Thomson and his colleagues analyzed sediments drilled from the ocean floor just offshore of Lambert Glacier, as well as from onshore moraines, the rock piles pushed up by glaciers. Tests on minerals in the sands and muds helped them figure out when and how fast the surface eroded.
Here's what the sediments say: From about 250 million to 34 million years ago, the region around Lambert Glacier was relatively flat, and drained by slow-moving rivers, Thomson said. About 34 million years ago, which coincides with a cooling of Earth's climate, big glaciers appeared, shaping the spectacular valley now hidden under thick ice.
"It seemed like it occurred very early on, 34 [million] to 24 million years ago," Thomson said. Erosion slowed dramatically as the ice sheet stabilized about 15 million years ago, he said.
Some 5,250 to 8,200 feet (1.6 to 2.5 kilometers) of rock have since disappeared, ground down by glaciers and carried away by the ice, according to the study.
"Glaciers can carve deep valleys quickly — and did so on Antarctica before it got so cold that the most of it got covered by 1 or 2 miles [1.6 to 3.2 km] of thick, stationary ice," Peter Reiners, a UA geologist and study co-author, said in a statement.
Clues to buried mountain range
Lambert Graben extends about 375 miles (600 km) inland, ending at one of Antarctica's most enigmatic features — an entombed mountain range called the Gamburtsev Mountains. Buried under the ice, the mountains rose during Gondwana's rifting. Geologic evidence suggests two pulses of upliftfrom rifting events about 250 million years ago and 100 million years ago pushed up the jagged peaks.
But Thomson and his colleagues did not find evidence in the sediments for a second uplift phase 100 million years ago. The river sands contain minerals from the Gamburtsev Mountains, and the tiny grains suggest the mountains got their height with one tectonic push.
"This underscores both the mountain range's remarkable age and the extraordinary degree of subglacial landscape preservation," writes Darrel Swift in an accompanying article in Nature Geoscience. Swift, a geologist at the University of Sheffield in the United Kingdom, was not involved in the study.
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