These lavas overlie steeply dipping Silurian sedimentary rocks. These strata unconformably overlie Archean strata of the Superior craton and Proterozoic Richmond Group. The plateau consists of thick sandstone formations that overlie granitic basement rock. Siliceous limestones belonging to the Silurian overlie the Ordovician rocks. Both residual ice caps overlie thick layered deposits of interbedded ice and dust. Beneath the surficial deposits are lake-bottom silt and clay, which overlie till and shale bedrock. Major lava flows have formed vast plateaus that overlie Paleozoic sandstone. Beachrock units form under a thin cover of sediment and generally overlie unconsolidated sand. Permian and Triassic strata overlie these older rocks in the east. Miocene basalts overlie much of the plateau, creating relatively fertile red/brown soils. Unusually for such mixed-composition ignimbrites, the rhyolites overlie the more mafic deposits. Drift deposits overlie the north-western flanks. Plateau glaciers are glaciers that overlie a generally flat highland area. Farther east in the Bauland, the muschelkalk deposits still overlie the Early Triassic layers. Sections of oceanic crust which overlie continental crust are known as ophiolites.