Fossilized Valleys? Ancient Lava Flows and Inverted Valleys in Utah

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.

Google maps screenshot of St. George UT. There are three flat topped hills here in this area one of which has been used as an airport. Google and the Google logo are registered trademarks and are used here with permission.
Google maps screenshot of St. George UT. There are three flat-topped hills here in this area one of which has been used as an airport. Google and the Google logo are registered trademarks and are used here with permission.

Below is a Google Streetview image from the airport at the edge of the hill with the city of St. George below.

A view from on top of the lava-capped hill overlooking St. George. This is from the airport road. You can see the dark basalt rock that forms the upper layers of the hill. Google and the Google logo are trademarks of Google Inc and used with permission.
A view from on top of the lava-capped hill overlooking St. George. This is from the airport road. You can see the dark basalt rock that forms the upper layers of the hill. Google and the Google logo are trademarks of Google Inc and used with permission.

Below is a Google Streetview image from the base of a flat-topped hill to the east of downtown St. George.

This is a view of one of the lava-capped plateaus on the east side of town. Again, you can see the dark rim of hardened lava (basalt) right at the top. Google and the Google logo are trademarks of Google Inc and use with permission.
This is a view of one of the lava-capped plateaus on the east side of town. Again, you can see the dark rim of hardened lava (basalt) right at the top. Google and the Google logo are trademarks of Google Inc and use with permission.
Former valley floor covered with lava is now a flat topped hill with an airport.
Former valley floor covered with lava is now a flat topped hill with an airport.

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.

This is a large overview of the St. George area showing the source of the lava flows. I have outlined the major lava fields that still remain today. The lava came from up north were there is a large moutain with massive lava fields. Google and the Google logo are registered trademarks of Google Inc and used with permission.
This is a large overview of the St. George area showing the source of the lava flows. I have outlined the major lava fields that still remain today. The lava came from further north were there is a large mountain with massive lava fields. Google and the Google logo are registered trademarks of Google Inc and used with permission.

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.

A bit north of St. George this is what these lava fields look like. Here it is clear that the lava (black basalt) are "poured" out over the red sandstones that are so common in this part of Utah. Google and the Google logo are registered trademarks of Google Inc and use with permission.
A bit north of St. George we can see what some more modern lava fields look like. Here it is clear that the lava (black basalt) are “poured” out over the red sandstones that are so common in this part of Utah. Google and the Google logo are registered trademarks of Google Inc and use with permission.

This is an updated version of a post first published in 2013.

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