[Illustration: Fig. 136. Chasms worn by Waves, Coast of Scotland] =Stacks and wave-cut islands.= As the sea drives its tunnels and open drifts into the cliff, it breaks through behind the intervening portions and leaves them isolated as stacks, much as monuments are detached from inland escarpments by the weather; and as the sea cliff retreats, these remnant masses may be left behind as rocky islets. Thus the rock bench is often set with stacks, islets in all stages of destruction, and sunken reefs,--all wrecks of the land testifying to its retreat before the incessant attack of the waves. [Illustration: Fig. 137. A Stack, Scotland] [Illustration: Fig. 138. Wave-Cut Islands, Scotland How far did the land once extend?] =Coves.= Where zones of soft or closely jointed rock outcrop along a shore, or where minor water courses conic down to the sea and aid in erosion, the shore is worn back in curved reëntrants called coves; while the more resistant rocks on either hand are left projecting as headlands (Fig. 139). After coves are cut back a short distance by the waves, the headlands come to protect them, as with breakwaters, and prevent their indefinite retreat. The shore takes a curve of equilibrium, along which the hard rock of the exposed headland and the weak rock of the protected cove wear back at an equal rate. [Illustration: Fig. 139. Coves formed in Softer Strata _S_, _S_; while the Harder Strata _H_, _H_, are left as Headlands] =Rate of recession.= The rate at which a shore recedes depends on several factors. In soft or incoherent rocks exposed to violent storms the retreat is so rapid as to be easily measured. The coast of Yorkshire, England, whose cliffs are cut in glacial drift, loses seven feet a year on the average, and since the Norman conquest a strip a mile wide, with farmsteads and villages and historic seaports, has been devoured by the sea. The sandy south shore of Martha's Vineyard wears back three feet a year. But hard rocks retreat so slowly that their recession has seldom been measured by the records of history. [Illustration: Fig. 140. A Pebble Beach, Cape Ann, Massachusetts] Shore Drift