 Most of Mt Imlay National Park was formed during the Ordovician Period, 500 to 435 Million years ago, from sedimentary and metamorphosed rocks of the Mallacoota Beds, part of the Southern Highlands Fold Belt, including greywacke, sandstone and shale.The summit of Mt Imlay and the upper slopes are younger, with Devonian (395 to 345 Million years ago) rocks of the Merimbula Group, lying above the Ordovician sediments.The Merimbula Group includes sandstone, conglomerates, quartzite, siltstone and shale.