In drift of pine needles, sneakers dry by the fire; harvested shells line window sills; the child, stroking a tiny sandpiper skull, ''I found this. It's mine. I'll take it home.'' Early the last morning, the car is packed: suitcases, fishing poles, cattails and bayberry (''No room for bayberry - '' ... ''I'll hold it on my lap'') slowly, slowly - the lopsided footbridge slippery. Sunrise checkers the woods. Suspended from juniper the wisp webs of spider. Gulls snatch breakfast scraps thrown from the rocks. The door (open all summer to sea) is closed; the grandmother wheedles the rusted lock. ''We'll shore up the cabin ... next year.'' Along the path, the child takes her hand. ''We'll all be back next summer - won't we?'' ''Before you know it, we'll be back.'' Hands warmly clasped, ''Mind the bridge now....''