What it means
The Marcus Landslide is two things locals use interchangeably: a prehistoric collapse of the eastern McDowell Mountains, and the easy, signed trail that walks you out across what it left. The slide is a jumble of weathered granite boulders and hummocks spread across the desert below the range’s high point, the East End, with a pale breakaway scar still visible on the ridge above. To the untrained eye it looks like any other lumpy, boulder-strewn McDowell foothill. That is exactly why nobody recognized it for so long.
The trail begins at Tom’s Thumb Trailhead on 128th Street, the same lot climbers and summit hikers use for Tom’s Thumb. It runs about 1.5 miles each way on wide, gentle tread, with interpretive signs that explain the geology as you go and an optional 0.7-mile loop that climbs onto the slide mass itself. It is the rare trail where reading the signs genuinely improves the hike.
The full trail guide, with parking and timing, is the Marcus Landslide Trail page, and the man behind the name has his own page. This entry is about the word.
Where you’ll meet it
Preserve maps, the Conservancy’s geology hikes, family hike lists, and the left-hand turn at Tom’s Thumb Trailhead that most visitors miss.
Where the word came from
The landslide was recognized in 2002 by Arizona State University graduate students, who saw that the hummocky ground, oversized blocks and scar on the ridge added up to a single event. The two, John Douglass of ASU’s geography department and Brian Gootee of its geological sciences department, wrote it up with ASU geographer Ronald Dorn as “A large landslide on the urban fringe of metropolitan Phoenix, Arizona,” published in the journal Geomorphology in 2005.
In that paper they “informally named the Marcus rock avalanche in honor of former Arizona State University professor Melvin Marcus.” Marcus was a glacier and mountain geographer who taught at ASU from 1974 until he died in 1997, and he led a generation of students into the field. Naming a collapsed mountainside after a mentor is a scientist’s tribute and a decent joke at the same time. “Rock avalanche” is the precise term for what happened; “landslide” is the word that stuck on the trail signs and in conversation.
The name reached the public through the Preserve. The northern McDowell bajada came into city ownership after the Preserve tax votes of 1995 and 2004, and when the permanent Tom’s Thumb Trailhead opened, the slide sat an easy walk away. The city signed the trail, the McDowell Sonoran Conservancy began leading geology hikes, and a specialist’s field site became a family outing.
What it is not
The Marcus Landslide is not a recent or ongoing hazard, and the trail does not climb the scar. Nor is it Tom’s Thumb, the granite pinnacle above the same trailhead; the two routes leave the lot in opposite directions.
The fine print
The paper’s numbers are the firmest ones available. It describes a granitic rock avalanche of about 5.25 million cubic meters, a little under half a kilometer wide, that ran roughly a kilometer from the mountain front, with lateral levees and pressure ridges typical of the long-runout slides geologists call sturzstroms. The slab failed along a major joint in the granite dipping about 44 degrees, probably during a wetter period than today’s, when downcutting in the Salt River system undermined the base of the slope.
The age comes from desert varnish. The researchers read the microscopic layers in the dark coating that has built up on the boulders since they stopped moving and, with other landform evidence, concluded the deposit is slightly older than about 500,000 years.
Figures elsewhere vary. The Conservancy calls it the second-largest landslide in Arizona; the paper is more cautious, calling it one of the largest Quaternary landslides in the state outside the Grand Canyon. Interpretive materials and the site’s guide describe a runout of about a mile from the breakaway, larger than the paper’s measurement along the deposit. We give the published figures where they differ.
The paper’s closing argument is the part worth remembering on the trail: a slide this large, unrecognized beside a growing metro, is a reason to study landslide hazards wherever development cuts into the base of steep granite slopes.