25,000 years of glacier history

It’s well documented that the glaciers in Glacier National Park are receding and many have vanished. But when they were bigger, they were likely far bigger — in fact, the iconic Grinnell Glacier once likely extended nearly to what is now Swiftcurrent Lake.

Kelly MacGregor, a geomorphologist and associate professor of geology at Macalester College in St. Paul, Minn., has been studying the ebb and flow of Grinnell Glacier over the past 25,000 years.

Giant masses of moving ice, glaciers are difficult to study on the ground — the carving, grinding and shaping of the landscape occurs beneath them and simply can’t be observed. Glaciers also have a tendency to destroy, or at least alter, their geological tracks.

But there is a way to track glaciers over time, MacGregor explained to a packed house at the Waterton-Glacier Science and History Day recently — by taking core samples from the sediments in the lakes that glaciers leave behind. She called the sediment in lakes “nature’s junk drawer” — heaped with remnants from the past.

MacGregor and her undergraduate helpers gathered core samples from Lower Grinnell, Josephine and Swiftcurrent lakes in 2005 and 2010. Lower Grinnell is the deepest of the three, at about 88 feet. They then analyzed the sediment from the lakes, dating samples as far back as 22,000-plus years.

The samples contain a treasure trove of information. Pollen found in the sediment tells researchers what plants were growing during a certain time period. For example, weed pollen starts showing up in samples about the same time white men settled the region.

Charcoal in the sediment shows the frequency and severity of wildfires in the valley — all told, about 50 fires have scorched the Many Glacier Valley over the past 13,000 years, most of them in the past 2,000 years.

Algae in the sediment tells researchers the water temperature and productivity of the lake — the warmer the lake, the more algae it produced. Minerals in the sediment also lend clues. Grinnell Glacier is eroding away dolomite — a pink rock that is 1.6 billion years old. By tracking dolomite, they can track the progress of the glacier itself.

Grinnell Glacier has moved significantly over the past 25,000 years. MacGregor noted that her research indicates the glacier was either at or very near Swiftcurrent Lake about 25,000 years ago.

The sediment cores also reveal past volcanic eruptions, including Mount St. Helens in 1980 and a huge Mount Mazama eruption about 7,000 years ago that created Crater Lake in Oregon.

MacGregor hopes to do more core sampling of the lakes next summer, looking to delve even further into the history of Grinnell Glacier and Glacier Park itself.