Much of the Pend Oreille valley consists of relatively ancient metamorphic rock, uplifted over 500 million years ago from the former floor of the Precambrian sea that covered the region during that period.
In the previous Ice Age, a massive glacier of the Cordilleran Ice Sheet advanced southwards through the Idaho Panhandle, burying the Lake Pend Oreille and upper Pend Oreille River valley with ice hundreds to thousands of feet thick.