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Radio echo soundings on Rusty Glacier, a small surge-type glacier in Yukon Territory reveal that the ice is considerably thicker than previously believed. A reinterpretation of deep ice-temperature measurements made in 1969 and 1970 suggests that a large zone of temperate basal ice exists. This result supports thermal instability as the surge mechanism for Rusty Glacier.

ABSTRACT. Radio echo soundings on Rusty Glacier, a small surge-type glacier in Yukon Territory, reveal that the ice is considerably thicker than previously believed. A reinterpretation of deep icetemperature measurements made in 1969 and 1970 suggests that a large zone of temperate basal ice exists. This result supports thermal instability as the surge mechanism for Rusty Glacier. REsuME. (Collins, 1972 ). Temperature models based on a gravity survey to determine ice thickness (Crossley and Clarke, 1970) and deep ice-temperature measurements (Classen and Clarke, 197 I , 1972 ) indicate a small zone of temperate basal ice. Radio soundings reveal that parts of the glacier are considerably thicker than previously believed. Basal ice temperatures calculated using this new information show that much of the bottom ice is at or near the pressure-melting point. This is consistent with the essential premise of thermal-instability surge theories: that surging glaciers are sub-polar glaciers with a basal temperature which oscillates near the melting point (Robin, 1955;Hoffmann and Clarke, 1973) . RADIO ECHO SOUNDINGSAttempts in 1968 and 1969 to measure the thickness of Rusty Glacier by the conventional seismic reflection method and by radio echo sounding with a 35 MHz Mark II Scott Polar Research Institute radar set (Evans and Smith, 1969) positioned on the glacier surface were unsuccessful. Since a typical thickness for Rusty Glacier is 100 m, the transmitted radio and seismic pulses completely obscured the reflections, preventing depth determinations. Owing to the proximity of the valley walls, it was not thought that airborne soundings at 35 MHz would be fruitful, although Evans and Robin (1966) successfully used this method over wide valley glaciers. By removing the transmitter from the glacier surface they were able to increase the time between the transmitted and reflected pulses and as a consequence were able to measure depths over a thin ice cover. Using a 620 MHz high-resolution radiosounding system developed for temperate glaciers (Goodman, 1970), depths as shallow as• Present address: Innovative Ventures Limited,

As part of a program to study surge-type glaciers, a radar-depth survey, using a frequency of 620 MHz, has been made of Trapridge Glacier, Yukon Territory. Soundings were taken at 26 locations on the glacier surface and a maximum ice thickness of 143 m was measured. A rapid change in surface slope in the lower ablation region marks the boundary between active and stagnant ice and is suggestive of an “ice dam” or the water “collection zone” postulated by Robin and Weertman for surging glaciers.