234: Campbell — Fractured Bowlders in Conglomerate. 



bowlders show similar results, the two on the right being 

 marked by especially deep bruises and great cracks which 

 extend entirely through them. The bowlder on top has*had 

 another bowlder thrust into it so far that a piece of the oppos- 

 ing bowlder is still contained in the upper part. The bowlders 

 in question are mostly tough quartzite and they present a 

 striking illustration of the enormous pressure that has been 

 brought to bear upon them. 



Similar fractured bowlders may be seen in figure 1, but in 

 this case they appear more like fractures due to shearing than 

 the result of one bowlder impinging upon another. A good 

 example may be seen above and to the right of the hammer 

 where a bowlder 8 or 10 inches in length is sheared in three 

 or four places. At first sight it might seem that only a few of 

 the bowlders in this view are broken, but close inspection 

 shows that almost all are more or less affected. 



Such pressures as are shown by these bowlders are naturally 

 associated in one's mind with intense folding, but this relation- 

 ship is questionable. The bowlders shown in figure 1, where 

 the bed is tilted at an angle of TO degrees, are not so badly 

 fractured as are those shown in figure 2 from the north side of 

 the basin, where the dip is only 10 degrees. It must be stated, 

 however, that where the bed occupies the center of the basin, 

 as for instance at the place where the Dudleyville trail first 

 reaches Deer Creek, a short distance east of the mouth of Little 

 Rock Creek, the bowlders are not fractured, or at least fractures 

 were not observed. The explanation of the phenomenon prob- 

 ably lies in the peculiar conditions under which the bowlders 

 were held. If the matrix had been solid and homogeneous and 

 closely attached to the bowlders, it is probable that no such 

 fractures would have been produced, but under pressure the 

 soft turf acted much like fluid material and all of the strain 

 came upon the bowlders themselves. Not only that, but the 

 bowlders were held so loosely that there was opportunity for 

 the sides to give underpressure and consequently cracks were 

 produced and even the entire side of the bowlder was shoved 

 off to an appreciable extent. A glance at figure 2 makes it 

 plain that the large bowlder could not have been crushed from 

 the spot in front unless there was room for the side to expand 

 on what is now the upper part. If this bowlder had been sur- 

 rounded by a strong homogeneous cement no such fracture 

 could have been produced. 



The conclusion is, therefore, that such fracture can be pro- 

 duced only where the matrix is soft and flows under pressure, 

 leaving the bowlders to take the brunt of the strain, and afford- 

 ing them no lateral support to prevent them being crushed 

 under the strain. 



