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). [Illustration: +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. [Illustration: +Fig.+ 318.—Map of shore-terraces, largely wave-built. Lake Bonneville. (Gilbert.)] [Illustration: +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.)] [Illustration: +Fig.+ 320.—Map showing that in the early stages of the simplification of a shore-line the irregularities are increased. The numbers indicate the depth of water in fathoms. (From chart of C. and G. Surv.)] The spit is usually either straight or in conformity with the general course of the shore-current, but since the littoral current itself is subject to alteration as the result of shifting winds, the spit may depart from straightness. Winds which simply reverse the direction of the littoral current retard its construction, but may not otherwise affect it; but if a strong current be made to flow past the end of a spit, it may cut away its extremity and rebuild the materials into a smaller spit, joining the main one at an angle. This gives rise to a _hook_ (Fig. 315). Successive storms may develop successive hooks along the side of a growing spit. The end of a hook may be so extended as to join the mainland, when it becomes a _loop_.