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 growth 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 narrowed, 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 channels 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. 317]).
Fig. 317.—Sketch of a portion of the New Jersey coast. The dotted belt next the sea is the barrier, modified by the wind. The area marked by the diagonal lines is the mainland. In the marshy area between, there are numerous channels or “thorofares” kept open by the currents. The figures show the depths of water in feet. Scale about ⅜ inch = 1 mile.
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 ([Pl. XX] and [Fig. 2, Pl. XXII]). In such cases the sediment of the beach has been shifted but a short distance; but in other cases it has traveled far.
Fig. 318.—Map of shore-terraces, largely wave-built. Lake Bonneville. (Gilbert.)
Fig. 319.—A portion of the Texas coast showing the tendency of shore-deposition to simplify the coast-line. The deposits (the narrow necks of land parallel to the coast) shut in the bays. (From chart of C. and G. Surv.)
