IP s t c Is ia 297 



The sandstone pools present the greatest 

 variety of structure,- and it is to be expected 

 that the greatest variety of causes also exists. 

 Here, as in the shales, the water striking 

 against a line of weakness, causes erosion, and 

 as sandstone is softer than shale, erosion occurs 

 on a larger scale. This eroded surface may 

 be the beginning of the pool. The action is 

 continued by the water, by the bits of broken 

 rock, and by the agency of plants and animals. 

 The roots of plants strike into the rock, and 

 both by the spreading of the cracks, caused by 

 the increase of the size of the holdfast, and by 

 the dissolving action of the acid formed by the 

 plant, the depression increases in size. 



Similarly, boring animals, as the rock clam, 

 and the sea-urchin, serve to increase the size 

 of a depression made by erosion or solution, 

 or, they may originate the depression by boring 

 into solid rock. A steep sandstone cliff, just 

 west of "The Point," shows the face of the cliff 

 honeycombed into such depressions. 



A rather insignificant cause for the forma- 

 tion of tidepools is the change of temperature 



