For this Earthcache you will take a short hike on the Mohave Trail (0.4 miles, Easy/Moderate, out and back, 308 elevation gain)
- Start Early - Beat the heat and enjoy the best light for photos — temperatures rise fast after 9 a.m.
- Bring Water - Carry at least 1 liter per mile; there are no water sources on the trail.
- Watch Your Step 直 - The rocky, uneven terrain includes loose gravel and steep spots — wear sturdy hiking shoes.
- Stay on Trail - Protect fragile desert soils, plants and insect life; stick to marked paths and avoid shortcuts.
- Respect the Desert ☀️- Use sun protection, stay alert for rattlesnakes and cholla cactus, and turn back if overheating.
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Logging Tasks
To claim this EarthCache 
, you must first visit the posted coordinates and then Send Answers via the geocaching.com Message Center to the following questions based on your observations and the information provided in the short Earthcache Lesson below:
- Landform Evidence: Describe how the Phoenix Mountains differ from the surrounding valley. What does their shape and height suggest about fault-block uplift?
- Rock Composition: Examine nearby outcrops. Do the rocks look crystalline, layered, or sandy? Based on their appearance, are you likely looking at Precambrian basement or younger sedimentary deposits?
- Subsidence Connection: How could groundwater pumping in the nearby basin relate to cracks or fissures near the base of these mountains?
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EARTHCACHE LESSON: The Phoenix Mountains rise abruptly from the flat desert floor in the heart of Arizona’s largest city. Peaks such as Piestewa, Shaw Butte, Dreamy Draw, and North Mountain create an island of rugged topography surrounded by sprawling suburbs and alluvial plains. To most visitors, they are simply a scenic backdrop or a place for early-morning hikes — but to geologists, these mountains are a window into the deep and dynamic processes that have shaped the Basin and Range Province.
The Phoenix Mountains occupy a special position within the central Basin and Range, the broad region stretching from Nevada to Sonora that formed as the Earth’s crust thinned and fractured over the past 25 to 35 million years. The crust here has been pulled apart by as much as 50 percent. As it stretched, large blocks of rock broke along normal faults: some blocks dropped down to form basins, while others rose as elongated ridges. The Phoenix Mountains are one of those uplifted blocks — technically a horst — bounded by faults and tilted slightly through time.
1️. Uplifted Fault-Block Structure
The first geologic distinction of the Phoenix Mountains is their fault-block origin. Mapping by the Arizona Geological
Survey shows that the range trends northwest–southeast and is bounded by normal faults that dip toward the surrounding basins. During Miocene crustal extension (about 15–25 million years ago), these faults allowed the central block to rise relative to the basins now filled with sediment.
Because of this faulting, the Phoenix Mountains display as much as 420 meters (≈ 1,380 feet) of relief above the surrounding city. Where you are standing, their linear ridges and steep flanks contrast sharply with the flat valley floors of Paradise Valley and the Phoenix Basin, demonstrating the alternating high-low topography typical of Basin-and-Range provinces. The South Mountains to the south and the White Tank Mountains to the west share this same structural ancestry — each representing a slice of crust that was uplifted or down-dropped as the region stretched.
2️. Exposure of Ancient Basement Rocks
A second distinction is the exposure of very old metamorphic and igneous rocks, in stark contrast to the younger volcanic and sedimentary units that fill the surrounding basins. Most of the Phoenix Mountains consist of Precambrian metamorphic basement that formed more than 1.6 billion years ago when ancient continental fragments collided and were buried deep in Earth’s crust, especially:
- Schist – A metamorphic rock with shiny, flaky layers formed under moderate heat and pressure.
- Gneiss – A banded metamorphic rock with alternating light and dark minerals, formed under high heat and pressure.
- Quartzite – A hard, glassy metamorphic rock made from recrystallized sandstone, highly resistant to weathering.
Later granitic intrusions crystallized within this metamorphic complex. You saw plenty of examples of all of these rock types your hike here.
These rocks record extreme heat and pressure long before the modern desert existed. Over hundreds
of millions of years, erosion stripped away the younger cover layers, and Basin-and-Range faulting uplifted these ancient cores to the surface again. Today, you are hiking across a crystalline basement that once lay miles underground. In road cuts and trail exposures you can see foliation — thin, streaky layering created by metamorphism — as well as dark intrusive dikes that cut through lighter-colored granite.
Because these rocks are so resistant to weathering, they have helped preserve the mountainous topography even as the softer basin sediments around them have been eroded flat. The Phoenix Mountains thus provide one of the few places inside a major U.S. city where billion-year-old crust is exposed.
3️. Connection to Modern Basin Subsidence and Fissures
The third distinction of the Phoenix Mountains is their relationship to modern subsidence in the surrounding basins. Although the mountains themselves are relatively stable, the adjacent Phoenix Basin continues to compact as groundwater is withdrawn for urban and agricultural use. The basin fill — unconsolidated sand, silt, and clay derived from erosion of these same mountains — loses pore pressure when water is pumped out, causing the land surface to
drop.
This subsidence is not uniform; it can vary by several centimeters across short distances, producing earth fissures and surface cracks. These fissures tend to form along buried fault lines or at basin margins — exactly where the Phoenix Mountains transition to flat valley terrain. The contrast between solid crystalline basement and soft alluvial sediment creates stress zones that localize cracking. Although fissures are more dramatic in the southern and eastern parts of the metropolitan area, the same geologic principles apply here.
Thus, the Phoenix Mountains embody a complete geologic story arc: deep crustal formation → uplift during Basin-and-Range extension → ongoing surface deformation in the neighboring basin. From the billion-year-old rocks under your feet to the modern fissures developing at the city’s edge, the landscape records a continuous sequence of geologic evolution.
From this location in the Phoenix Mountains Preserve, notice how steep slopes rise abruptly from the flat desert. Imagine the underlying faults dipping away beneath you, forming tilted blocks that descend under the city. Each hiking trail, each wash, and each skyline ridge traces the shape of ancient geologic forces still subtly active today.
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