

 the Lincolnshire Wolds the strike of the Chalk is south-east, in the 

 Cliiltern Hills it is south-west, in the Forth Downs it is east and 

 west. But the strike is entirely independent of the contour or eleva- 

 tion of the ground, and equally independent of the direction along 

 which the edge of a stratum can be followed over the surface of the 

 ground, for that direction may vary with denudation, but no amount 

 of denudation can affect the direction in which the plane of a stratum 

 is inclined. 



Outcrop. The area occupied by a stratum on the surface of a 

 country is termed its outcrop. The line of outcrop or basset is the 

 line where the bed comes to the surface from beneath an overlying 

 deposit. The line of outcrop of an inferior bed is the denudation 

 line, or limit of the outcrop of the stratum which rests upon it. In 

 level country the outcrop usually runs straight, but every hill and 

 valley, every variation in the texture of the stratum tends to make its 

 direction variable and sinuous, because outcrop lines are determined 

 by the ways in which the overlying strata are removed by the action 

 of frost, rain, and the sea, so as to uncover the layers beneath. The 

 general direction of outcrop follows the direction of strike, but the 

 details are the conseqtien'ees of denudation. The nature of the 



outcrop may be in- 

 fluenced by the 

 mineral character of 

 the deposit. Thus 

 since clays are 

 easily worn away, 

 they form valleys 

 or low level plains. 

 But limestones and 

 sandstones being 

 more durable, often 

 form terraces or 

 ridges of hills which 

 extend in the direc- 

 tion of the outcrop. 

 When a stratum in 

 this way rises up 

 like a sloping cliff, and exposes a large part of the thickness of a 

 stratum along the limiting line of outcrop, such exposure is called an 

 escarpment. These exposures, which often resemble inland cliffs, have 

 a base-line which varies in level, and is therefore regarded as having 

 been determined by frost and rain rather than by the sea. 



Two modifications of outcrop called " outlier " and " inlier " often 

 occur. An outlier is a portion of a stratum which has become sepa- 

 rated from the principal mass by denudation, and remains isolated 

 like an island. 



Map. 



Section. 



Fig. 19. Map showing lines of Outcrop. A P; C D. 





