314 GEOLOGY 



even without land-drainage. The scour of the tides often pre- 

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

 nels or "thorofares" 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 fill up the lagoon 

 behind a barrier, a spit, or a bar, converting it into land (Fig. 258). 



Since spits and bars are built only where there is shore-drift in 

 transit, they are always built out from a beach or barrier. The 

 distal end of the bar may also join a beach or barrier. Traced 

 back to its source, the beach from which a spit leads out is often 

 found to terminate in the cliff from which the material of the beach 

 and the spit were derived. In such cases the sediment of the beach 

 has been shifted but a short distance; but in other cases it has 

 traveled far. 



The off-shore movements of shore-waters may leave the sediment 

 of the shore in the form of a wave-built terrace, which is really a 

 seaward extension of the beach. The wave-built terrace often 

 borders the wave-cut terrace along its seaward margin (Fig. 247). 

 Terrace-cutting and terrace-building are both involved in the devel- 

 opment of the continental shelves. 



Beach ridges, spits, bars, etc., like sea-cliffs and wave-cut 

 terraces, are often preserved after the relative level of sea and land 

 has changed. If the shore has risen, relatively or absolutely, these 

 features are evidence of the change. If shore features are sub- 

 merged instead of elevated, they furnish less accessible though not 

 less real evidence of the change of level. Similar features 

 about lakes have a like significance, but in this case it is often 

 demonstrable that it is the water rather than the land which has 

 changed its level. 



Effect of shore-deposition on coastal configuration. The ten- 

 dency of shore-deposition is to cut off bays and to straighten and 

 simplify the shore-lines. This is abundantly illustrated along the 

 Atlantic and Gulf coasts of the United States (Figs. 240, 254, and 

 PI. XX). It is to be noted, however, that in the simplification of 

 the shore-line through deposition, the initial stages often result 

 in great irregularity (Fig. 254 and PL XIX, Fig. 2). 



1 For map work on topographic features of shores, see end of Chapter. 



