220 



PRINCIPLES OF STRATIGRAPHY 



differential decrease in velocity. Where the shore projects into 

 headlands or points, the energy from a considerable crest line is 

 concentrated, resulting in intensive destructional work. In bays, 

 on the other hand, the energy of a small portion of the crest line is 

 stretched, as it were, over a broad area, and hence becomes rela- 

 tively weak. This is shown in Fig. 34, copied from Davis, where 

 the energy of the crest line D F is concentrated on d f , whereas 

 at A C only the energy of the short portion of the crest lying 

 between a c is felt. The bay head thus becomes a place of relative 

 safety for vessels and a region of slight erosion. 





Fig. 34. Map showing concentration of energy from a considerable crest 

 line upon a narrow headland and its diffusion in the bay or 

 harbor. (After Davis.) 



An on-shore wind produces a shoreward "drift" of the water 

 aside from the movement due to waves. The "undertow" is the 

 return current flowing outward at the bottom. This is, of course, 

 greatly increased during strong wave action. When the wave 

 strikes the shore obliquely, a movement parallel to the shore is inau- 

 gurated, and this is the longshore or littoral current, the chief 

 agent in the transportation of material along the shore. 



Destructive Work of the Waves. Wherever the waves beat 

 upon a shore, erosional work of some kind is going on. If the 

 shore is sandy, the waves may only stir up the sand, which will then 

 be carried seaward by the undertow, or along the coast by the 



