How does a hill get a flat top? The picture below is a Google Maps satellite image of St. George Utah. To the left you can see an airport. This airport is on top of a long hill whose top is well over 100 feet above the city below. The airport was built here because this hill very conveniently had a very flat top. To the left of the airport and to the far right there are two other naturally flat-topped hills which are even higher. What is the origin of these strange features?
If you prefer not to read this post I have put together a 15 minute YouTube presentation that explores the information on this and two other related posts.
Below is a Google Streetview image from the airport at the edge of the hill with the city of St. George below.
Below is a Google Streetview image from the base of a flat-topped hill to the east of downtown St. George.
So how did these hills get their tops chopped off? I’ve written about a similar feature elsewhere in Utah (Fossilized River? The Exhumed Paleochannels of Utah). But these flat top hills are not the fossilized remains of an ancient river bed as we saw further north in Utah. Rather the flat tops are what is left of lava flows that once filled ancient valleys in the area. After the lava filled the valley it hardened into basalt. Later, the whole area experienced a long period of erosion. The hard basalt cap prevented the rock under these ancient lava-filled valleys from eroding resulting in what was once a valley floor now being above the surrounding landscape. The residents of St. George Utah can literally look up and see what had been the bottom of a former valley.
Similar flat-topped hills are found north of St. George, UT. The black rock is the basalt which lies on top of the red sandstones.
In our next post we will explore what these preserved valleys tell us about age of these landscapes.
This is an updated version of a post first published in 2013.