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On the Maine coast Carson visits a small pool inside a low sea cave. She can reach it only at the lowest spring tides of the year, and the returning sea closes it off again within minutes. She uses this brief, rarely seen scene as her image of the shore: a world that shows itself only at the extreme of the tide.
On rocky shores, life sorts into horizontal bands according to how long each level sits exposed to air between tides. From top to bottom the bands are black lichens and blue-green algae, then barnacles and periwinkles, then rockweeds, then Irish moss, and finally kelps at the lowest tide line. Each band's upper edge is set by how much drying, heat and cold its organisms can tolerate. Its lower edge is often set by competition and predators from below.
Read a shore by its ground first, because the substrate decides what kind of life can exist there. On wave-battered rock the rewarded strategy is to hold on, through holdfasts, cement, byssus threads and suction. On shifting sand, where nothing can be anchored, the rewarded strategy is to dig, so life goes underground in burrows and tubes and is almost invisible from the surface.
On the coral and mangrove coasts of southern Florida, the shore is made by organisms rather than by rock or sand from elsewhere. Coral animals and algae secrete limestone that becomes the reefs and the Keys. The arching prop roots of red mangroves trap drifting sediment and debris until mud builds up into new land that pushes seaward. Here the coast is a living construction that keeps extending itself.