The Marcus landslide, one of the largest such features in Arizona, is described on a new webpage on the AZGS Phoenix office site by AZGS geologist Brian Gootee.
Brian writes that about 500,000 years ago, a portion of the east-central summit of the McDowell Mountains about 20 miles northeast of Phoenix, suddenly collapsed into a catastrophic rock avalanche, moving an estimated 16-44 miles per hour downslope, carrying 5.5 million cubic meters of rock and soil. The energy released was roughly equivalent to that of an atomic bomb.
The slide was only discovered in 2002 by Dr. John Douglass and Dr. Brian Gootee.
There is also a fantastic virtual field trip to the Marcus landslide in the McDowell Mountain Regional Park put together by John Douglass of Paradise Valley Community College. Links to this and more photos are on the AZGS Phoenix office page.