by Mark Milligan

Geologic information: The St. George area of southern Utah exhibits many classic geologic features of the Colorado Plateau, including flat-lying layers of red sedimentary rock carved into buttes, mesas, and narrow canyons.

However, unlike the rest of the Colorado Plateau, some of the small (less than 1,000 feet high) isolated bluffs in the St. George area are capped by black lava rock, called basalt.

Approximately 1.2 million years ago, lava flowed down a small valley or canyon, that today is the basalt-capped bluff of the St. George Municipal Airport. View looking south.Photo courtesy of the Civil Air Patrol.

Examples include the bluff above Bluff Street (West Black Ridge, the location of the St. George Municipal Airport) and the bluff behind the outlet mall directly east of I-15 at Exit 8 (Black Ridge).

The basalt on top of these bluffs originated as lava that intermittently flowed from small local volcanoes approximately 2.3 million to 20,000 years ago.

Like other liquids, the basalt would flow downhill and into low areas such as stream beds, valleys, and canyons. This presents a puzzle: how does lava flowing downhill end up on hilltops?

1. Former canyon in layered sedimentary rocks.
2. Basalt flows down bottom of canyon.
3. Erosion removes former canyon walls, leaving resistant basalt flow topographically higher.

Unless it defied gravity, the lava should have flowed to low areas, not on bluff tops. And it did; the basalt that caps today's bluffs was originally deposited in canyons and channels.

However, basalt is more resistant to erosion than the surrounding red sandstone and shale. Over time, erosion lowers the surrounding red rock faster than the basalt, leaving the basalt flows high and dry – inverted topography.

How to get there: The bluffs in St. George above Bluff Street and behind the outlet mall are great examples of inverted topography. However, one of the best places to see that the lava originally flowed down streambeds or channels is in a road cut near Hurricane. From I-15 take the Hurricane exit (16) and travel east on State Route 9 for approximately 4 miles. The channel is exposed on the south side of the road before you reach Hurricane.

State Route 9, west of Hurricane, looking south at a road cut exposing a basalt-capped bluff. The red sandstone and shale on the left and right of the photo contained a channel that filled with lava approximately 350,000 years ago. This lava flowed from Volcano Mountain, southwest of Hurricane. Since the basalt is harder and more resistant to erosion than the sandstone and shale, it now stands out in relief as a bluff. What was low is now high - inverted topography. If you look closely you can find some channel-bottom gravel and pieces of sandstone that were trapped in the lava as it flowed.

Geosights article, Survey Notes, v. 34 no. 3, September 2002