Granite Boulder in the Scablands: Mountain Top or Glacial Erratic?

Due to the ice age floods, when a large granitic rock is found near the Spokane Airport it can be hard to determine if it is an old mountain top surrounded by younger geological features or rafted in on glaciers during the ice age floods. This may seem like an academic question, but understanding the way groundwater works in the arid West Plains or Spokane County is important to all. The deposits storing water in this region are generally composed of two types of materials: 1) the 12,000 to 16,000 year old gravel and sand deposits from the floods and 2) the approximately 15 million year old Columbia River Basalts. Hamilton and others (2004), mapped some small (approximately 60’ diameter) granitic features on the surface near Hayford road southwest of the Spokane International Airport as basement exposures. This puts constrictions on groundwater flow as granite is not readily transmissible for water.

In order to better understand the local geology and its effects on ground water, the Spokane County Water Resources decided to drill a series of small cores near several of the granite exposures in the region. This allows us to get a glimpse of the subsurface and determined whether these granitic rocks protrude upward from the basement or are from some other form of emplacement. The borings encountered basalt just beneath the granite boulders (GSI, 2014). This indicates that these large granite boulders were deposited much more recently, and most importantly, they probably do not impact groundwater hydrology of the area.

This goes to show you the picture science paints is always adapting to new knowledge. Issues like this one can have a significant impact on groundwater and painting the picture as clear as possible helps government agencies to properly allocate resources. Projects like this are a wonderful example of the Spokane County government trying to understand the resources around them and aiding in the discovery of science.

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