Huge tidal waves of water once went through the very area were you standing - destroying everything in its path! The rushing water swept over the area at 60-80 miles an hour! The flooding happened 15,000 years ago - perhaps up to 100 times for the next 5,000 years, when the Clark Fork ice dam holding back the ancient Lake Missoula would break over and over and over again. (1)
Post Falls is about 30 miles downstream from the Clark Fork ice dam and 15 miles from the end of Lake Pend Oreille, where most of the flood waters were channeled. The Lake Missoula Floods charged southwestward down the Rathdrum Prairie toward Post Falls, and into the Spokane River in eastern Washington. (2)
These great floods wore away at the sediments and exposed some of Earth’s oldest rocks:
The Precambrian era spans about 88 percent of Earth history, beginning with the formation of Earth about 4.5 billion years ago and ending 540 million years ago with the diversification of life that marks the start of the Paleozoic era. It is the least understood span of Earth's history because most Precambrian rocks are buried from view.(3)
You are standing on some of the oldest rocks on Earth! The bedrock is a mixture of of very hard stone (gneiss, schist, and granite) that were formed by intense heat and pressure over BILLIONS of years.
There is a Park Geology informational sign at the listed coordinates (4). Nearby (to the left) is a steep cliff overlook. You can use this page, the sign, and the nearby rock formations to answer these questions:
Questions:
- The bedrock at this location is made from what three types of rock?
- This bedrock was formed by what?
- “They are some of the oldest rocks in Idaho.” What period in geological history were these rocks formed?
- As you watch the water flow through this gorge why do you think that these cliffs were left after the Lake Missoula Floods charged through this area?