156 PHYSIOGRAPHY 



the bay, but tends rather to cross it in the direction in which it was 

 previously moving. Under such circumstances it may build an 

 embankment or spit of gravel and sand across the bay. Currents 



Fig. 159. Map of the head of Lake Superior. (U. S. Geol. Surv.) 



do not build spits above the water, but waves may build them up 

 into land by washing material from their slopes up to their tops 

 (Fig. 158). After they become land, the wind may build dunes 

 upon them (p. 21). When spits cross bays they become bars 

 (Fig. 159). 



Reefs and spits and the land to which they give rise often in- 

 crease the irregularity of the coast-line greatly for a time; but they 

 represent the first step toward regularity, for after the reefs have 

 become land, the lagoons behind them are likely to be filled with 

 sediment washed down from the land or blown in by the wind 

 (Fig. 160). When the lagoon is filled, the shore line is much more 

 regular than before ; but the first effect of the making of the reef- 

 land is to make the coast more irregular. 



Deposits of gravel and sand are sometimes made between a 

 mainland and islands near it. Nahant Island, on the coast of 

 Massachusetts, and the Rock of Gibraltar, on the coast of Spain, 

 have been thus " tied" to the mainland (Fig. 2, PL XXXIX, p. 140). 



