Bivachny Glacier, Tajikistan, from space. Astronaut photograph of the Bivachny Glacier, Tajikistan, showing evidence of surging as it meets MGU glacier. Most glaciers flow at a fairly steady rate, but surging glaciers lurch forward in bursts, in some cases flowing 10 to 100 times faster than normal for short episodes. Though it varies by individual glacier, surges usually last less than a year and occur every 15 to 100 years. Surging glaciers typically show distinctive curved lines called looped medial moraines (ridges of debris), on their surface. Non-surging glaciers typically contain straight medial moraines. The loops of surging glaciers are caused by the alternation between surging and non-surging behaviour compressing and stretching the moraines.