Geologic History

Geologic history (photo)The region around Lake Sullivan has come about as a result of several geologic processes. In order from oldest to youngest:

  1. The oldest period of geologic history having a relevant effect on the rocks, topography and the minerals found in the area is the tropical marine environment the area enjoyed during the Precambrian Eon and into the Early Cambrian Period (see stratigraphic section below) of the Phanerozoic Eon. This is when the North American Continent rifted apart, creating a new coastline. The rocks that can be used to prove the geologic setting are phyllites of the Maitlen Formation (Alt, Hyndman, 1986). They date to around Early Cambrian or around 540-500 MY ago. Within this formation are phyllite rocks, which dominated the samples found at every site we visited.
  2. During the Jurassic Period, the exotic terrane Quesnellia (also called the Intermountain Terranes microcontinent) crashed into the North American continent (Alt, Hyndman, 1995). Most people don't realize that the coast of North America during the Jurassic Period - yes, the same period that the dinosaurs were roaming the lands and seas - was as far east as Idaho in places. The site Spokane now occupies was then on a westward-reaching portion of the continent.
    1. The crashing in of the Quesnellia terrane created an uplift region in Washington and western Idaho called the Kootenay Arc. This is a "belt of rock jammed hard into the former western edge of the continent..." (Joseph, 1990). This "belt" is highly deformed and contains many thrust faults. This is the first of the uplifts that affected the Lake Sullivan area.
    2. There were a several collisions from 570 to 100 million years ago between the advancing North American continent and a few subcontinents. The next major accretion was the North Cascades subcontinent (Orr, Orr, 1996). The Insular Terrane is the succeeding terrane that had an effect on the region.
    3. The collision with these exotic terranes caused not just uplift but the continuous pressure resulting in folding and faulting of the Earth's surface. This is how rolling hills and most mountain ranges are made. This small mountain range paved the way for glaciers to scour the landscape when they came through.
  3. The third major event affecting the Lake Sullivan area was the Pleistocene ice age continental glaciation.
geologic history (diagram)
Stratigraphic section of rocks in the Lake Sullivan area