Department of
Environmental Quality

The Belt Supergroup

The geology of the Rathdrum Prairie and the surrounding area consists of bedrock and unconsolidated sediments. The oldest bedrock in the area, referred to as the Belt Supergroup, comes from sediments that were deposited over a billion years ago in a narrow ocean basin called the Belt Basin. The sediments are believed to be over 10,000 feet thick in some areas. Over time, the weight of the overlying material hardened the sediments into fine-grained rock such as siltstones and sandstones.

The rocks in the Belt Supergroup have different characteristics depending on the environment the original sediments were deposited in. Sediments deposited in water may have ripple marks caused by wave action or polygonal cracks caused by drying in the sun. Sediments deposited in deep water would not have any of these characteristics due to the lack of sun or wave action.

Sediment deposited in shallow water may also contain fossil remains from a type of algae called stromatolites. Fossils of stromatolites can form thin mats or round masses that resemble heads of cabbage. There are no other types of fossils in the Belt Supergroup rocks, since animal life at the time was very primitive.

Approximately 70 to 80 million years ago, a large granite mass called the Kaniksu Batholith (the pink area in the figure below) pushed up from underneath the Belt Supergroup rocks, pushing them upward and to the east. The Purcell Trench separates the two rock masses and runs approximately north-south. The Purcell Trench, located mainly in Canada, extends south through Bonners Ferry, Sandpoint, and Coeur d'Alene. In a type of rock called mylonite, there is evidence of the movement that occurred when the Belt Supergroup rocks were pushed up and eastward. Mylonite is formed when large rock masses slide over each other and is present along the west side of the Purcell Trench.

The Kaniksu Batholith granite formed when portions of continental crust were pushed down to great depths and exposed to very high temperatures and pressures, causing portions to melt and then rise up through the overlying rock to cool and harden near the surface. The hardened granite formed into large masses of rock covering hundreds of square miles, called batholiths.

Basalt Flows

About 16 million years ago, massive quantities of basalt began to flow from an area near the Idaho-Oregon border. It is estimated that over a period of about 1.5 million years, almost 41,000 cubic miles of basalt flowed to the surface, covering nearly 63,000 square miles (United States Geological Survey 2007). It is believed that the basalt flowed from the Spokane area east into the Rathdrum Prairie, then north as far as present-day Lake Pend Oreille.

At the time of the basalt flows, surface water generally flowed toward the south in the Pend Oreille area, creating the Rathdrum River. Geologists believe the Rathdrum River flowed south through the Rathdrum Prairie and parallel to what is present-day Lake Pend Oreille. As the basalt flowed northward, it dammed the flow of the Rathdrum River, ponding the water behind it to form Rathdrum Lake. Deposits of fine-grained lake sediment were formed throughout the area at this time and are referred to as the Latah Formation. Some water well reports around the Rathdrum Prairie today show silt or fine-grained sand units between layers of basalt, evidence of the successive basalt flows and damming events that took place millions of years ago.

Glacial Lake Missoula

Between 10,000 and 15,000 years ago, large ice sheets moved south into north Idaho, Washington, and Montana. As the ice moved south, it dammed the Clark Fork drainage near the Idaho-Montana border. The water began to pond behind the ice dam, eventually creating Glacial Lake Missoula. At its largest, the lake was 2,000 feet deep and contained over 500 cubic miles of water. This is about 40 times more water than Lake Pend Oreille currently contains. As Glacial Lake Missoula increased in height behind the ice dam, it put more pressure on the dam and began to push water between the ice and surrounding bedrock. Eventually, the ice dam floated upward and failed catastrophically, releasing enormous volumes of water.

During this time, Lake Missoula flooded about 40 times, with flood events occurring about every 50 years. During each flood, the wall of water would travel down the Clark Fork Valley and the Rathdrum Prairie, across eastern Washington, eventually entering the Columbia River Valley and emptying into the Pacific Ocean. The painting below depicts the flood waters flowing down the Columbia River Gorge. Each flood carried huge quantities of sand, gravel, cobbles, and boulders and deposited them along the flood path. The Rathdrum Prairie received enormous volumes of flood sediments that blocked tributaries flowing into the Rathdrum Prairie.

After the flood waters receded each time, the flood sediments remained and caused the water flowing down the tributaries to begin to pond and form lakes. These water bodies still remain in north Idaho as Spirit, Twin, Hauser, Hayden, and Coeur d'Alene Lakes. The area of eastern Washington that was flooded has significant erosional channels, along with giant potholes and large ripple marks. Because of its appearance, the area is known as the channeled scablands.

The concept of ancient catastrophic floods in the Pacific Northwest was originally introduced by J Harlen Bretz in 1923. Bretz was a geologist from the University of Chicago who spent considerable time in eastern Washington, meticulously analyzing and studying the geologic evidence indicating enormous floods. Bretz’s colleagues were highly critical of his findings, since the accepted thinking of the time explained all geological formations through very slow and gradual processes over millions of years. However, based on the geologic evidence, Bretz was convinced he was correct and would not change his position, even though he endured continued criticism and, at times, significant attacks from his critics.

One of the most important questions Bretz could not answer was where all the flood water came from. The answer finally came in 1942 when J.T. Pardee, a USGS geologist who had completed field work around Lake Missoula during the 1930s, published a paper presenting evidence of the lake and answering the question of where the water came from. Even with Pardee’s evidence, Bretz's catastrophic flood theory still was not widely accepted until the early 1960s.

The entire Rathdrum Prairie today is made up of glacial flood deposits from Lake Missoula thousands of years ago. The geology map below reflects the major geologic events in the RPA over the last billion years. The Belt Supergroup rocks are seen on both sides of the Rathdrum Prairie, along with the younger gneiss and granite rocks that pushed up along the Idaho-Washington border. The Purcell Trench cannot be seen on the land surface, but the location is projected onto the map. Basalt rocks can be seen in a number of areas around the cities of Hayden and Coeur d'Alene.