318 C. K. Weniworth— Wedge Work of Pebbles. 



If the cobble is taken as a sphere and if the assumption 

 is made that no shearing deformation takes place between 

 successive zones of each segment of the sphere the equi- 

 valent elastic cylinder of the same height will have a 

 diameter of approximately 2.4". Inasmuch as there will 

 be some deformation of this sort the diameter of the 

 equivalent cylinder will be somewhat less and may be 

 roughly taken as 2" and its end area as 3.14 square inches. 

 This gives a pressure of 6280 pounds exerted by a 4" 

 cobble under the conditions assumed. Such a pressure is 

 likely to cause local rupturing of the cobble if concen- 

 trated on a very small area at the contact. This will 

 tend to broaden the area of contact to a competent value. 

 "With an area of ^ square inch at the contact the pressure 

 computed above is not likely to produce crushing in 

 cobbles of the stronger rocks. 



From the considerations set forth above it appears that 

 the limit of pressure developed by any single pebble or 

 cobble is that required to compress it by the amount of 

 the thermal expansion caused by the maximum diurnal 

 temperature range. This pressure may be reached only 

 when the area of contact is sufficiently large to transmit 

 it. Most commonly the position reached by the cobble in 

 its fall will be such that there is considerable readjust- 

 ment and local crushing while the tirst part of the expan- 

 sion takes place. If it be assumed that one half of the 

 expansion takes place before the cobble is adjusted to 

 assume a full load there will remain expansive effect 

 sufficient to produce a pressure on a 4" cobble amounting 

 to over 3000 pounds or 1% tons. 



It is apparent that the above computations are based on 

 a number of assumptions which are only approximately 

 correct but they serve to show that even after making 

 ample allowance on the conservative side the pressure 

 developed by the expansion of pebbles in cracks is 

 sufficient to produce very considerable disruptive effects. 

 When it is considered that this process is ever active at 

 all temperature ranges it seems that in some situations 

 even where there are freezing temperatures in winter it 

 may be more potent in splitting the rocks than the ice 

 which forms in the cracks, though the writer is by no 

 means disposed to underestimate the effect of the latter. 



