The Kootenay Arc is located in northeast Washington and southern British Columbia. During the Paleozoic, this area was a coastal plain and continental shelf but now it is a belt of tightly folded Precambrian and Paleozoic sedimentary rocks. How did this area begin this way? Approximately 900 to 700 million years ago, there was a split known as the continental rifting event of Rhodinia in the continent from the north to south of eastern Washington. We are not sure where the old piece of continental crust is located. We do know that the oldest rocks are of the ancient continental crust consists of granite, gneiss and schist, aging over two billions years ago and are formed in the northern Idaho. How was the Kootenay Arc formed?
There was another continental split that separated North America from Europe and Africa along what is now the middle of the Atlantic Ocean. The split marked the breakup of the ancient Supercontinent, Pangaea. The North American continent began to move west over the floor of the ancient ocean that sank along the western edge of the continent, which is now eastern Washington. Therefore, the Kootenay Arc formed by a collision between the continent and the floor of the ancient Ocean jamming against the old continental shelf.
The folds seen in the Kootenay Arc are from the telescoped layering of sedimentary rocks. They formed as the oceanic crust began to slide into the trench and crushed the continental shelf into the edge of the continent. What is observed is that many of the layers of rocks stand nearly vertical. If these folds were laid out flat it would stretch out like an accordion into a quite respectable coastal plain (Alt, Hyndman, 1984, ref. 1). Then one might ask, why mine there?
Much younger granite intruded both the old continental crust and the ancient sedimentary rocks about one billion years ago. The basalt that was there began to rise above the slab of ocean floor that sank into the trenches in eastern Washington. During the Cretaceous Period, the magma crystallized so that it consisted mostly of granite and granodiorite. Younger granite intruded the ancient continental crustal rocks about 100 million years ago. The granite magma brought uranium, which was mined in the Kootenay Arc area. There was hope in discovering silver in the mines but it only produced enough lead and zinc to keep in the mine running.