 Subsequent to the Miocene lava flows, the volcanoes of the Cascade Mountains actively erupted, depositing ash, cinders, pumice and mudflows that eventually inter-fingered with the alternating basalt layers throughout the region.Interstate 90 drops through the Thorp Drift, which marks the oldest and furthest reaching known glacial moraine in the Kittitas Valley.Changes in the types of vegetation become more evident in this area.