Jefferson River - Geology

Geology

The geology of the Jefferson River and the surrounding mountain ranges includes some of the oldest rocks found in North America, dating back to the Archean Eon, 2.7 billion years ago. Found primarily in the Tobacco Root and Ruby ranges, these ancient rocks are metamorphic, having been highly compressed and nearly re-melted by geologic forces over eons of time. Frequently found along the Jefferson River, these rocks include layered feldspars, gneiss, glassy quartz, heavy dark amphibolite, and sometimes marble.

About a billion years ago, the Willow Creek Fault, north of the Jefferson River canyon, dropped down deeply and filled with seawater, stretching north to Alberta and British Columbia. Eventually, the sea receded and erosion wore away intervening geologic history until about 530 million years ago, during the Cambrian Period of the Paleozoic Era.

A new sea encroached on the land, depositing sedimentary layers of limestone, dolomite, shale, and sandstone over several hundred million years. Limestone is generally made of calcium from marine animals that have been compacted and cemented together. Dolomite is similar but has more magnesium. Shale is formed from fine-grained mud, silts, and clays that have been compacted and cemented together. The sandstone is made up of quartz and feldspar.

By the Mississippian Period, 340 million years ago, much of western North America was covered with a warm, shallow sea, much like the Gulf Coast of Florida today. Small marine fossils can be found in the Madison Group limestone that makes up the steep, narrow section of the Jefferson River canyon today.

Gentle uplift eventually raised the region above sea level again. Rainwater percolated down through cracks in the limestone, dissolving rock and creating caves such as those found at Lewis and Clark Caverns State Park.

Local mountains, such as the Tobacco Roots were formed from the Boulder Batholith. The batholith is composed of at least seven, and possibly as many as fourteen, discrete igneous rock masses called plutons, which formed beneath the Earth’s surface during a period of magma intrusion about 73 to 78 million years ago during the Late Cretaceous. The rising buoyant plutons resulted from subduction along what was then the west coast of North America. Regional uplift brought the deep-seated granite to the surface, where erosion exposed the rocks and the mineral veins they contained. The granite generally consists of quartz, hornblende, and feldspars. Gold, silver, and other semi precious minerals are also associated with batholiths.

The ancient metamorphic and more recent sedimentary layers above the batholiths eroded away as the magma pushed up through the crust. Thus, the granite batholiths are typically found at the center of local mountain ranges, while the much older metamorphic gneiss is usually found lower in the mountains, and limestone layers are mostly found in the foothills nearest the Jefferson River.

The Rocky Mountains began a new and continuing phase of crustal stress 5 to 10 million years ago as tectonic forces began to pull the region apart. Blocks of earth dropped down to form valleys, and the Jefferson River eroded a channel through rock to form the Jefferson River canyon.