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Deceptively similar to a lava flow, the grey mass flowing at the base of Sourdough Peak near McCarthy, Alaska, is actually a glacier blanketed by rocky debris. Jack Holt and his team at the Lunar and Planetary Laboratory have spent years hiking and skiing (Photo: Eric Petersen/HiRISE) Deceptively similar to a lava flow, the grey mass flowing at the base of Sourdough Peak near McCarthy, Alaska, is actually a glacier blanketed by rocky debris. Jack Holt and his team at the Lunar and Planetary Laboratory have spent years hiking and skiing across its surface to survey it with ground-penetrating radar and other techniques.