Below him, a major wildfire was raging, devouring trees and brush.
But what caught his eye was the black volcanic rock along the South Fork of the Boise River. On both sides of the canyon, the rock was on fire. Then, it came to him. It wasn't the rock that was on fire. It was lichen. Even the tiny rock-hugging lichen were burning.
That is how it went with the 1992 Foothills fire - one of the most savage and memorable blazes to sear Idaho in years.
"That fire was like a blowtorch," said Stephen Mealey, supervisor of the Boise Forest from 1991 to 1994. "It traveled 18 miles in 10 hours. That's an incredible rate. And it went with incredible force and incredible heat."
Today, the Foothills fire has become a textbook example of the environmental damage inflicted by modern-day, super-hot wildfires.
It steamrollered across a quarter million acres, reached temperatures of more than 2,000 degrees - hot enough to melt glass bottles and metal culverts - and triggered horrific soil and stream damage. The largest ponderosa pine in Idaho, one that had survived centuries of natural fire, was fried by the Foothills fire.
Today, the Foothill's footprints remain painfully visible from the air. Charred hillsides seem to roll on forever. Ghostly white-ash images of trees linger on the blackened ground - a signature of the fire's fury. Damage was so extensive that "it will impose limits on forest ecosystems for centuries," the National Commission on Wildfire Disasters - a panel of fire specialists - said in a report last year.
One especially hard-hit piece of scorched earth lies near the headwaters of the South Fork of Sheep Creek - a tributary of the Middle Fork of the Boise.
Fire burned so fiercely it stripped steep slopes of soil-gripping vegetation. The area was reseeded but - for reasons still unknown - seedlings didn't grow. For two years, the area remained barren, as vulnerable "as a crab out of its shell," said Thornton.
Then, on July 31, 1994, disaster struck.
A thunderstorm pounded the area. It wasn't a big storm but it did a lot of damage. Up to 3.5 inches of soil washed away in a few hours - soil that had taken 1,500 years to form. Steep parts of the creek were scoured by mud and ash; gentler sections were buried by them. Many aquatic species perished, including an entire spawning class of rare bull trout.
No one knows why the area suffered so extensively. But it may be due to a freakish phenomenon called "hydrophobic soil" that can be caused by very hot fires.
"You literally get an impermeable waxlike layer below the surface," said Mealey. "It resists water. If you were to scratch the surface of the soil and pour water on it, the water would bead up and run off just like the hood of your car."
On the South Fork of Sheep Creek, the waxlike layer may have prevented young trees from growing. Almost certainly, it hastened erosion. "When water doesn't penetrate, it runs off," said Mealey. "And as it runs off, it takes everything that's loose. And there's a lot that's loose."
Sheep Creek is no isolated incident. Similar fire-related damage is happening throughout the region.
After the Tanner Gulch fire in Oregon in 1989, mudslides occurred along a pair of creeks so small they didn't have names. That was just the beginning.
The erosion led to a very major "debris torrent" in the Grand Ronde River which was measured 36 miles downstream, said Oregon fisheries biologist V.K. Kaczynski.
It loaded the river with sediment, increased the temperature of the water, and depleted it of oxygen, Kaczynski added. The result: "A total fish kill in that stretch of river, including the endangered spring chinook salmon. I was up there in 1993, and you still had excessive sediments in spawning areas."
"Something is crazily out of kilter," said Mealey, who now heads the Upper Columbia River Basin project, a long-range federal land-management plan for a huge chunk of the Northern Rockies and Pacific Northwest.
"The problem is we've interrupted the natural processes. We've altered fire regimes which were nature's way of portioning out limited water and nutrients," Mealey said.
"Over so many acres, much of the coniferous vegetation has been removed. It's difficult to make the argument there's going to be much biodiversity. Most of the vegetation will all be the same age. And because of that, there won't be much variety. That's a frightening prospect: to see that much of this forest in such a monotonous ecological condition. It didn't evolve that way."
* Tom Knudson