136 ON SOME PLATEAU DEPOSITS AT FELSTEAD AND STEP.BING. 



most persons. These agencies cause a complete disintegration of 

 the soil, and there is a sensible movement of the mass due to the 

 redistribution of the molecules. If there are lumps of chalk in the 

 soil (Boulder Clay) the change is more striking. In the case of 

 complete penetration by frost, sufficient moisture being present, 

 the lumps of chalk suffer disintegration, and when the thaw comes 

 they are quickly broken up and diffused through the soil (see " Notes 

 on the Prolonged Frost," Essex Naturalist, vol. v., page 66). 

 The effect of this diffusion is to make the soil afterwards more 

 pervious to water ; and the effect of rain-water containing carbonic 

 or other acids then percolating through the soil is to attack the 

 calcareous matter and gradually take it away in solution. Boulder 

 Clay is ordinarily almost or quite impervious to water ; but if, by any 

 means, it can be completely penetrated, by frost, a door is at once 

 opened for the admission of atmospheric agencies, and this door 

 will not again be closed until most or all of the calcareous matter 

 is removed. 



In order to apply this reasoning to the cases under considera- 

 tion, we must suppose longer or colder winters than we have at 

 present. My experience is that our winters make no appreciable 

 difference to the weathering of the Boulder Clay when it is kept 

 moderately dry.^ The conditions required for change seem to be such 

 as those now obtaining in some parts of Siberia or Canada, but not 

 where the soil is permanently frozen, a thaw being essential to the 

 work of decalcification. Cases are quoted where the loams extend 

 to a depth of ten feet. In order to affect the Boulder Clay to such an 

 extent, it seems that the winter temperature must have been much 

 lower than at present, and the country correspondingly sterile ; 

 moreover, we know that the drainage was less perfect than at 

 present, and the swamps may have assisted the frost materially in 

 its work. 



In inquiring as to when such arctic conditions prevailed, we 

 must take into account the work done by erosion since. This is 

 considerable, for some loams now occupy such a position that, 

 supposing those conditions of frost to return again, the loams would, 

 on account of the loss of their lateral supports, slide down the slope 

 on the return of the thaw. In fact, a frost' and thaw of that 

 character would be prolific of landsh'ps in FLngland. The extent to 



5 a p-rpeniiculir section of Boulder Cay m.ide here about twenty years .tgo, witli slightly 

 overhingins ledge, has not suffered appreciably by weathering. 



