BREADCRUMBS FROM THE PAST:

Glacier-deposited rocks aged younger than expected

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Story and photos by Lucas Boland for The Mercury

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Five to six hundred thousand years ago, instead of windmills and grain silos rising out of fields near Wamego, hundreds of feet of glacial ice would have towered above.

“If you look across the Big Blue it lies there; it’s glistening. You can see it, and it might be halfway dirty with rocks on top of it, too. It’s blocking to the south the Kansas River Valley and it creeps up on the other side and bulldozes all sorts of stuff in front of it,” imagined K-State geographer Arnaud Temme, who led a recent effort to determine more precisely when this ice cap retreated.

A massive glacier once covered much of North America, extending all the way from the North Pole to its southern edge within a few miles of Mount Mitchell Heritage Prairie. As the frozen ice sheet’s weight and imperceptibly slow movement shaped the northern U.S. landscape, it covered Riley and Pottawatomie counties in a shimmering white. The glacier was taller than the Manhattan Housing Authority’s 12-story Apartment Towers, and so immense that it dammed the Kansas River and created a massive lake stretching from Salina to Manhattan and north past Tuttle Creek Lake.

“This is an inner sea, this is not a lake,” Temme illustrates.

In 2019, Temme and his team sought to more accurately determine how long ago the glacier covered the now grassy and abundant Kansas landscape. As if the glacier knew that one day humans would wonder about its existence, it left behind quartzite rocks that contained the answer, like pebble-to-car-sized breadcrumbs of the geologic past.

“We really want to know how long those rocks have been there, because that should be the same as how long ago the glaciers retreated,” Temme said. “And when I say glaciers, I mean the entire continental icecap.”

Other geological records and past studies indicate that the last glacier to reach Manhattan did so around half a million years ago.

“We do know that (it) should be more than 500,000 and probably less than 600,000 years ago from other research,” Temme said. “And so we were looking for the perfect opportunity to try and nail down that number a lot more accurately with this modern technique of cosmogenic exposure dating.”

But upon receiving the test results of rock samples two years after they were collected, Temme was puzzled to see the rock ages return about 40% to 80% younger than expected. Temme anticipated the rocks to be around 500,000 years old or a little older, but instead multiple came in around 285,000 years old and one around 85,000 years old.

“If we would have gotten 580,000 plus or minus 5,000, I’m fine. But the problem is we actually got way younger ages. That’s not a problem, that’s a fact,” Temme said. “Our expectations were not met from what would have happened, so we’re learning from that now.”

When glaciers travel along the earth, the great weight and movement of the ice drags debris composed of sediment and rocks with it.

For a slow-moving but persistent glacier, transporting a rock weighing thousands of pounds is a small task. Temme said the quartzite rocks found in the Manhattan region and sampled by his team originated in what is now South Dakota. As the ice sheet grew from its northern origins and extended south into Kansas, it picked up these quartzite hitch hikers along the way.

Among the beige limestone of the Flint Hills, the red rocks stand out by their color and commonly large size. Like icebergs, much of the rock is often hidden underground, so the true mass is much greater than what meets the eye. The rocks hide in plain sight in front lawns and at public use areas, especially Mount Mitchell.

“If you walk up to the summit, you will have crossed multiple of these rocks,” Temme said. “They’re all angular, they’re a little bit reddish, pinkish. ... The cool thing about being in that location is you’re kind of where the glacier stopped. If you look back north across the Kansas River Valley, you can really imagine all of that space would have been filled with ice 200 feet thick.” 

For local rancher Stephen Russell, the glacier rocks can be both a friend and a foe when navigating pastures. On one property, Russell uses the rocks as landmarks to wayfind through the field, but the smaller rocks hidden in the grass can be a hazard when driving all terrain vehicles.

“If you run into one, they can be really annoying, because I’ve done it. But they are a unique feature and I think need to be appreciated for the awesomeness of how they got there,” Russell said. 

By cutting samples from these glacier-deposited rocks and testing them in a lab, the team learned the amount of time they had been exposed to cosmic radiation. The rocks were exposed to radiation only after they were no longer covered by ice. The amount of radiation measured in the rock effectively gave the date the ice that carried them retreated.

Temme explained the testing method, saying that when cosmic radiation hits the rocks, “one kind of atom hits another kind of atom, and under the right conditions, produces a third kind of atom, and that’s the one that we have here.” “Quartz consists only of silicon and oxygen atoms almost exclusively,” he said. “This rock, because it’s been on the surface for a very long time, has also accumulated beryllium. … The longer it’s outside, the more beryllium there is.” 

So, why the young rocks?

Temme settled upon the only explanation for the deviant ages that seemed plausible: the rocks were buried under way more glacial debris than anticipated, so much that it took about 200,000 to 400,000 years to uncover them.

“It wasn’t just a big block on the surface the day the glacier left,” Temme said. “That big block was covered under potentially meters of other gunk and junk that took hundreds of thousands of years to be removed and be eroded itself.”

The new information gleaned from the tests further testifies to the size and power of the glacier that once covered Kansas, that it could move such an unfathomable amount of material.

“I was surprised, but in a good way. … Now in my mind there is a much wider, thicker, more prominent ridge, end moraine basically, of that icecap all along its edge, whereas initially I was thinking, ‘Okay a bunch of blocks, not much else,’” Temme said. “That picture I have only because we have these new dates now.”

That improved image can help researchers and all Kansans to better understand the landscape in which they reside, and ultimately, appreciate and protect it.

“One major part of the Flint Hills region is knowing how fragile it is and how unique it is. And so the more we learn about the makeup of the soil and the soil types, and the grass types allows you to be a better manager of the range,” Russell said. 

Whether a person uses this knowledge to inform land management decisions depends on the individual. But Temme says that one has to admit, a near football field-thick ice sheet extending northward as far as the eye can see is worth imagining on a hike with a good view, preferably from the not-so-comfortable seat of a quartzite rock.

“I know Kansans, maybe more than other people, really are connected to their landscape and are proud of their landscape, and I think knowing more about it for that reason alone is really cool and valuable.”

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