Four main geological units occur in the area between Moscow and Lewiston. The oldest unit, dating back 1.47 to 1.4 billion years (Precambrian time), is known as the "Belt" Supergroup. The Belt Rocks are comprised mainly of sands, silts, and clays. They suggest a low- to moderate-energy environment of deposition and probably accumulated in a large basin or inland lake. These were very extensive deposits, covering an area from here into southern Canada, and from about the western border of Idaho east into Montana, and they can locally be more than 18 miles thick! These are the basement rocks in this area.
As the Belt Rocks accumulated back in the Precambrian, new sediment was transported in and accumulated on top of older sediment in horizontal layers (think Principles of Original Horizontality and Superposition). No fossils are found within the Belt Rocks because animals and plants did not exist in the Precambrian.
The sand, silt, and clay layers of the Belt Rocks show low-grade metamorphism and are resistant to erosion, making such prominent landscape features as Steptoe Butte, Kamiak Butte, and Paradise Ridge (these are predominantly made up of quartzites).
The next oldest formation is the Idaho Batholith, which is primarily granitic. Moscow Mountain is an exposure of the Idaho Batholith. The age of the Batholith ranges from about 85 to 65 million years (Cretaceous). The granite formed deep underground (~2 miles beneath the land surface) when magma intruded the Belt Rocks.
There is a HUGE gap in the geologic record between formation of the Belt Rocks and the intrusion of the Idaho Batholith. What was going on? Why are there no rocks today from that time period?
The next group of rocks to be preserved in this area are the Columbia River Basalts (CRBs). These rocks are approximately 17-15 million years old (Miocene) and represent one of the largest deposits of flood basalts in the world. The CRBs cover most of eastern and central Washington, northern Oregon, and the western fringe of Idaho.
Clearly, the CRBs indicate an intense episode of volcanic activity, but these basalts were not blown spectacularly from a volcano. Instead, lava erupted through large cracks (fissures) in the Earth's crust and flooded the landscape - first, filling in the topographic lows, then rising like water in a bathtub to cover larger areas of land. Here, between Moscow and Lewiston, the CRBs are up to 3,000 feet thick.
After deposition of the CRBs, the Snake River developed and began carving a valley through the CRBs. Today, the entire thickness of the CRBs has been eroded away along portions of the Snake River.
The last main geological unit found here in northern Idaho was deposited during the Pleistocene, from about 2 million years ago to 10,000 years ago. These are the Loess deposits of the Palouse. Loess is the term used to describe windblown silt. You've heard of sand dunes - well, the Palouse Hills are giant silt dunes. It is believed that winds from the northwest, blowing across ice sheets extending down from Canada, picked up fine-grained sediment along the glacial margins, carried it here, and drapped silt deposits over CRBs to form the Palouse Hills. Recent studies of dune shape and mineralogical make-up offer an alternative explanation, suggesting that the source of the Palouse loess lies to the southwest and was not of glacial origin. Regardless of source area, the loess covers much of the CRBs in the Moscow-Pullman area and makes for prosperous farming (love those lentils!). In this area, the loess deposits are up to 300 feet thick.
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