Either Side of the Strand. Alysha Forrest, #4

When a routine mapping expedition uncovers what looks like ruins on the planet Theth-129, the crew of the Stardancer is thrilled to have something more exciting to do than catalogue worlds only a terraformer could love. Their trip to the world's surface confirms their wildest hopes: their ruins really were made by natives and abandoned long ago, a find of incredible significance. But there's more to this world than an obelisk, a set of painted caves, and a mysterious shoreline.
When the sea claims one of the 'Dancer's senior officers, their investigation reveals that the lost civilization might not be quite so lost after all. Alysha Forrest and the crew of the Stardancer have no idea that their way of life is on trial. But before it's over, they'll be asked to prove themselves, and the fate of an entire world is resting on the outcome.... This first full-length novel in the Alysha Forrest series stands on its own, or can be read as part of the series (beginning with Alysha's Fall).
It is part of the same universe (the Pelted) as Earthrise and Mindtouch.