 The emplacement of these lava flows spanned at least three periods of glaciation, evidence for which is preserved in the form of tuyas, ice-ponded valley deposits, and subglacial mounds.The few tuyas in the region, such as Gage Hill, Hyalo Ridge, McLeod Hill and Mosquito Mound, were formed when magma intruded into and melted a vertical pipe in the overlying glacial ice.The partially molten mass cooled as a large block, with gravity flattening its upper surface.