650 



PRINCIPLES OF STRATIGRAPHY 



beaches of cobbles or shingles are chiefly characteristic of steeply 

 sloping and exposed rocky shores, where the finer product of ero- 

 sion is carried by the undertow to deeper water, the coarser alone 

 remaining. The character of the pebbles will of course partake of 

 that of the cliff from which they are formed, those derived from 

 the more easily shattered rock, as well as the most difficult to grind 

 to powder, predominating. Thus on the north shore of Massachu- 

 setts felsite pebbles predominate, on Lake Michigan limestone peb- 

 bles, and on the shore of Lake Erie flat shale pebbles. These peb- 

 bles are often piled up into extensive terraces, especially after heavy 

 storms. These terraces may show on section a rude and irregular 

 bedding, but the regular cross-bedding (torrential type) found in 

 many old conglomerates was not formed in this manner. 



Fig. 129. Diagram illustrating the deposits in the littoral district of the sea. 

 At the shore gravelly and sandy facies occur, these shading off 

 seaward into lutaceous, and, finally, calcareous sediments. The 

 latter are derived from the coral reef C. R., /. c, they are thalassi- 

 genous, while the others are terrigenous. The overlapping of 

 the formations is also shown. 



Organic remains in pebble beds. On the beach, where the peb- 

 bles undergo much movement, most organic remains, such as shells, 

 bones, etc., are rapidly destroyed. Nevertheless, they are occa- 

 sionally preserved, as is shown by the occurrence of worn shell 

 fragments even in the high gravel terraces. 



That organic remains are common in older boulder and pebble 

 beds has been the experience of many geologists, though some of 

 the so-called conglomerate beds are due to causes other than those 

 active on sea beaches. A comparatively modern example of a pebble 

 and sand beach now abandoned by a slight elevation of the coast 

 is found in eastern Scotland. Near Goldspie on the Moray Firth, 

 the lower of the elevated beach lines abounds in entire shells of 

 Acmaea and other shore forms, as well as in worn fragments of 

 this and other shells. Boulders up to the size of six inches or more, 



