Kathree’s eye went to a detail she had noticed a minute ago: an unnatural-looking deposit of flat black rocks half buried in wave-driven sand. As the remainder of the party made their final descent to the water’s edge, she used a piece of driftwood to scrape sand and gravel away from these until she could worry one loose. Though it was rough around the edges, it had clearly been shaped by humans: a slab of black rock about as thick as her finger, big enough to hold in the crook of an arm, smooth enough to write on. Scattered in the muck around it she’d seen lumps of calcium carbonate: chalk. Traces of it were still visible on the slates. Not writing but a fragment of a diagram, a map perhaps, and a few numbers.