SHORE DEPOSITS 187 



(I'l. XV, I iu f . i and Fig. 187). The same phenomena are to be 

 seen along many lakes (Fig. 193). Bars may tie islands to the 

 mainland i PI. XV, Fig. 2). If the bay across which the bar is built 

 reeeives abundant drainage from the land, the outflow from the 

 I >ay may be sufficient to prevent the completion of the bar, for 

 when the growth of the bar has narrowed the outlet of the 

 bay sufficiently, the sediment brought to the end of the spit by 

 tin- littoral current will be swept out by the current setting 

 out from the bay. The completion of a bar may be interfered with 

 also by tidal currents, even without land-drainage. The scour of 

 the tides preserves deep entrances (inlets) to bays in some places, 

 and maintains definite channels or "thorofares" in the lagoon 

 marshes behind barriers and spits (Fig. 191). The sediment brought 

 down from the land, as well as that washed in by tidal currents and 

 wavc-s, tends to fill up the lagoon behind a barrier, a spit, or a bar, 

 converting it into land (Fig. 191). 



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 also may join a beach or barrier. Traced 

 back to its source, the beach from which a spit leads is in many cases 

 found to end at the cliff from which the material of the beach and 

 spit were derived. 



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* A wave-built terrace borders 

 many wave-cut terraces along their seaward margins (Fig. 181). 

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

 opment of continental shelves. 



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

 races, are preserved for a time after the relative levels of sea and 

 land have changed. If the shore has risen, relatively or absolutely, 

 these features are evidence of the change. If shore features are 

 submerged instead of elevated, they furnish less accessible though 

 not less real evidence of the change. Similar features about lakes 

 have a like significance, but there it is demonstrable, in many 

 cases, that it is the water rather than the land which has changed 

 level. 



Shore-deposition and coastal configuration. The tendency of 

 shore-deposition is to cut off bays and to straighten and simplify 

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



