The Geology of the McDowell Mountains: A Field Guide for Hikers

The McDowells look like one brown wall from Scottsdale Road. They aren’t. Walk from Tom’s Thumb to Bell Pass to Lost Dog Wash and you cross three different kinds of rock, laid down hundreds of millions of years apart, and the best recent work on them was done by retirees with hand lenses. Here’s how to read the range from the trail, what the volunteer surveys found, and where to get the maps, which are free.

Three ranges in one

The city’s Cultural Resource Master Plan, summarizing Arrowsmith and Péwé (1999), splits the mountains into three parts:

• The north end is weathered granite, “most evident” at Tom’s Thumb. This is the rock of the rounded towers, the boulder piles and the climbing walls.
• The middle, between Windgate Pass and Bell Pass, is metamorphic: rock that was buried, cooked and squeezed until its layers bent.
• The south end is volcanic and sedimentary rock.

North of the mountains is a second landscape, a broad pediment sloping away to the southwest and southeast, studded with low granite mountains (Fraesfield, Granite, Pinnacle Peak) and smaller mesas (Brown’s, Cholla, Cone). In all, the Preserve runs from 1,690 feet at its southern boundary to 4,059 feet on East End, per the city’s 2024 Ecological Resource Plan.

The timeline, short version

Pieced together from two Arizona Geological Survey reports done with Conservancy volunteers (CR-10-E in 2010 and OFR-15-03 in 2015), with ages in millions of years ago (Ma):

1. About 1,750 to 1,650 Ma. Sand piles up and is buried. It will become quartzite.
2. About 1,700 Ma. Volcanoes along the edge of an ancient continent pour out thick flows of rhyolite, a silica-rich lava. These are probably part of the Red Rock Group mapped elsewhere in central Arizona.
3. About 1,650 to 1,600 Ma. A mountain-building collision, the Mazatzal Orogeny, buries the rhyolite 6 to 12.5 miles down. Heat and pressure fold it and turn it into metarhyolite.
4. About 1,425 Ma. Granite intrudes. The big body to the north is the Carefree or Pinnacle Peak granite, dated to 1,422.5 ± 2.2 Ma. The same coarse granite runs from Tom’s Thumb north across the piedmont toward Cave Creek.
5. After 1,425 Ma. Milky quartz veins form. OFR-15-03 puts them after the granite, possibly tied to the Grenville Orogeny around 1,200 to 1,100 Ma, though that is flagged as uncertain.
6. 541 to 65 Ma. Sedimentary layers like those of the Colorado Plateau were probably laid across the area, then stripped off during the Laramide uplift (80 to 40 Ma). None survive here.
7. About 22 to 10 Ma. Basin and Range stretching pulls the crust apart, and the McDowells rise as one of the blocks.
8. The last 5 million years. The basins fill and drain into the Verde, Salt and Gila systems, carving today’s washes. About half a million years ago, part of East End’s northeast face collapsed into the Marcus Landslide.

So the “1.7 billion years” you’ll see quoted for the McDowells is the age of the volcanic rock. The granite is a few hundred million years younger.

What to look at from the trail

Granite at the north end. On the Tom’s Thumb and Marcus Landslide trails, notice how the granite rots into loose crystal-sized gravel, which geologists call grus. That’s the decomposed-granite tread under your shoes and the reason the Tom’s Thumb descent feels like ball bearings. The towers are what’s left when the grus washes away. The Marcus Landslide signs are the best interpretive geology in the Preserve; read them.

Bent rock in the middle. On the Windgate and Bell Pass loop you’re in the metamorphic core, below Drinkwater Peak and McDowell Peak. The Survey’s 2018 geologic map of the range names structures around you: the Bell Pass anticline, the Windgate Pass fault and the Thompson Peak thrust fault.

Quartz everywhere. White veins of milky quartz run through all parts of the Preserve. The Quartz Trailhead takes its name from them. Quartz resists weathering much better than granite, so where a big quartz body sits in granite, the granite wears down around it and leaves a cone-shaped hill with knobs of white quartz on top. Once you see one, you’ll see them all over the north pediment.

The southwest foothills. Around Lost Dog Wash the dominant rock is the metarhyolite: gray-black and greenish, often finely layered, with reddish stains where iron-bearing minerals oxidized.

The volunteers who rewrote the map

In 2008 a few Conservancy stewards looked at rock samples displayed in a Conservancy class, labeled as quartzite, and decided they didn’t look like quartzite. One of them sent photos to Brian Gootee, a research geologist at the Arizona Geological Survey who had co-discovered the Marcus Landslide as a graduate student. He agreed they probably weren’t quartzite and guessed greenstone, an altered basalt. That guess launched a permitted survey of the southwestern foothills: about 300 recorded bedrock observations by volunteer teams with loupes, GPS units, compasses and rock hammers, eight field visits by Gootee, and thin sections cut for the microscope.

The thin sections settled it. Basalt is iron-rich and looks dark under the microscope; these slides were almost transparent with quartz. The rock was metarhyolite, not greenstone, which, the report notes, is consistent with a 1979 description by Welsch and Péwé. The project leaders, Dan Gruber, Larry Levy, Joni Millavec and Bill Ruppert, published it with Gootee as AZGS Contributed Report CR-10-E in 2010. The editor’s note calls it a model of citizen science. The report is also honest about its method: it tells the story phase by phase, wrong guesses included.

The limestone that shouldn’t be there

On a field trip in late January 2009, Gootee looked again at a whitish outcrop the volunteers had seen before and written off as caliche, the crusty calcium carbonate that forms in desert soil everywhere around here. Acid made it fizz, as caliche would. But it had layers of chert in it, some weathered orange, and odd little tubes. Thin sections showed it was travertine, a limestone deposited by a mineral spring.

That was news. The 2009 report (AZGS CR-09-B) says limestone “has not been identified previously in the McDowell Mountains or in the metro Phoenix area,” and the 2010 report adds that no limestone deposits are known in the other ranges nearby either. The tubes in the chert might be plant fossils, from plants growing around the spring, though the authors say that needs more work. On age, the two reports differ in emphasis: the 2009 report’s preliminary estimate is 2 to 20 million years ago, during the uplift of the range, and the 2010 report says only that the weathering points to “more than a million years.” Both leave the biggest question open: where did calcium-rich water come from in a range with no limestone to dissolve?

Where is it? Both reports say plainly that the outcrop is not on any trail, and the work was done under a city permit. We’re leaving it at that.

Giant crystals and orbicular granite

In 2015 Gootee and Gruber published Quartz Vein Investigation (AZGS OFR-15-03), done with the Conservancy’s field institute. Volunteers first scanned the whole Preserve on Google Earth, divided into twenty sections, for white quartz outcrops. Nine sites got field visits, and two, called Fraesfield and Paraiso in the report, were mapped in detail. Neither is on a trail.

At Paraiso, in the Carefree granite, the mapping showed that the “veins” were actually the quartz cores of pegmatites, bodies of enormous crystals. Feldspar crystals there run a meter (about 3 feet) or more, and the quartz crystals are thought to be at least as large, weighing tons. The team also found a patch of orbicular granite, which the report calls “relatively rare”: granite holding round orbs wrapped in concentric shells of different minerals. The outcrops are a few meters long. The report thanks the volunteers for “hundreds of hours” of photography and fieldwork.

Rules, and how to see more

Everything above was collected under permits. For the rest of us, the Preserve code forbids removing, damaging or disturbing any rock or “geologic site or feature,” and forbids digging or removing sand, gravel, rocks or soil. Those are misdemeanors, and so is leaving the trail. Look, photograph, put it back. The Gateway Loop protocols apply to geology too.

The better way to learn the rocks is with a guide. The McDowell Sonoran Conservancy runs free geology hikes, and the Marcus Landslide is where they pay off most. If you want to go deeper:

• Geologic Map of the McDowell Mountains (Skotnicki, AZGS Contributed Map CM-18-B, 2018). The units and contacts were digitized by Conservancy volunteers.
• Preliminary Geologic Survey of the Lost Dog Overlook Vicinity (CR-10-E, 2010) and Limestone Discovery in the McDowell Sonoran Preserve (CR-09-B, 2009).
• Quartz Vein Investigation (OFR-15-03, 2015).
• Mesoproterozoic Rocks of the McDowell Mountains, Arizona: Journey into the Magmatic Gap (Skotnicki and Gruber, Geological Society of America Field Guide 55, 2019), cited in the city’s ecological plan.

The AZGS reports are free to download from the Survey’s online repository at data.azgs.arizona.edu, under a Creative Commons license. Print the map, take it to Bell Pass, and the mountain stops being one brown wall.