Built between 1993 and 1998, the Robertson Tunnel runs for through the Tualatin Mountains. Located underground, it marks the deepest train station in North America. The tunnel displays a core that exhibits Boring Lava deposits. For the first of the tunnel, the core shows Boring lava flows with cinder, breccia, and loess dated from 1.47 million to 120,000 years ago, which have been deformed by the Sylvan fault. With the Oatfield fault, the Sylvan fault trends to the northwest, extending northwest and southeast of the tunnel. It is of Quaternary age and lacks surface expression, possibly as a result of its extensive burial by loess along its length.