Geology 



be horizontal. This horizontal direction at right angles 

 to that of the dip is called the "strike" of the bed. In 

 cases where a bed of rock is of considerable thickness, 

 lies in a horizontal position or dips at a low angle, and 

 occurs in a comparatively flat country, it may cover a 

 very wide area of the surface of the ground ; but where 

 the dip is steep, the rock soon plunges beneath the 

 surface, and is lost to view, the width of the strip of 

 land formed by it decreasing as the dip becomes steeper, 

 until, when the angle is 90, the width will be equal to 

 the thickness of the bed. 



The area over which a rock appears at the surface 

 is called its "outcrop," and the form and area of this 

 depend not only on the angle of dip, but also upon the 

 form of the ground. These technical terms are intro- 

 duced as their use will greatly simplify much that is 

 to follow. 



Since the rocks of a country have been laid down 

 layer upon layer in an approximately horizontal manner 

 it follows, of course, that the lowest layers are older 

 than those above, and in places where the horizontality 

 has been disturbed, and the rocks possess a dip, newer 

 and newer rocks will be traversed as one walks in the 

 direction of the dip. 



Often in making a traverse of a country it is found 

 that a sequence of rocks is repeated in inverse order, 

 and in this case the beds have been folded into either 

 a trough (or syncline) or an arch (or anticline), and the 

 upper portions of the folds subsequently removed by 

 the action of the weather (Figs. 6 and 7). 



In other cases the rocks in place of folding have 

 broken through in such a manner as to allow the beds 



64 



