C. K. Wentworth— Wedge Work of Pehhles. 317 



yield in the rock. If the rock yields in part by rupture 

 the effect will be so much the more rapid. The foregoing 

 analysis shows adequately that pebbles can exert cumu- 

 lative stresses in widening cracks and disrupting the 

 rocks. That the pebbles in cracks are usually tightly 

 wedged is proof that they do exert pressure on the walls. 

 It remains to inquire what the quantitative importance 

 of this process is. The factors involved include thermal 

 expansion, thermal conductivity, thermal capacity or 

 specific heat, elasticity, crushing strength, tensile strength 

 and density. It is not essential nor possible here to treat 

 the problem exhaustively and certain approximate 

 assumptions will be made. The following values are 

 taken as sufficiently typical for the common rocks : 



Coefficient of expansion 0000025 per 1 degree F. 



Compression elasticity 00025 per 1000 lbs. to the sq. in. 



Let a maximum diurnal change of 100 degrees F. be 

 assumed and let it be considered that this temperature 

 change penetrates only a foot or two. Considering tem- 

 perature changes alone, then, two points on either side of 

 a crack and each several feet within the rock mass will be 

 separated by a distance which remains a constant. Let 

 it further be assumed that the temperature gradients on 

 either side of the crack and extending into the rock be 

 such that the total thermal expansion on both sides is just 

 equal to that wrought on a 4" length by a change of 100 

 degrees F. If then a 4" cobble be allowed to fall during 

 the time of minimum temperature into the crack, the total 

 expansion effect of an increase in temperature of 100 

 degrees will be twice that wrought on the cobble or 



2(4" X 100) (.0000025) =.002" 



The pressure required to produce a .002" diminution 

 of a 4" length will be 



1000 = 2000 lbs. per sq. in. 



It is valid to conclude that the pressure exerted by the 

 cobble will equal 2000 times the mean or equivalent cross- 

 sectional area in square inches. This conclusion is based 

 on the assumption that the elastic yielding of the 

 strongly supported regions of contact of the rock walls 

 is negligible. 



