358 GEOLOGY. 



its surface approaches the level of effective agitation, the waves may 

 begin to work on it, as on a barrier, and may build it up to, and even 

 above, the surface of the water. So long as the end of such an embank- 

 ment is free, it is a spit (Fig. 315 and PL XXI). If the spit be lengthened 

 until it crosses, or nearly crosses, the bay, shutting it off from the open 

 water, it becomes a bar. Bars 

 have shut in lakes (ponds) on the 

 coast of Martha's Vineyard, Mass. 

 (Fig. 1, PL XXII), and lakes and 



lagoons at numerous points both 



., A XT X* J xi, T) •^ Fig. 316. — Cross-section of a bar. 



on the Atlantic and the racanc (Gilbert) 



coasts (Fig. 2, PL XXII, Rodeo 



lagoon). The same phenomena are to be seen along many lake 



shores. Bars sometimes tie islands to the mainland (PL XXIII, Fig. 



1, Nahant, Mass.; Fig. 2, near Biddeford, Me.). The structure of a bar 



as seen in cross-section is shown in Fig. 316. 



The construction of a spit has been aptly compared to the construc- 

 tion of a railway embankment across a depression. The material is 

 first carried out from the bordering upland (shallow water) and dumped 

 where the slope to the depression (deep water) begins. The embank- 

 ment thus begun is extended by the carrying out of new material, which 

 is left at the end of the dump already made. 



If the bay across which the bar is built receives abundant drainage 

 from the land, the outflow from the bay may be sufficient to prevent 

 the completion of the bar (Fig. 2, PL XXII), for when the gro\\i;h of the 

 spit has sufficiently narrowed the outlet of the bay, the sediment brought 

 to the end of the spit by the littoral current will be swept out beyond 

 the spit by the current setting out from the bay. 



The completion of a bar may be interfered with by tidal currents, 

 even without land-drainage. Currents generated by the tides may 

 sweep in or out of the bay with increased force as the entrance is nar- 

 rowed, carrying in or out the sediment which the littoral current 

 would have left at the end of the spit. The scour of the tides often 

 insures deep entrances (inlets) to bays, and maintains definite chan- 

 nels or ' ' thorof ares " in the lagoon marshes behind barriers and spits. 

 The sediment brought down from the land, as well as that washed in 

 by tidal currents and waves, tends to hll up the lagoon behind n 

 barrier, a spit, or a bar, converting it into land (Fig. 317). 



