390 Proceedings Portland Society Natural History 



formed by these species in such areas are so dense that the 

 popple and birch do not become established and con fers are 

 able to succeed the brambles directly. 



C. PRIMARY FORMATIONS OF THE HYDRARCH SERIES 



The formations of the hydrarch series include lakes and 

 ponds, which differ from one another only in the matter of 

 size; and swamps of several different kinds. These two 

 groups are not absolutely distinct and usually show inter- 

 gradations, the lakes and ponds in general tending to be- 

 come filled up and converted into swamps through the action 

 of various agencies. 



As already indicated (page 308), the lakes and ponds of 

 the Penobscot Bay region are for the most part of glacial 

 origin, occurring either as morainal lakes which are con- 

 nected in some manner with glacial debris, or as scoop lakes 

 which occupy depressions scooped out of the undei'lying 

 rock itself. Other ponds are coastal in nature, occurring 

 behind barrier beaches, while a fourth group has resulted 

 from the cutting off of indentations along the shore of a 

 larger lake by the formation of a bar or sand spit. 



The swamps may originate through the filling in of a lake 

 or pond, in which case they are known as lake-swamps ; or 

 they may result from the seepage of perennial springs in 

 locations where the topography prohibits pond formation. 

 These latter arc known as spring-swamps. A third type — 

 the precipitation swamp — which is common in more north- 

 ern regions and depends on direct atmospheric precipitation 

 for its water supply, is practically absent from the Penob- 

 scot Bay region. Coastal swamps, however, developed by 

 the deposition of sediment in shallow places along the shore 

 are of frequent occurrence. 



1. Formations of Lakes, Ponds, and Swamps Inland 



The nature of the vegetation in lakes, ponds and swamps 

 inland depends on the character of the drainage and the 



