12 



A NATURALIST IN THE GREAT LAKES REGION 



might when it fragments the cement sidewalk or tears the 

 plaster from the exterior of a house. But it is quite as powerful 

 when it operates on the rock of a cliff or the disintegrating 

 bowlder. At the base of a rock cliff is often found a pile of rock 

 debris, the talus, the accumulation of the incessant disintegra- 

 tion of the rock above (Fig. 12). 



The moisture-laden atmosphere is always busy producing 

 destructive changes on the exposed rocks. Crack open any 



/ .,^«j 

 ^<? 







m 



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-ii_ 





^ -<:''-! 

 "'??'*■ 



. « 



Fig. 12. — A talus at foot of clifif and outwash from the hills 



field bowlder and you will see a layer of weathered material 

 on its outer portion that crumbles readily. The rock piles, 

 excavated in mining or quarrying, show nicely the results of 

 this weathering. The great angular blocks that were dumped a 

 decade or more ago are crumbling into heaps of rock fragments, 

 many of them sufficiently disintegrated to make coarse soil 

 where weeds and some trees are beginning to root. This process 

 of the transformation of blocks of rock into soil may readily be 

 seen on the dumps of the local quarries or along the line of the 



