Monday, January 31, 2011

A Look at the Marcus Landslide in the McDowell Mountains of Arizona

For those aware of the special appeal in "seeing" long-gone events and the power of geologic observation in resurrecting such events, the Marcus landslide is a truly wonderful story. Imagine being a young geology student out on a spring field trip with your professors and other researchers. The day drags on as a few of the "big-wigs" co-opt an otherwise valued discussion, endlessly debating some arcane or miniscule detail. Predictably, you wander off away from the group with your best friend and ultimately stop to enjoy lunch.

Image courtesy of AZGS
With the wide expanse of the Sonoran Desert as your backdrop, you suddenly look up and notice an unusual and  vibrant concentration of golden brittlebush, back lit perfectly on the slopes of the McDowell Mountains outside Scottsdale, Arizona. Other parts of the range do not have this density of flowers and something appears unusual. You begin to also notice clusters of large granite boulders that seem to trend up the slope towards the colorful brittlebush. A lively exchange of ideas ensues and soon the light clicks on inside both of you - you have just detected the initial evidence for a giant Ice Age landslide that occurred here. No one else has ever seen this before!

Such is a description of how Brian Gootee and John Douglass discovered the Marcus landslide in 2002. (The feature was named in honor of the late
Melvin Marcus Professor of Geography at ASU). Gootee (now with the Arizona Geological Survey) and Douglass (now a professor at Paradise Valley Community College) completed subsequent research on the evidence left by this humongous slope failure. Admittedly subtle at first, the evidence is convincing that something big happened here about 500,000 years ago.
Aerial view of the Marcus landslide (AZGS)
View of the east side of the McDowell Mts. (AZGS)
Digital elevation image of the slide (AZGS)


I was fortunate to accompany my good friend Brian Gootee on a field trip to the Marcus landslide in January of this year. Accompanying us were his wife and duaghter, Rachel and Zoe, and my wife, Helen. Below are some of the photos I took on this glorious winter's day in the desert.

This is the east side of the McDowell Mountains looking south. Fortunately, this patch of desert is protected by the City of Scottsdale and Maricopa County in two nature preserves. The city and county are working closely with Brian Gootee to develop an interpretive trail to the slide.

A closer look to the south reveals a famous Arizona landmark in the Superstition Mountains - Weavers Needle. According to legend, the Lost Dutchman's gold mine was located within sight of this feature, meaning that the treasure could lie among the boulders in this slide. However, most of the people who believe in this legend scratch and scour the Superstition area.

As we hiked to the south we began to see the slope of the collapsed debris pile. You can see it in the middle distance between the sunlit foreground and the shaded slope of the mountain range.

Helen listens to Brian as he gives an excellent description of the features present and the sequence of observations that led him and colleague John Douglass to the conclusion that a giant slide mass is present here.

These are some of the granite boulders that are located on the north levee of the slide. This material likely originated high up on the slopes of the McDowell Mountain in a rock failure that was activated along joints (fractures) in the granite. It was then entrained with other smaller grained components into a debris flow that traveled almost one mile in length and 1/3 of a mile in width.

Detailed mapping has shown that the tilted boulder (above) is part of the slide material, but the granite it rests on is in place and not part of the slide. The evidence will be shown below.

Looking southeast across the Marcus landslide towards the Superstition Mountains.

Here is a close-up (with lens cap for scale) of the clay and sand size material that makes up the matrix of the slide mass. As it came down the slope of the McDowell's, at an estimated speed of 16 to 44 miles per hour, the huge boulders would have been "cushioned" by this fine material.

Here is a view of the eastern terminus of the slide. Note that the boulders are not found farther out in the valley and outboard of the slide front. One of the boulders at this location is where Gootee and Douglass shared lunch on a field trip in the spring of 2002 and first pondered the possibility of a huge slide here. And if you are still wondering what the heck "an unusual concentration of brittlebush flowers" had to do with their discovery, note that the scar left on the side of the mountain front from the slide left the perfect substrate for the enhanced growth of brittlebush. They just happened to be in the right place at the right time of day and year!
 
Here is a close-up of the mega-crystic granite that forms the backbone of the McDowell Mountains and was caught up in the Marcus landslide. The quartz crystals are quite durable chemically but the feldspars weather more rapidly and certainly helped to facilitate the slide. Remember, this all happened just prior to 500,000 years ago (a date based on rock varnish micro-laminations) during the Ice Age, when Arizona was much wetter and cooler. These conditions certainly played a big role in the development of the slide.

How did Gootee and Douglass determine what granite was involved in the slide and what granite remained in place as bedrock. Look closely at the granite above and notice that the upper boulder has no joints in it, while the lower granite has at least three joints running through it. As the granite came down the slope catastrophically, it would have broken into its constituent pieces along joints. So the material arrived here already "chopped up". The lower granite wasn't involved in the slide and so still shows its obvious jointed nature.

A nice view of the slide front to the south - it is highlighted by the shade of the late afternoon sun. The timing of my visit did not allow for me to take pictures of the pocket or headwall area of the slide up in the mountains - it was looking right into the late afternoon sun.

A zoom view of the immense fountain in Fountain Hills, which can be seen during the first 15 minutes of every hour.

Please read Brian Gootee's official description of the Marcus landslide here on the AZGS official web site. You can also go on a virtual field trip to the slide on John Douglass' web site here. I can highly recommend both sites to learn more about this fascinating discovery.

If you would like to visit the Marcus landslide be aware that there are no signs yet, nor official trails to it. Use your ingenuity to find the trails on the east side of the McDowell Mountains that approach Tom's Thumb. Walk south from there and keep your eyes open for a train of debris that looks anomalously strange.

2 comments:

Lucy (aka rharper) said...

What a great blog you have!

Tom Mcmaster said...

Excellent post, hiked there this morning, what a beautiful place. Thanks for the insight