small fragments of the oceanic and continental lithosphere called terranes created the North Cascades about 50 million years ago. During the Pleistocene period dating back over two million years ago, glaciation advancing and retreating repeatedly scoured the landscape leaving deposits of rock debris. The "U"-shaped cross section of the river valleys are a result of recent glaciation. Uplift and faulting in combination with glaciation have been the dominant processes which have created the tall peaks and deep valleys of the North Cascades area. See also Geography of the North Cascades Geology of the Pacific Northwest References External links North Cascades National Park National Park Service Mantis Peak weather: Mountain Forecast Mantis Peak/Distal Phalanx (photo): Flickr Mountains of Washington (state) North Cascades National Park Mountains of Skagit County, Washington North Cascades Cascade Range North American 2000 m summits