During the Pleistocene period dating back over two million years ago, glaciation advancing and retreating repeatedly scoured and shaped the landscape. Glaciation was most prevalent approximately 18,000 years ago, and most valleys were ice-free by 12,000 years ago. 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. Subduction and tectonic activity in the area began during the late cretaceous period, about million years ago. Extensive volcanic activity began to take place in the oligocene, about million years ago. Glacier Peak, a stratovolcano that is southwest of Cloudy Peak, began forming in the mid-Pleistocene.