A limber pine, Pinus flexilis, on an escarpment near Pawnee Buttes. Photo by Jeff Mitton

Entrance to a packrat cave. Photo by Jeff Mitton

Fossil teeth of the early horse, Merychippus, which lived 17 million to 11 million years ago. Photo by Jeff Mitton

History Buried in the Cliffs

(Appeared May 27, 2005 in the Daily Camera)

An isolated stand of limber pines clings to an escarpment near Pawnee Buttes. The age of this stand is controversial, for some biologists believe it is many thousands of years old, while others believe it was established within the last 200 years. We organized an expedition to resolve the controversy.

Dave’s Draw is 70 miles east of Fort Collins, 8 miles north of Keota, and 3 miles west of Pawnee Buttes. Today, 377 limber pines, Pinus flexilis, cling to the escarpment, taking water that flows horizontally through the rock strata and seeps to the surface at eroded hillsides and cliffs.

When the glaciers were full, approximately 20,000 years ago, the southern Rocky Mountains were strikingly different. Glaciers had displaced the spruce and fir forest from the high mountains to the plains of Oklahoma and Texas. Hardy limber pines were the most common trees in the foothills and on the plains. As the glaciers retreated, spruce and fir moved off the plains, and recolonized the mountains. Some biologists believe that it was at this time, approximately 15,000 years ago, that limber pines were stranded at the escarpments near Pawnee Buttes.

Historical anecdotes and age distributions suggest that the stand at Dave’s Draw is recent. A cavalry officer rode through the area in the early 1800s, and wrote in his journal that there was no firewood available for fires. His troops cooked their meals over fires of buffalo dung. Dr. Bill Schuster, who studied this stand for his doctoral degree at CU, found that the oldest tree was 175 years old, and most were much younger. Perhaps an unusual flight of Clark’s nutcrackers brought limber pine seeds from the mountains around 1830.

Plant material entombed in packrat middens resolved the controversy. Packrats have the curious habit of collecting all sorts of debris from their local area and hiding it in their burrows or caves. They urinate on the debris, and the soggy mass dessicates to a solid midden, preserving the debris.

Dr. Julio Betancourt, with the US Geological Survey, dissects packrat middens to reconstruct ancient plant and animal communities. He uses carbon dating to estimate the age of the debris, and identifies the fragments to determine which species of plants and animals were present. Betancourt analyzed middens to determine whether the pines had been there many thousands of years, or less than two hundred.

As Betancourt slowly and laboriously excavated a midden at the foot of a cliff, I sat nearby, wondering about the history of the site. I sat on a pile of loose material that had eroded from the cliff, and absent-mindedly sifted it through my fingers. An unusual shape woke me from my daydream; I was holding a fragment of fossilized bone. I searched more methodically, and found more bone fragments, then two fossilized teeth.

The teeth were from Merychippus, a small horse (40 inches tall at the shoulder) that lived 17 million to 11 million years ago. Species of this genus had three toes on each foot, but only the center toe touched the ground. They also had high-crowned cheek teeth, indicating that they were the first of the grazing horses. Species of this genus were the ancestors of all modern horses.

We sought packrats to learn about the history of limber pines. We found the packrats scurrying through caves lined with the bones and teeth of horses that lived more than 10 million years ago.

Packrat middens allowed us to look sever thousand years into the past, but limber pine needles, seeds, and pine scales were limited to the most recent layers. The stand of limber pine was established recently, since 1800.