Glacial moraines product of last Ice Age

The moraines of Wallowa Lake last saw glacial action as recently as 17,000 years ago.

It is well to remember that winter once was a constant here, when glaciers last reigned. Only 17,000 years ago, Wallowa Lake and the area between the east and west moraines were full of moving, groaning, gritty glacial ice. That was the most recent of many times during the last million years that glaciers advanced from their alpine fortress.

The geologically young date of 17,000 years comes from boulders atop the innermost east and west moraines. Joe Liccardi of the University of New Hampshire devoted the summer of 2002 sampling the largest granodiorite erratics (or, what most of us see as "granite boulders") atop the moraines. He chose stones with the least lichen. He looked at surfaces exposed flat-side up, to the sky. He was looking for rocks that had become, in a geologic way, "sunburned." Liccardi wanted to identify and sample rocks that were last moved by glacial ice, and had remained in the same position, exposed to sun and cosmic rays, as the ice began its most recent retreat.

When rocks are exposed to high-energy cosmic rays, the rock surface begins a slow chemical reaction. The element silicon changes slowly and steadily to the element beryllium. Specifically, it transforms into type, or isotope, of beryllium called Be10. The longer the surface of rock is exposed to cosmic rays, the more Be10 is produced. By measuring the amount of Be10 on the surface of the rock, Liccardi could determine how long the rock had remained undisturbed. Or, how long it had been since the Wallowa glacier had abandoned it atop the moraine.

The careful work by Liccardi and his colleagues, including Peter Clark at Oregon State University, revealed two late Pleistocene advances of the ice. The last was about 17,000 years ago. Undisturbed boulders at the northern edges of the west moraine have rested in place for an average of 17,200 years Directly across the lake on the east moraine, the average is a bit younger - 16,600 years. But another advance, predating the 17,000-year event, left clusters of boulder in place at 21,300 years ago on the West Moraine, and 20,600 years ago on the east moraine. The averages suggest that late in the Pleistocene, there were two major advances of glaciers in the Wallowa Mountains: one at 21,000 years, followed by a warming period and glacial retreat, leaving a lake in place of glaciers. Then, at about 17,000 years, the climate cooled, precipitation increased and glaciers once again advanced and reclaimed the lake basin.

And there likely were additional small advances for glaciers in the Wallowas. Liccardi mapped and dated a small, recessional moraine at Glacier Lake. Its boulders had remained undisturbed for the last 10,200 years. So we know that only about 10,000 years ago, the Wallowas sported a strong component of active, though small, glaciers that may have occupied the Lakes Basin. And beneath the waters of today's Wallowa Lake, geology students and Jay Van Tassel from Eastern Oregon University have mapped two small moraines left by more anemic advances between 10,000 and 17,000 years ago.

The classical moraines of the Wallowa Lake system indicate that at least seven, and possibly 10, major advances and retreats of ice during the Pleistocene's million-year tenure. Glacial advances - periods of higher precipitation, colder temperatures, and low concentrations of atmospheric greenhouse gases - alternated with times of mild weather, when atmospheric greenhouse gases were high and plant growth prospered. We do not have solid dates for all the advances and retreats here - yet. But at least we know it was winter here once - 17,000 years ago.

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