The Oregon Coast between Yachats and Newport is a narrow, slightly elevated coastal plain. With the exceptions of basalt rock at Yachats and Seal Rock, the bedrock along this segment of the coast is sedimentary. Several Pleistocene marine terrace levels are discernible at places along the plain, and sand dunes, both active and stabilized, impart a rolling topography to most of it. Numerous streams have incised small valleys and ravines into its surface; at Waldport the plain is interrupted by the estuary of the Alsea River and at Ona Beach State Park by the alluvial plain of Beaver Creek. Except in the two localities where basalt is exposed, the shore is marked by long stretches of sandy beaches bounded by low sea cliffs. Where the basalt is exposed, the shore has the rugged features that are characteristic of Oregon shores bounded by this type of rock.