SNOWMASS, 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....