Lewis and Clark AAPG GeoTour
slated for July 12-17

Some sites along the tour. Photos provided by Bill Hansen, Jireh Consulting Services.

Click on picture to get a closer look.

On entering the stretch of the river known as the Gates of the Mountains, Meriweather Lewis writes on July 19, 1805:

"This evening we entered much the most remarkable cliffs that we have yet seen. These cliffs rise from the water’s edge on either side perpendicular to the height of 1200 feet. Every object wears a dark and gloomy aspect. The towering and projecting rocks in many places seem ready to tumble on us. The river appears to have forced it’s way through this immense body of solid rock for the distance of 5 3/4 miles and where it makes its exit below has thrown on either side vast columns of rocks mountains high.

"The river appears to have worn a passage just the width of its channel or 150 yards. It is deep from side to side, nor is there in the first three miles of this distance a spot except one of a few yards in extent on which a man could rest the sole of his foot. Several fine springs burst out at the water’s edge from the interstices of the rocks. It happens fortunately that although the current is strong it is not so much so but what it may be overcome with the oars, for there is here no possibility of using either the cord or setting pole. It was late in the evening before I entered this place and was obliged to continue my route until sometime after dark before I found a place sufficiently large to encamp my small party. At length, such an one occurred on the larboard side where we found plenty of lightwood and pitch pine. This rock is a black granite below and appears to be much lighter color above and from the fragments I take it to be a flint of a yellowish brown and light creamy-colored yellow. >From the singular appearance of this place I called it the gates of the rocky mountains."

The rocks Lewis is describing ( shown left and right) are the Mississippian Madison Limestones exposed in the footwall of the Eldorado-Lewis Thrust system in the Montana Thrust Belt. Farther upstream, the Geotour will also examine the Precambrian Belt Series rocks thrusted over the Madison and exposed in the hanging wall of the thrust. Lewis describes the limestones as being dark and foreboding, which is probably a result of his seeing the rocks initially in the deep canyon in the early evening after a rain storm, which tends to make the rocks look darker.