The Valley of the Sun is home to many interesting rock formations forged long ago by heat, cold, water, pressure, wind and weathering. One unique area is on the east side of the McDowell Mountains where you can climb into the middle of an ancient landslide known as the Marcus Landslide, which also happens to be the second largest landslide in Arizona.
Boulders the size of a small house dot the landscape, haphazardly situated in the path of the slide. Tracing the jumble of boulders up the slope, it is easy to spot the concave basin where the rocks originally broke away from the mountain. The break away scar begins almost at the top of the mountain, where rocks and debris broke off and fell down a steep 300 foot fall zone. The momentum, coupled with the finer matrix material beneath, allowed the boulders to slide an additional 4,000 feet to the east.
One of the most notable boulders in the area is "submarine rock," aptly named by those who originally studied the landslide. Erosion has hollowed out a little open grotto at the base of the rock, accessible by a short trail and a great place to stop for lunch or a snack. Many of the boulders in this particular area are cylindrical in shape and are lying in an east-west direction. Scientists still do not understand how the boulders lined up during the slide.
The Marcus Landslide trail starts at the Tom's Thumb Trailhead in the McDowell Sonoran Preserve in Scottsdale. It is a relatively flat trail, sloping southeast past a magnificent stand of chain fruit cholla and many interpretive signs about the interesting geology of the area. At about 1.5 miles, a .7 mile loop trail climbs into the heart of the landslide. Be sure to take plenty of time to complete the 4 mile roundtrip trek and take lots of water as there is no water at the trailhead. As always, good shoes with good traction are a must.
Exploring this area will trigger a land slide of wonder and questions about geologic processes.
Questions for Young Trackers
- What are these rocks made of? Study the composition up close.
- What caused the strange formations? Why do some rocks look like mushrooms?
- Why do you think such a huge chunk of rock broke off the mountain? What would cause that to happen?
- Why did the geology researchers name this rock "submarine rock?" Can you think of names for other rocks in the area?
- Why is there a crack in submarine rock?
- Why are some boulders so large? What kept them from breaking apart during the landslide?
- Can you make up a mythical story about the geological events that happened here?
References and Resources
Informational signs on the trail
Images: Sharon Pegany
Images: Sharon Pegany
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