Hill: Penobscot Vegetation 367 



with a profusion of large boulders, formerly present in the 

 deposits, but which were too heavy to be transported away 

 from their source. These are usually covered with a dense 

 growth of rockweeds as is usual on rocky bottoms. 



b. Association-complexes of depositing areas along 

 the coast 

 Association-types of shingle beaches 

 Shingle beaches, composed of characteristic rounded 

 wave-worn rocks of all sizes are of frequent occurrence in 

 the region. They are usually found on the more exposed 

 shores and in most instances are barrier beaches, cutting off 

 small ponds or swamps from the ocean. On these beaches, 

 as on sandy and gravelly ones as well, three more or less 

 distinct zones are discernible. These comprise the lower 

 beach, which lies between low and high tide marks and con- 

 sequently is subjected to alternate submergence and ex- 

 posure; the middle beach, which extends from the limit of 

 summer tides to that of the winter storms; and finally the 

 upper beach, which ordinarily is not submerged at any pe- 

 riod of the year. 



The loiccr beach is usually steep and absolutely devoid of 

 vegetation. The rocks of which it is composed are con- 

 stantly being rolled back and forth by the waves and this 

 incessant motion naturally prohibits any plant growth 

 (Figure 19). At its lower limit this beach may tend to be- 

 come sandy or gravelly. 



The middle beach is in general similar to the preceding in 

 structure. The rocks however are not in motion during the 

 growing season, since it is free from waves in the summer, 

 and consequently some vegetation is able to develop. The 

 most conspicuous plant on such a beach is the sea lungwort 

 (Mertensia maritima) growing in great prostrate clumps, 

 while the beach pea (Lathyrus lnaritimus) and the wild 

 morning glory (Convolvulus sepium var. pubescens) are 

 also common. 



