Two prospectors were found at work in a spruce forest near which the fire started but which it did not reach for a week. These men said that, an hour or so after a thunder-shower of a few days before, one of the brown beetle-killed pines had sent up a smoke-column. Apparently lightning had struck this tree. The following day a small fire was burning near it. This expanded into the forest fire. Commonly it is a standing dead tree that is set on fire by the lightning, but the bolt sometimes fires accumulated trash around the roots where it enters the earth.
Within this extensive burn the trees had stood from thirty to one hundred and forty feet high and from two hundred to three thousand to the acre, and they were from thirty to four hundred and fifty years old. A majority were about two centuries old. The predominating kinds were yellow pine, Douglas spruce, Engelmann spruce, and aspen. Different altitudes, forest fires, and a variety of slope-exposures, along with the peculiar characteristics of each species, had distributed these in almost pure stands, an area of each kind to itself. There was some overlapping and mixing, but lodge-pole pine noticeably stood by itself.
Where first encountered, this fire was roaring through a thick second growth of lodge-pole pine. Scattered through this young growth were hundreds of dead and limbless trees killed by a fire of thirty years before. The preservative effect of their fiery death had kept these great pillars sound, though they had become checked and weathered. They burned slowly, and that night while the fire-front was storming a ridge, these columns spread sparks and flames from split sides, or as gigantic candles blazed only at the top. Yellow pines and Douglas spruces killed in an intensely hot fire are so cooked and preserved that they will resist weathering or rot for decades. I have seen a few of these pitchy broken fellows standing erect in the depth of a century-old second generation of forest with the arms of the living trees about and above them.
Down a slope a fire moves more slowly and with lower temperature than upward on the same slope. A fire may rush in a minute up a slope which it would require a day to creep down. A fire is more all-consuming in going up, and even after years have passed, the remains left on a slope will often enable one to determine whether a fire swept up or crept down. One peculiarity of flames in young growths on steep slopes is that they sometimes dart up the heights in tongues, leaving narrow ragged stretches of unburned trees! Usually these fiery tongues sweep in a straight line up the slope.
The intense heat of a passing fire-front is withering at long distances. I have known a fire to blister aspen clumps that were seven or eight hundred feet from the nearest burned trees. The passing flames may have been pushed much closer than this by slow heavy air-swells or by the brief blasts of wild wind rushes.
The habits of forest fires are largely determined by slope-inclination, wind-speed, and the quantity and quality of the fuel. In places the fire slips quietly along with low whispering, then suddenly it goes leaping, whirling, and roaring. A fire may travel less than one mile or farther than one hundred miles in a day. The ever varying slope and forest conditions in the mountains are constantly changing the speed and the enthusiasm of a fire. Where all conditions are favorable, it sweeps level stretches at a mile-a-minute speed and rolls up slopes with the speed of sound!
One evening I climbed a high ridge that stood about half a mile in front of a heavily forested peninsula which the fire-front would reach in a few hours. The fire was advancing across the valley with a front of about two miles. On arriving at the top of the ridge, I came up behind a grizzly bear seated on his haunches like a dog, intently watching the fire below. On discovering me he took a second look before concentrating his mind on a speedy retreat. Along the ridge about a quarter of a mile distant, a number of mountain sheep could be seen through the falling ashes, with heads toward the fire, but whether they were excited or simply curious could not be determined.
The forested peninsula which extended from between two forested cañons had a number of meadow openings on the slopes closest to me. Around these were many brilliant fiery displays. Overheated trees in or across these openings often became enveloped in robes of invisible gas far in advance of the flames. This gas flashed and flared up before the tree blazed, and occasionally it convoyed the flames across openings one hundred feet or so above the earth. Heated isolated trees usually went with a gushing flash. At other places this flaming sometimes lasted several seconds, and, when seen through steamy curtains or clouds of smoke, appeared like geysers of red fire.
At times there were vast scrolls and whirling spirals of sparks above and around the torrential, upstreaming flames of the fire-front. Millions of these sparks were sometimes formed by high outflowing air streams into splendid and far-reaching milky ways. In moments of general calm the sky was deeply filled with myriads of excited sparks, which gradually quieted, then floated beautifully, peacefully up to vanish in the night.
Meantime the fire-front was pushed by wind-currents and led by ridges. By the time the fire-line had advanced to the steeper slopes it was one vast U about three miles long. Its closed end was around the peninsula toward me. The fire-front rushed upward through the dense forest of the peninsula steeps more swiftly than the wildest avalanche could have plunged down. The flames swept across three-hundred-foot grassy openings as easily as breakers roll in across a beach. Up the final two thousand feet there were magnificent outbursts and sheets of flame with accompanying gale and stormy-ocean roars. Terrific were the rushes of whirled smoke-and-flame clouds of brown, ashen green, and sooty black. There were lurid and volcanic effects in molten red and black, while tattered yellow flames rushed, rolled, and tumbled everywhere.