In Search of Layered Pond Muck

There's a story here.

We were making slow but steady progress up the narrow valley of Libby Creek, across unvegetated hummocky ground covered in boulders and snow, through a network of cold sediment-laden creeklets. But just as we caught sight of the snout of the glacier ahead, we were stopped by icy torrents emerging from the fractured dirty ice. Water came roaring down the canyon, rolling and bashing boulders. It was only June, but the onset of melting had opened tunnels and drainages in the ice, and the annual flood was underway.

We headed back, stopping at one of the ponds among the hummocks of glacial till, where we ate lunch perched on quartzite boulders above the cold wet ground. The pond was mostly ice-free, the water murky with sediment carried in by streams. I tossed a penny into the pond—not for a wish, but to confuse future glaciologists.

Then my ice age dream vaporized and I was back in the aspen stand, staring at layered dirt.

From gravel pit near Barber Lake, 1972; photo by Wayne Sutherland, pencil for scale (Mears 2001).

Learning geology is learning to read ... not books, but landscapes, rocks and dirt. Like books, they take us into other worlds—even the far distant past. But the entryways aren’t always obvious. For example, few people would stop to examine the drab dirt slope in the photo above.

That photo has appeared in several publications about the geology of the Medicine Bow Mountains of southeast Wyoming. The outcrop was said to be located just west of Centennial, which is just 30 miles from where I live. Of course I wanted to see it, so I read the description carefully.

“… recession of the ice in the broad canyon west of Centennial left hummocky till behind. Trapped meltwater created a pond here which is recorded by thin varves of sand and pebbles washed in during summers, and silts that slowly settled when the pond’s surface was frozen. The varves, now exposed in an old gravel pit near Highway 351 [Barber Lake Road], are overlain by till of a second advance.” (Mears 2001)

Medicine Bow Mountains during the Pleistocene. Arrow marks location of pond sediments exposed in an old gravel pit. Diagram and drawings by SH Knight (1990).

I drove the Barber Lake Road from top to bottom, searching for an abandoned excavation, but found nothing. Then I checked around the various campgrounds … still nothing. Maybe it’s gone, I thought—buried, eroded away, or turned into a campsite. After all, 45 years have passed since that photo was taken.

But I didn’t give up. Instead, I did what any wise person would do: I asked a librarian, specifically the Centennial librarian …

The librarian didn’t recognize the layered dirt in the 1972 photo, and didn’t know of an old gravel pit near Barber Lake, but she did know several people who might well know, and indeed one of them did know: “just off the Willow campground road near the entrance gate, on the left.”

A week later, I parked near the campground gate. Through aspen trunks, I spotted a promising pale outcrop and headed over for a closer look. Yes! This was the old gravel pit, now overgrown with aspen. Even after 45 years of weathering and erosion, layers were still distinct in places, though not as fresh as in the old photo.

Gerard De Geer of the Geological Survey of Sweden was the first to figure out the stories told by what Swedish geologists called hvarfig lera (layered clay), now called “varves.” Varves are like tree rings. Each one represents a year of activity—growth in the case of tree rings, and sedimentation in the case of varves. De Geer constructed the first varve chronology, published in 1912. Since then, the Swedish Varve Chronology has grown into a 14,000-year record of floods, drought, rains of volcanic ash, deglaciation, plants (pollen), and more.

The varves in the Medicine Bow Mountains date to roughly 10-15,000 years ago, when the crest of the range was covered by an ice cap, and a glacier extended down the Libby Creek drainage. During the warm season, streams from the glacier brought down sediment—more or less depending on the amount of melting and vigor of the streams. Some of the sediment ended up in ponds in the moraine and hummocky till (unsorted sediment) below the glacier. After a short “summer,” the ponds froze over. Then the finest particles slowly settled out, forming a thin band of clay on top of summer sediments. Gravel quarrying exposed part of the varve record of one of these ponds.

A layer is a year’s worth of sediment. Arrows point to clay bands marking end of annual cycles.

Do thinner varves above the thicker one tell of less melting and sedimentation? Or was the glacier farther away, melting back to the high country? Our chronology is too short to say. We need more varves, probably a lot more.

More than once I’ve wondered if rocks and dirt are as easy to read as their stories in print suggest, but rarely can I judge. However, in this case I know there's another version of the story. The great Samuel H. Knight—Mr. Geology of Wyoming—concluded that the varves formed in a moraine-dammed lake rather than in a pond in hummocky till.

“Accumulations of unsorted rock fragments transported by ice were deposited in moraines on valley floors, behind which lakes were impounded in which distinctly laminated, fine-grained sediments (varves) were deposited.” (Knight circa 1974, published 1990)

Libby Creek terminal moraine and impounded lake, in which varved sediments were deposited. Cross section shows relationships of various glacial sediments (Knight 1990).

Whether deposited in ponds or lakes, surely more varves lie hidden under today’s soil and plants. So hikers and mountaineers, keep an eye out. Among all those lodgepole pines and grouseberry bushes, there may well be entryways to other worlds!

Some climb mountains “because they are there”

Others climb mountains their secrets to share,

For mountains hold in their massive grasp,

Profound records of the infinite past. 

– Samuel H. Knight

Sources

Thanks to Deb and Lowell for directing me to the varves.

Knight, SH. 1990. Illustrated geologic history of the Medicine Bow Mountains and adjacent areas, Wyoming. Geological Survey of Wyoming Memoir 4. PDF

Mears, B, Jr. 2001. Glacial records in the Medicine Bow Mountains and Sierra Madre of southern Wyoming and adjacent Colorado, with a traveler's guide to their sites. Geological Survey of Wyoming Publ. Info. Circ. No. 41. PDF

North American Glacial Varve Project, Tufts University.