AN ILLUSTRATION OF THE WEDGE-WORK OF ROOTS 



A. O. THOMAS 



The wedge-work of roots is a powerful and effective agent of 

 weathering and performs an important part in breaking up soHd 

 rock. Since the indurated rocks, as a rule, are covered by mantle 

 rock of considerable thickness the actual penetration of their seams, 

 joints, and crevices by roots is concealed. In some cases, too, the 

 mantle rock is of so great thickness that even the longest roots fail 

 to reach the solid rock below. On bare rocky cliffs where there is 

 sufficient moisture, trees, in some cases of large size, grow from 

 crevices and joints where their seeds have sprouted. As they grow 

 their roots penetrate the openings and wedge their sides apart some- 

 times prying off unstable rock masses and toppling them over — the 

 final overthrow occurring oftentimes during a windstorm which 

 may blow down the trees. 



Not only does the wedge-work of roots shove the rocks apart but 

 the enlarged crevices, becoming the repositories for soil and debris 

 blown in by the wind or washed in by the rain, are further widened 

 when the water and soil thus admitted freeze. Furthermore, when 

 the tree eventually dies the decaying roots form passageways which 

 water readily enters and assisted by the acids of decay may further 

 disintegrate the rock by solution. Even the living roots secrete 

 acids which have a very great effect on the undecomposed minerals 

 or rocky materials. 



The wedge-work of roots as a mechanical action is not of the 

 same strength in all plants though all roots possess the power to 

 some degree. In some of the tropical and subtropical parts of the 

 world man makes use of the prickly pear cactus in preparing soil 

 on stony land. The roots of this plant have a remarkable power of 

 penetrating and wedging rock apart and especially in converting 

 fresh lava rock in this manner into soil. Ordinary weathering re- 

 quires a century to accomplish on such lava what the cactus can do 

 in a few years. ^ Everyone is familiar with the wedging power of 

 roots in heaving walks, in cracking walls of masonry, and in break- 

 ing, entering, and even clogging sewer and drain tile. This force. 



iMcConnell, P., Elements of Agricultural Geology, London, 1902, page 48. 



