The Monmouth Creek complex on the west side of the Squamish River mouth is a prominent and enigmatic edifice composed of basaltic andesite and dacite of unknown age. It may represent a group of dikes and lava domes that formed subglacially. At least four dikes protrude its summit. These form the ribs of high lava spines, the tallest being The Castle, which contains horizontal and radiating columnar joints. The spines are covered by welded breccia close to their bases and columnar jointing extends into the welded sequence. The most elevated lava flows and spines are composed of dacite.