The Peters and the Muldrow Glaciers on the north side of Mt. McKinley are known to be surge-type glaciers, which means they get up and go suddenly after years of slugging along like other glaciers. The Muldrow Glacier last surged in 1956 and the Peters Glacier surged in 1986. These surges are known to last 1 to 2 years and may result in an advance of a glacier's terminus by several miles. During the most recent surge of Peters Glacier, the terminus advanced nearly 3 miles from July to the following May. The glacier may have moved 150 feet each day during part of the surge. Surging glaciers are one of the unsolved mysteries of glaciology. Many surge-type glaciers in Alaska, lie along or adjacent to the 500 mile-long Denali Fault. The Muldrow and Peters Glaciers are directly atop portions of this prominent break in the earth's crust, which arcs across Alaska and crosses the northern slopes of Mt. McKinley. Most recently, in 2000, surges occurred on the Tokositna Glacier within the park and the nearby Yanert Glacier. Today there is still visible evidence of the recent surges on these glaciers. Their formerly smooth surfaces are now slashed with huge crevasses, and the upper portions of the valleys are marked by a "bathtub" type ring caused by the sudden drop of the glacier's surface as the ice surged out below. Today if you stood on the upper part of the Yanert Glacier you would be 300 feet lower than you would have been prior to its surge!