You would think that the rocks along the California Trail would look exactly the same to us today as they did to the emigrants, but that’s not true in the case of Chimney Rock! Based on diagrams and photographs we have from the time, it looks as if some kind of force—erosion or a lightning strike, perhaps—reduced the spire from its earlier height to its current measurement of 300 feet.
Chimney Rock, located at the western edge of modern-day Nebraska, marked a big change for the emigrants as they went from the mud and grass of the Platte River valley into the western half of their journey—a journey that would be full of remarkable rock formations. Up until Chimney Rock, the landmarks described in the various emigration guidebooks were water features, or trees. From Chimney Rock onwards, they tended to be remarkable rock formations. Perhaps for this reason, a survey of over 300 emigrant diaries revealed that Chimney Rock was the most-mentioned landmark on the trail.
Chimney Rock is a good example of what geologists call “differential erosion.” There is a layer of hard rock called a “cap rock” that sits on top of softer rocks and protects them from eroding away as quickly as they would without the cap rock. The cap rock on Chimney Rock is a 25 million year old sandstone, and the older layers underneath are clays and volcanic ash particles that were piled on top of each other over millions of years of volcanic eruptions nearby. Differential erosion is responsible for many strange rock features along the trail. Keep an eye out during your travels—you might see some!