FORM OF NANTASKET BEACH 165 



tombolos are described, and Nantasket Beach, which might be 

 described as a complex tombolo, is briefly mentioned. 



In common with all students of shoreline topography, we are 

 indebted to Dr. G. K. Gilbert's classic studies of lake shores for 

 the elucidation of many of the principles upon which all shoreline 

 studies must be based. 



A brief note on "The Geology of the Nantasket Area," contain- 

 ing an outline of the physiography of the district, was published by 

 Professor D. W. Johnson in Science three years ago. 



THE PRINCIPLES OF SHORELINE DEVELOPMENT 



Physiographers recognize two distinct classes of shorelines — 

 those formed by a (relative) elevation of the land, called shorelines of 

 elevation; and those formed by a (relative) depression of the land, 

 called shorelines of depression. It is not necessary to repeat the 

 characteristic features of these two classes of shorelines, nor to trace 

 the successive stages by which young shorelines of each class acquire, 

 by the time they reach maturity, curves of a relatively simple pattern, 

 marine cliffs more or less bold, and shelving beaches at the foot of the 

 cliffs. Initial characteristics and stages of development are both set 

 forth in our best textbooks on physiography. 



We may note, however, that the processes of shoreline develop- 

 ment involve both wave and current action. It has been shown that 

 wave action is largely confined to the erosion of the land margins, to 

 the transportation of the eroded material a short distance from the 

 shoreline, and to the deposition of the material in the deeper 

 water; and to the heaping-up of sand, gravel, and cobbles into long 

 ridges or beaches, where the conditions favor wave building more 

 than wave erosion. Current action, on the other hand, effects but 

 little erosion, and is mainly effective in the 'longshore transportation 

 of material previously eroded by the waves or brought in by rivers. 

 It has been shown that the combined effects of these two processes is 

 to produce, in time, a shoreline characterized by long, simple curves, 

 and free from sharp angles or other irregularities. Headlands are 

 cut back, or retrograded, and re-entrants are built forward, or pro- 

 graded, in the attempt made by waves and currents to straighten 

 out the initial irregularities of the shores, and thus to establish a 



