162 THE BOOK OF NATURE STUDY 



pointing in the direction of dip (Fig. 49), and marked with its 

 amount in degrees, are placed so that their points are exactly 

 at the place where observations have been made. A map now 

 becomes not merely a record of the outcrop of certain rocks, but 

 it shows the run of the rocks in the crust beneath, that is, the 

 rock structure of that crust (Figs. 50 and 51). The importance 

 of such symbols will be seen if it is realised, for instance, that a 

 seam of coal is not generally in good condition for working at its 

 actual outcrop. If the direction of dip is known, the side of the 

 line on which a shaft should be sunk is at once indicated, while 

 the angle of dip will tell the depth to which the shaft must be 

 sunk. 



Outcrops in some cases consist of precipices, the strata being 

 broken right across, more or less perpendicular to their surfaces. 

 The sea, on the other hand, would plane off the projecting strata 

 to a horizontal surface. Average denudation on land comes 

 between these two extremes, producing varying slopes, those in 

 hard or porous rocks being generally steeper, and those in soft 

 rocks gentler : Resistant rocks usually make hills ; less resistant, 

 valleys. 



As the outcrop of a band runs along the strike-line across a 

 country, the hill it gives rise to will similarly tend to stretch as 

 a chain or ridge parallel to the strike (Figs. 64 and 65). The 

 importance, height, breadth, and relief of the ridge will depend on 

 several factors the resistance of the rock, its thickness, its angle 

 of dip, and on the relative softness of the strata above or below it. 

 Excavation of valleys by streams along the strike of the soft rocks 

 above and below a hard band, and parallel to the ridge formed 

 by it, will give rise to the longitudinal valleys already noticed : 

 these may be alternatively named strike-valleys (Figs. 44 and 48). 

 Typically the intervening ridges will become wolds or cuestas. The 

 soft stratum, being stripped by denudation from the top surface 

 of the hard bed, will allow that surface to govern the outline of 

 one slope, which will therefore correspond in angle with the dip 

 of the bed : hence it is called the dip-slope. The denudation of 

 the outcrop of the softer bed below, on the other hand, will 

 undermine the hard rock, and cause it to break away across the 

 stratum. Thus there will be formed a steep slope, tending to be 



