Discontinuities in a Transverse Velocity Profile of a Valley Glacier Measured by Photogrammetry

Abstract

The surface movement of part of a Nanga Parbat (Himalaya) glacier immediately below an ice fall has been investigated by terrestrial stereophotogrammetry. This method provides an opportunity of measuring many more points than by using other procedures. The shape of the transverse velocity profile indicated normal stream-like flow and not “blockschollen movement”, as might have been expected, because of its high velocity (45 cm/d). But, one feature of the ice fall above had been carried down-glacier to the location of the profile; namely, several discontinuities along the velocity diagram. The ice fall had transmitted a small part of its splitting into seracs to the flatter area below, but not its blockschollen movement. There seems to be a contradiction in that on one side there are discontinuities, i.e. the glacier is torn up into blocks by shear planes, but on the other side the glacier moves en bloc. In fact, a short-term investigation over hours or just a few days discloses cracks but, after time periods of a week or more, these irregularities compensate for one another so that they merge into a single unit block motion.