CHAPTER X 

 DENUDATION 



IN studying the denudation of natural surfaces it will be best to 

 begin so far as possible with disintegration. The work of frost 

 is perhaps that which lends itself most readily to both ex- 

 periment and observation. 



The fact that water expands on freezing may be demonstrated 

 by the breaking of a bottle, or even of an iron receptacle, in a 

 freezing mixture. In most places displacement of stones in walls, 

 disintegration of roads and paths, and bursting of water pipes, 

 during frost may be observed. Use can be made of the fact that, 

 though the damage is done on freezing, the results become apparent 

 only with the ensuing thaw. In quarries it will be possible to 

 show that rocks are traversed by natural fissures into which water 

 can penetrate and then expand with irresistible force on freezing ; 

 the stages in detachment can be seen in Fig. 8. In a hilly 

 country there will usually be screes, or heaps and trails of 

 angular stones broken from the crags by frost (Fig. 9). If these 

 screes are traced upwards to the crags which feed them evidence 

 of frost action will generally be apparent ; and it may even be 

 possible to show that crags and peaks are the residual forms 

 resulting from the wedging action of frost. The larger stones of 

 the upper part of the screes will also be found to be split again 

 and again by the same agency during their downward travel. In 

 towns flagstones generally demonstrate the action of frost by the 

 formation of hollow centres. The water saturating the cement 

 between the grains of the upper layers expands on freezing, 

 causing the layers to bulge upwards as they are held tightly at 

 their ends. These layers, then unsupported by the rock beneath, 

 are easily crushed, and the fragments brushed or blown away. 

 During rains water settles in the hollows so produced, and the 

 action is repeated. The disintegrating effect of alternately 



