A TOPOGRAPHIC ORIGIN FOR DOUBLE-RIDGE FEATURES IN VISIBLE IMAGERY OF ICE DIVIDES IN ANTARCTICA

The appearance of double-ridge features on visible imagery of the ice divides of Antarctic ice rises has often been noted but, largely due t o a lack of adequate ground truth, their origins have remained enigmat ic. We present several examples of ice rises and other isolated ice-fl ow centres that apparently show double ridges. We investigate one of t hese in particular: Fletcher Promontory, Antarctica. A digital-elevati on model (DEM) of the summit region is derived from surface profiles o btained using the Global Positioning System (GPS) and this is correlat ed with Landsat MSS satellite imagery. Precise registration is achieve d by correlating image brightness values with surface slope calculated along the direction of the Sun azimuth in the image. Using a simple b i-directional relation, the DEM data are used to model the Landsat ima ge. We therefore demonstrate that the double ridge is a product of a s ubtle concavity parallel to the ridge and is unlikely to be dependent on other factors. This concavity is not predicted by steady state mode ls of ice divides and so we suggest that the ridge may not be in a ste ady-state but responding to changes in the glaciological boundary cond itions. We speculate that this may be an indication of ongoing migrati on of the ice divide.