By KIRK JOHNSONSNOWMASS, Colo. — Two different time scales collided in this place.
More than 130,000 years ago in the chilled depths of the Illinoian ice age, an errant glacier left a hole atop a 9,000-foot-high ridge near what would become the town of Aspen in the central Colorado Rockies. The depression filled with snowmelt, and for tens of thousands of years, the little lake attracted the giants of the Pleistocene — mammoths, mastodons, ground sloths half again the size of grizzly bears, supersize bison, camels and horses — that came to drink, and in many cases to die, in the high alpine mud.
The second time scale was more like a runner’s sprint. Scientists had only 70 days — a number framed by mountain winter weather and lawyerly fine print — to search the old lake bed sediments for remnants of these ancient animals.
That was from Oct. 14, when workers on a reservoir dam turned over the first fossil bones (of a young female mammoth, promptly nicknamed Snowy) to last weekend, when work on the reservoir resumed. A tight contract schedule dictates that the reservoir, which will supply the condos and ski lodges of Snowmass, must be completed by late this year. The result was a frantic race to find and catalog everything possible before the site was entombed once more by water.
The breakneck pace of the fossil dig was matched only by what scientists said was the extraordinary richness of the site, one of the best windows into the thundering megafauna of its time. “The speed of this thing is so unlike normal science — from discovery to completion of one of the biggest digs ever in less than nine months,” said Kirk R. Johnson, the chief curator of the Denver Museum of Nature and Science, who oversaw the project. (He is no relation to this reporter.)
“Typically, you write a grant proposal and wait nine months to hear anything,” Dr. Johnson said. “We couldn’t wait — in a single day, we were finding a couple hundred bones.”
The ancient Snowmass clock was measured in the untold lives of the creatures that roamed and roared in a place and period poorly recorded in the scientific record: The high reaches of Rocky Mountains during the Sangamonian interglacial, a time of very warm weather around the globe, 75,000 to 125,000 years ago.
Other well-known ice age fossil sites, by contrast, like the La Brea Tar Pits in California and Hot Springs, S.D., have been dated to between 10,000 and 40,000 before present, and no well-preserved site has ever been found, scientists said, at this altitude in North America.
Here at Snowmastodon, as the site is called, the human clock ran partly on adrenaline, with 50 or more shovel-wielding scientists, volunteers and interns from the Denver Museum pawing the lake bed on a typical day. Their goal: sift 7,000 tons of sediment — 35 feet worth to the bottom of the glacial scrape — by the deadline.
Something very big — a mammoth tusk taller than LeBron James, a partial mastodon skull half the size of a Smart Car — was turning up every few days. By the end, more than 4,500 fossil specimens from 20 different animals were hauled out.
“Bone up!” Dr. Johnson shouted on recent, brilliantly sunny day, as a cheer rose across the pit. “Arm bone of a sloth,” Dr. Johnson said casually from a practiced distance, when the huge humerus was held aloft by its finder.
Preliminary estimates say the ancient ridge-top lake — unusual in having no stream inlet to bring in sediment — might have persisted for as long as 100,000 years before windblown dust filled it in to become a typical-looking alpine meadow, a state it had reached 50,000 years or more before humans came to the Americas.
The resulting fossil bed thus has a long climate record in its pollens, buried plants and windborne particles, as well as a long yardstick of the animals and what might be deduced about their lives. The sediment layers suggest periods when the lakeside landscape was tundra — too cold for trees — and others when great forests hugged the shore.
“I think at the end of the day that’s what’s going to be so valuable — you’ve got this crystal-clear glimpse into the Rockies before humans show up,” said Ian Miller, curator of paleobotany at the Denver museum. “We’re sitting here at almost 9,000 feet, and climate is driving ecosystems up and down. It’s a window, and you just watch it go by.”
A businessman from Wisconsin, R. Douglas Ziegler, bought the lake bed in 1958, when it was just an old meadow being used for grazing sheep.
The growing water needs of Snowmass Village, founded in the 1960s, eventually led engineers to look for a reservoir site, which led to the backhoes, and the first discoveries last fall, and which will lead, in a grand circling back of history, to an eventual restoration of meadow’s use as a watering hole. The accelerated pace was partly because the Snowmass Water and Sanitation Department District, under its contract with the Ziegler family, which still owns the land around the lake, faced substantial financial penalties if the work wasn’t completed on time. The reservoir must be up and running by next spring under the contract, but because winter will close down the work late this year, just as it did on the dig, that means finishing up before snow flies.
The dig, partly supported by a grant from the National Geographic Society, will be featured in a National Geographic-Nova special on PBS next year.
“We cross-country skied over where those creatures once roamed, and we never had any idea,” said Peter Ziegler, 62, who spent two days at the dig in June, laboring with a shovel. There are still many unanswered questions about what happened here — most pointed is, when did the animals actually die?
Because the site is too old for radiocarbon dating, which is only useful to about 50,000 years before present, other more complicated methods, all of which take longer to work out, will have to be used. Ancient pollen, for example, was collected from the mud to compare against other climate indicators. Core samples will be examined for markers like volcanic dust, which might be dated using radiometric dating techniques based on argon 40-39 or uranium-lead geochronology.
Secondly, the animals did not march to their deaths in a steady procession over the centuries. There are sediment layers with few bones, followed by layers with many bones — indicating, Dr. Johnson said, that the lake may have multiple stories to tell. The remains of young animals found in the pit could suggest, for example, that through at least through part of its history the lake was a trap, with slippery slopes or lethal leg-sucking goo, like the La Brea Tar Pits.
The third great question, connected to everything else, is how the great shifts of climate recorded by the mud affected the lives and habits of the creatures that roamed here. Was the climate warm enough in the interglacial period, which peaked in temperature around 110,000 years ago, that elephant-family relatives and other animals like camels and sloths could live year round at high altitude, or were there migratory patterns — highlands in the summers, lowlands in winter — that might emerge? For instance, will the growth rings of mastodon or mammoth tusks found here differ from those of cousins found at low altitude sites, hinting at permanent mountain residence?
“That’s the kind of question we couldn’t even ask before this site was discovered,” Dr. Johnson said. Some researchers are hoping the finds will yield DNA that might give a glimpse into the genes of ice age mammals. Genetic diversity, or uniformity, can suggest how big a population was at the time of an individual’s death.
“The interglacial period wasn’t a great time for stuff to be preserved,” said Beth Shapiro, an associate professor of biology at Pennsylvania State University who studies ancient DNA. “So this is not just a window into a time, but a whole group of animals we’ve never been able to get before.”
“Scale” is the word that researchers and volunteers used over and over, from the timeline to the volume of specimens to the crush of pressure to the trove of data that will fill research agendas for years to come.
Chris Faison, a schoolteacher from Aspen and a volunteer at the dig, said he hoped the story would resonate into the future, too. After the news broke last fall about the archaeological treasure trove, his school built a mastodon model to scale, 12 feet tall at the shoulder — and Mr. Faison said the first and second graders he teaches shared his awe.
“I never thought I’d see this kind of stuff,” he said, pushing his shovel into the mud.