Geology of the Ironman St. George 70.3 

The St. George 70.3 serves as the North American Half Ironman Championship. The course is a great place to enjoy the colorful and striking geology of the Colorado Plateau.  The geology of the Ironman St. George 70.3 highlights Triassic and Jurassic sedimentary rocks, and young basalts.

The triathlon includes a 1 mile swim, 55 mile bike, and 13.1 mile run in the vicinity of St. George, Utah.

Annotated geologic map of the Ironman St. George 70.3 course on a geological map.
This is just a small section of the geologic map of the St. George and East Part of the Clover Mountains. Each color and pattern represents rock units of different ages and types. The approximate course is marked on the map. Map source: Utah Geological Survey.


The swim portion of the Ironman St. George 70.3 takes place in the Sand Hollow State Park Reservoir. The reservoir sits in a bowl of Navajo Sandstone . Gravels and other Quaternary age surface deposits overlie this bedrock for much of this valley. On the geologic map, this makes the reservoir look surrounded by yellow speckled markings – a geologists way of showing that gravels are at the surface. To look at the rocks in cross section, or to think about taking a slice into the earth, this is what the rock layers beneath the town of St. George look like. 

Directly to the east of the reservoir are flat-topped hills. The hills are topped with basalts that erupted between 2.3 million and 20,000 years ago. This feature is very common common throughout the area. 

Google maps satellite image of the st. George municipal airport built on a basalt flow.
The long flows of basalt make very stable surfaces – so flat and stable that the St. George Municipal Airport is built on one! (Image: Google Maps)

Interestingly, these hills are what are called “Inverted Topography.” By this, geologists mean the landscape in which the hills exist have been flipped. To start, basalt is a highly fluid lava, and after erupting from a volcano, it flows downhill, particularly liking river and stream beds. The basalt lava then cools. Because basalt is more resistant to erosion than the sandstone that surrounded it, after many years of erosion, all the surrounding sandstone has been eroded away, and the basalt – once filling the lowest parts of the landscape now is one of the tallest landscape features. 

The steps to create "inverted topography" shown in three sketch diagrams.
A diagram the formation“Inverted Topography”. We see examples of this type of topography all around St. George. Source: NPS.


The bike course will take athletes around the south end of the reservoir, after which athletes pedal north through the basalt ridges (mile 10). Cyclists pass a number of volcanic eruption centers before reaching West State Street. Miles 12-15 take you over basalt ridges and flows, visible as jagged outcroppings of black rock. 

Geology of the Ironman St. George 70.3 continues as you head west, and cross through a ridge of Triassic-aged sedimentary units: the upper parts of the Moenkopi Formation and lower Chinle Formation near mile 20. These red and tan beds look blocky to crumbly, and tilt (or “dip”) to the southwest. The Moenkope and Chinle Formations were deposited in tidal flats and braided river or stream/floodplain environments. Long basalt-capped ridges are visible, protecting those soft sedimentary units beneath.

You will cross a the Washington Fault Zone around mile 22-23. A fault zone is where two sides of crustal rock have either been pushed toward or pulled away from each other. Here, we see the Jurassic and Triassic rocks at the top. Older rocks are deep below the surface, all the way down to Precambrian bedrock shown at the very bottom in brown. Geologists map faults and fault zones to show where and how the earth’s crust has moved in the past. In this view, we can see that the Washington fault zone extends way down through all the layers of rock. Geologists show which way the rocks move relative to each other with little arrows.

Geologic cross section of the St. George region.
This is a small part of the geological cross sections that are part of the St George geological map. A cross section is like a “slice” into the earth along a certain line. Source: Utah Geological Survey.

You will stay in the red beds of the Moenkopi Formation as you continue west. At about mile 30 athletes cycle through the Quaternary alluvium-filled valley that the city of St. George is built upon.

The Jurassic age Kayenta and Navajo Sandstone Formations are the dominant sandstone units to the north of town. The Kayenta Formation were deposited in river, playas, dune field settings. These variable settings are seen today as differences in rock from conglomerates, siltstones, sandstone, and mudstones. The Navajo Sandstone records a much different landscape: a massive eolian (wind-blown) dune field. The Navajo Sandstone literally are petrified sand dunes! In places, you can see the curving cross-bedding of the dune shapes.

To the southwest at mile 35, we see our Triassic units, the Moenkopi and Chinle. To the north/northeast, the landscape is dominated by basalt flows that overlie older sedimentary units. 

Heading west at Center Street, leading up to mile 40, the tall red, layered cliffs are the Kayenta Formation, with Navajo Sandstone overlying these lower units. Basalt is visible in the valley floors.

The southward return towards the cycling-run transition takes you past the long, curving basalt-capped mesas and back into St George. The city is built on young gravels, called “Quaternary” gravels, meaningi they were brought here by erosion from higher topographic areas in the past 2 million years. Directly west of the bike-run transition zone is yet another tall basalt-capped mesa. The basalt here is 2-14 million years old, adn sits on the Jurassic Kayenta Formation, which peaks out in as red sandstone ledges below the basalt. 

Geology of St George: red beds of the Moenkopi Formation overlain by Chinle Formation.
The red beds of the Moenkopi Formation are below the Chinle Formation. You can see the muddy, softer sediments in the Moenkopi result in it being more crumbly and erodible. Photo: Rick Miller.


The run course of the Geology of the Ironman St. George 70.3  takes you north across the valley fill of St. George and up into the foothills of the tilted sedimentary units. Each loop takes the runner progressively through the geologic section. The runner progresses from Quaternary sediments through the red Kayenta up onto the Navajo Sandstone, and returns back down into the young, Quaternary sediments. 

The finish brings the athlete back to the city itself. The St. George triathlon takes you through some of the greatest geology on the Colorado Plateau. I hope you enjoyed it!

Good luck to all triathletes and spectators!

This is a brief overview of the geology of the Ironman St. George 70.3 – do you have any other questions or thoughts?

What questions do you have about the geology of where you run, bike, or swim?

For more great reading about specific details of the geology of the St. George region, check out the Our Geological Wonderland series by Rick Miller.

4 thoughts on “Geology of the Ironman St. George 70.3 

  1. leowatchman

    Fascinating! The geology of the Ironman St. George 70.3 course is truly remarkable. The variety of rock formations, from the Triassic and Jurassic sedimentary rocks to the young basalts, creates a visually stunning backdrop for the race. It’s amazing how the landscape has been shaped over millions of years. Best of luck to all the triathletes taking part in this unique and geologically rich event!

    1. magdalenadonahue

      It is a fascinating landscape, and a lovely one to experience up close during the triathlon!

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