The Reynolds Creek Gorge has been blocked at the southern end by Moogerah Dam a mass concrete double curvature arch dam with a un-gated spillway across the Creek.
Moogerah Peaks National Park was once beneath the belly of a volcano—the ancient Main Range volcano—which erupted some 24 million years ago. The eastern flank of this volcano once spread across the Fassifern Valley, probably as far as Mount Maroon, Boonah and even to Ipswich, while on the western side it spread to at least Warwick. It erupted mainly basalt lavas, which may have been as thick as 1000m near the volcano's crest.
The distinct peaks of the Moogerah Peaks National Park had their origins deep below the volcano. Composed of different rock types, separated from basalt magma at great depths, they formed as plugs, dykes or sills when magma entered numerous cracks and weaknesses in underlying older rocks, as well as moving up the main vents.
Prolonged erosion over 20 million years has removed mainly the eastern side of the volcano. The relatively steep slopes and greater power of the east-flowing streams rapidly eroded the volcanic lavas to create a retreating escarpment (long, cliff-like ridge), which is now the edge of today's Main Range. As the volcano and some of the underlying rocks were eroded away, any plugs, dykes or sills composed of resistant rocks—particularly those of rhyolite and trachyte—remained as steep peaks.
Mount Edwards is a large trachyte plug, which formed when magma filled vertical pipe-like fissures. The plug is cut by Reynolds Creek to form the mountain's distinctive two peaks ( Mt Edwards and Little Mt Edwards).