Welcome to McGuireville Rest Area!
Located along Interstate 17, this roadside rest area offers a place for travelers to stretch their legs - and learn a bit more about the local geology!
Sandstone Banding of the Colorado Plateau
The sandstone boulder before you may seem like an ordinary rock, but it preserves a remarkable record of ancient environments that existed hundreds of millions of years ago. This rock is part of the geologic story of the Colorado Plateau, a region famous for its colorful sedimentary rocks and spectacular landscapes. Long before the forests surrounding Flagstaff existed, this area experienced changing environments that included rivers, coastal plains, and vast deserts. Each change left behind a new layer of sediment that eventually became the sandstone visible today.
Although this boulder is no longer attached to the bedrock from which it came, it still preserves the original layering that formed during its deposition. By closely examining the rock, you can observe differences in color and texture that provide clues about changes in the environment millions of years ago.
What is Sandstone?
Sandstone is a clastic sedimentary rock composed of sand-sized grains ranging from 0.0625 to 2 millimeters in diameter. Most sandstone is made primarily of quartz, a mineral that is highly resistant to weathering, although feldspar and fragments of other rocks may also be present.
The grains that make up this rock were once loose sand transported by wind or flowing water. As additional sediment accumulated above them, pressure compacted the sand while groundwater deposited minerals between the grains, cementing them together into solid rock. The result is a durable rock that can survive for millions of years while preserving evidence of the environment in which it formed.
What Causes Sandstone Banding?
One of the most noticeable features of this sandstone is its banding, also called bedding. Each band represents a separate period of sediment deposition. As conditions changed over time, new layers formed with slightly different grain sizes, mineral compositions, or cementing materials.
Some bands appear lighter because they contain little iron, while reddish or orange bands owe their color to iron oxide, commonly called rust. Other layers may appear darker because they contain different minerals or have been altered by groundwater after the rock formed.
These bands are much more than decorative patterns - they are a record of changing environments. A shift in color or grain size may represent a change in river flow, wind strength, sediment source, or groundwater chemistry. By studying these subtle differences, geologists can reconstruct ancient landscapes that disappeared millions of years ago.
LOGGING TASKS
Please submit your answers within seven days of your online log.
0) What is the name and GC Code of the EarthCache you're submitting answers for? GCBT94T - Sandstone Banding at McGuireville Rest Area
1a) Examine the sandstone depicted above and describe the color, thickness, and texture of two distinct bands.
1b) Using the graphic above, what number best represents each band?
2) Of the two bands you've examined, which contains more iron oxide? What led you to this conclusion?
3) Using the two bands you've examined, what environments could they have been deposited in?
4) REQUIRED - Snap a photo of yourself (or a signature item) at the rest area, and attach this photo to your "found" log.