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‘A 36 million-year-old mystery solved’: Gem reveals secret to Castle Rock’s formation in Colo.

By Seth Boster, The Gazette
Published: March 30, 2024, 5:28am

COLORADO SPRINGS, Colo. — One spring day last year, Mark Longman was hiking around Castlewood Canyon State Park with questions that mounted with every boulder and rock wall he passed.

A career geologist and longtime Denver resident, Longman knew it to be conglomerate rock populating the park near Franktown. He knew the rock at Castlewood to be the same, 34 million-year-old rock defining Castle Rock and other buttes and mesas along Interstate 25 between Denver and Colorado Springs.

But Longman did not know any conglomerate rock — any of the varieties across Colorado or anywhere else — to be so prominent, so clearly resistant to millennia of weathering and erosion. Conglomerate, after all, is not granite or other major, mighty types forming the most exposed, 14,000-foot likes of Pikes Peak.

As an associate of Longman’s, James Hagadorn, put it: “Mother Nature usually renders conglomerate into a pile of rubble.”

And yet there’s that castlelike landmark and regal relatives, very much intact across the Front Range.

How? Longman kept asking during that hike.

He now knows.

“A 36 million-year-old mystery solved,” Hagadorn said.

Longman, Hagadorn and another Denver Museum of Nature and Science associate, Joan Burleson, recently published a study addressing a mystery as old as the 36 million-year-old Wall Mountain Tuff. That refers to the rock layer before Castle Rock Conglomerate that formed from mineral-rich, volcanic debris left by eruptions around the modern-day Sawatch Range.

The Denver colleagues suspect an ancient process involving those minerals and groundwater led to the “cement” they discovered in rock samples.

It’s “cement” of a surprise gem.

Through the intense scrutiny of an Olympus Vanox microscope, researchers determined opal to be coating the conglomerate’s inner grains and pebbles. Around the opal, they noted encasings of crystalline chalcedony.

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“Like a rind on an orange,” Longman called the layers in a news release — responsible for “cement(ing) these particles together to make the rock harder than most concrete.”

Opal acted “like nature’s super glue,” Hagadorn said.

But before you go looking for some sort of glimmer on Castle Rock, or consider crossing some sort of ethical or legal line, Hagadorn emphasized: “This is microscopic opal. You’re not gonna chip away this rock and make a piece of jewelry from it.”

This is opal invisible to the naked eye, detected only by the right technology and well-rehearsed professional. The study, published in The Mountain Geologist, suggests reasons for the Castle Rock mystery taking this long to solve — “due in part to the difficulty of studying a conglomerate in thin sections,” it reads.

That’s where Longman came in. “A Jedi of the microscope,” Hagadorn called him.

Longman took wafer-like samples of the rock — collected at Castlewood Canyon with permission of Colorado Parks and Wildlife — and placed them on a tiny, glass tray under the microscope. He then shined polarized light through the rock.

“When that light hits the minerals in the rock, it gets bounced off the mineral, or it bends, or it slows down, or it speeds up,” Hagadorn said. “Wavelengths change such that different colors come out the other side.”

Such scientists like Longman and Hagadorn know certain colors and certain textures to represent certain products. Opal appeared “kind of like icicles that form on your windshield,” Hagadorn said, “or like the trees on a day we get early morning frost.”

Icicles might be a helpful picture in trying to describe the ancient, complex process believed to have created the opal.

Think of stalactites and stalagmites in a cave, those iciclelike formations occurring when water dissolves limestone then redeposits calcium carbonate. In the case of Castle Rock Conglomerate, it seems groundwater met underlying, silica-rich volcanic rock. “Like a cave, only this is happening on a microscopic level,” Hagadorn said, “and the minerals that happen to be growing are opal and chalcedony.”

The ultimate translation, he said: “Your Castle Rock isn’t going anywhere any time soon.”

The study suggests another takeaway. It reflects back on a “relatively random hike in Castlewood Canyon State Park.”

It was a “surprising” start to the revelation, the study reads, “and shows how much remains to be learned about certain aspects of Colorado’s diverse geology.”