"The wrinkled sea" could be a wonderfully imaginative description of the peaks and valleys formed by the waves below the eagle's viewpoint as the tide "crawls" in and out along the shoreline. Particularly large wrinkles would be trimmed in foamy white as the sea crashes against itself or against the rocks on the shore.

Since the eagle is watching from a vantage point on "mountain walls," it is possible that the water is clear enough to see to the bottom of the sea in that area, so the wrinkles could be uneven rocks below the surface of the water. They might be broken up and jagged, might be folded into creases by the movement and formation of the earth's crust, or they may be worn into smooth contours by the action of the waves over many years.