he mountain ranges of central Montana appeared about 50 million years ago, fairly early in Tertiary time. Some of them owe their existence to large movements of the earth's crust, others to large intrusions of molten magma at depths of no more than a few thousand feet, still others to volcanic eruptions. No coherent or widely accepted explanation for their existence has emerged from years of debate.
Meanwhile, large masses of molten granite magma rose through the earth's crust along a broad trend from the vicinity of Boise to that of Helena. That also remains unexplained. Much of that magma erupted, exploded in great clouds of steam loaded with volcanic ash. They spread generous blankets of pale rhyolite (RYE-o-lite) ash across wide expanses of the region. Other enormous volumes of molten granite crystallized at shallow depth of a few thousand feet to make large masses of pink granites. The expedition camped amid very picturesque boulders of pink granite at Lolo Hot Springs, and crossed large masses of it on their way across Idaho.
"...Rocks, which appear above the timber like Towers."
--Private Joseph Whitehouse, Sept. 13, 1805
Granite outcrop at Lolo Hot Springs.
Both varieties of granite, as well as pale rhyolite ash--a volcanic rock similar to granite--weather into sandy soils, which typically are short of the essential fertilizing nutrients, especially potassium, iron, calcium, magnesium, and phosphate. Even if those minerals are present, rain rinses them out of the soil in regions with a climate as wet as that of the Bitterroot Mountains. Most of the soils in the Bitterroot Mountains are fertile enough to support dense forests, but not enough so to support many edible and nutritious plants. Neither game animals nor people can find much to eat there. Those grudging soils explain why the Corps of Discovery nearly starved in the Bitterroot Mountains. It is a matter of bedrock and climate.
A specimen of pink granite, its source identified by codes applied by a geologist.
As the Corps of Discovery reached a high point on K'useyneiskit near the western edge of the Bitterroot Mountains on September 19 they saw a welcome sight--"a level part of the country about 40 miles ahead." Sergeant Gass dramatically recorded the men's elation: "When this discovery was made there was as much joy and rejoicing among the corps, as happens among passengers at sea, who have experienced a dangerous and protracted voyage, when they first discover land on the long looked for coast." Actually, they were only looking at the broad, rich Nez Perce and Camas Prairies south of the Clearwater Canyon, which stand more than 3,000 feet above the camp they would occupy on the Clearwater River near today's Kamiah, Idaho. Still, they saw flat land, and that was encouragement enough for them.
On the very next day, though, the farmers in the party began to notice changes closer at hand. As Lewis reached the vicinity of Hungry Creek, observed, "the soil as you leave the hights of the mountains becomes gradually more fertile. the land through which we passed this evening is of an excellent quality tho very broken. it is a dark grey soil." It was essentially the same stuff of which those distant prairies were made--rhyolite mixed with basalt.
--David D. Alt, 10/04
Supported in part by a grant from the Idaho Governor's Lewis and Clark Trail Committee