The Crestone Eagle, August 2005:

“Rock of Ages”
The Crestone Conglomerate

by P.J. Smith

Long, long ago, and very, very close by, there rose up a mighty mountain range, the Ancestral Rockies. They overlooked a vast inland sea fed by glaciers. All are long gone.

But ample evidence of the Ancestral Rockies dots today’s local landscape. They’re those fascinating rocks that are colorful collections of all different rocks rolled up into one, and range in size from hand-held to big as a house. Many locals tell visiting friends and family that they are unique. They may not be one-of-a-kind in the world, but they are rare and the formation they come from is very rare.

These rocks-of-rocks, so familiar in local yards, parks, hiking trails and all around town, come from the Crestone Conglomerate, a member within the Sangre de Cristos formation. “It’s about 5 to 6 miles long and 3 to 4,000 feet thick and was deposited at the same time as its host formation—the Sangre de Cristos—but it’s much coarser grain,” explains Dr. Jim McCalpin, head of Geo-Haz Consulting and the Crestone Science Center. “Crestone Peak, Kit Carson, Challenger—all the highest peaks are made of this rock. And one of the reasons they are the highest peaks is because this is the hardest rock. It’s more resistant to erosion.” The Crestone Conglomerate is widest underneath all the 14ers, thinning out to the north and south. It outcrops near Kit Carson and Crestone Peak.

Crestone Conglomerate rocks are a mix of quartzite, pink granite, sandstone, schists, and gneisses cemented together in a matrix of fine-grained silica. What’s the process? “Look at the San Luis Valley: it’s a big basin with thousands of feet of sand and gravel,” McCalpin explains. “The pressure of the sand at the bottom is intense, and the groundwater is slowly flowing through the bottom gravels. The groundwater has dissolved chemicals in it, which are present in solution, and they precipitate in the pore spaces of the gravel, and cement it like concrete. It takes a long time, but it will eventually cement any rock that’s at the bottom of a pile.”

A combination of its component rocks and its matrix of silica—which has a hardness of 7 on scale of 10—gives the Crestone Conglomerate above-average hardness. “You see huge boulders that have been sitting on the glacial outwash fan for 15,000 years and don’t look like they weathered hardly at all,” says McCalpin. “They’re sitting out there thumbing their nose at erosion.”

They also haven’t gone very far from home, and are a good indication of what the landscape looked like 200-300 million years ago when they were formed. “There’s not enough precipitation in the mountain range to make big enough rivers to carry that gravel far away from the mountain range,” says McCalpin. “All the Baca is built on alluvial fans that are deposited by these streams coming out of the mountains. Alluvial fan deposits flank most mountain ranges in semi-arid climates. It’s all the waste debris.”

Why are some of them so big and so round? “What controls size and shape of gravel particles on alluvial fans is how close together the fractures are on the mountains, because that’s what defines the initial blocks that weather out and fall out. In the Crestone Conglomerate, unlike most rocks, the fractures are spaced very far apart. That’s where you get these giant hill-size boulders. Some of the fractures are actually curved fractures, so some of the rounding may be from that,” explains McCalpin. “So, when they fell off the outcrop, they weren’t square. If you go up Willow Creek, the fractures aren’t straight for long periods.”

Tumbling in water also helped create the globular shapes as the boulders broke off and were washed down in the glaciers of the last ice age, that are today’s South Crestone, Willow, Spanish, Cottonwood and Deadman Creeks.

So, how exactly did all those different rocks get rolled up into one? It helps to look at what geologists call the Rock Cycle. McCalpin explains: “Basins are where sediments are deposited, where all the rivers are carrying all the sediment, and these basins are subsiding into the crust and receiving lots of sediments, like these gravels in the Crestone Conglomerate. Around the world there are mountain ranges made of cemented sediments. Those mountain ranges at one time were basins.”

“If you look at the northern part of the Sangre de Cristos, north of Valley View Hot Springs, the range up there is made of sedimentary rocks which include limestones, which were deposited on the floor of the ocean. They were at one time in a topographically low place receiving sediments; now they’re up in a mountain range. Those limestone pieces are being eroded, and they’re being recycled into another basin. So, the rock cycle means the rocks are deposited in basins—they’re cemented and turned into hard rock. Later in time that area is uplifted and becomes a mountain range, and those sediments are now exposed to erosion; pieces of it break off and are deposited into yet another basin; that basin subsides; etc.”

Although the Crestone Conglomerate was formed 300 million years ago, it contains much older rocks. “When you look at the Crestone Conglomerate, you’re actually looking back several rock cycles,” McCalpin says. “The boulders on the alluvial fan are conglomerate that was made of old alluvial fan deposit that is now exposed in a mountain range and being eroded into a basin. And it is made up of metamorphic rocks that were once sediment from an even earlier basin that was eroded from an earlier mountain range. There’s three rock cycles that you’re standing looking at. You begin to get kind of dizzy as you look back in time and realize how many things had to happen to explain what’s in this rock.”

And, as locals have learned, although these “rocks of ages” aren’t so great to build with, they definitely make interesting conversation pieces, and decorative and unusual landscape additions. Some of them are quite beautiful in their colors and works of “rock art” in their own right.