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?
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. If you saw my previous NH Note. The Exhumed Paleochannels of Utah and Mars, then you have a good clue but these flat tops are not the fossilized remains of an ancient river bed. Rather these flat tops are the remains 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 these valleys now being inverted and found above the surrounding landscape. The residents of St. George Utah now can literally look up and see what had been the bottom of a former valley and where they are standing had once been higher ground.
Below is an image of some of the remnants of lava flows that lie to the north of St. George. The black rock is the basalt which lies on top of the red sandstones.