Essentially Nantucket is all beach -- a boomerang-shape sand spit consisting of detritus left by a glacier that receded millennia ago. Off Cape Cod, some 26 mi out to sea, the island measures 3 1/2 by 14 mi at its widest points, while encompassing -- such are the miracles of inlets and bays -- about 80 mi of sandy shoreline, all of it open, as a matter of local pride, to absolutely everyone.