EarthCache: Sandstone Boulders at Vicksburg
Buffalo Valley Rail Trail — Union County, Pennsylvania
Welcome to the EarthCache!
At the Vicksburg parking area of the Buffalo Valley Rail Trail, take a closer look at the sandstone boulders. These rocks are an opportunity to explore sedimentary geology, mineral composition, and the forces of weathering that continue to shape the landscape.
The Buffalo Valley Rail Trail follows a former railroad corridor between Lewisburg and Mifflinburg. Today, the trail provides a place to enjoy the outdoors while learning about the natural and human history of the region.
For this EarthCache, your task is to examine the boulders, observe their physical characteristics, and use those observations to understand how sandstone forms and weathers.
What is sandstone?
Sandstone is a sedimentary rock made primarily of sand-sized mineral and rock fragments. These grains were originally produced by the weathering of older rocks and transported by water, wind, or other natural processes.
When the sand settles, it can accumulate in layers. Over time, additional sediment buries the layers below, compacting the grains. Minerals carried by groundwater may precipitate between the grains and cement them together. This process, known as lithification, transforms loose sediment into solid sandstone.
Sandstone can vary considerably in appearance. Some varieties are pale gray or cream, others are yellow, tan, brown, pink, or red. Its texture depends on the size, shape, sorting, and composition of its grains, as well as the type of cement holding them together.
What minerals make up sandstone?
Examine the surfaces of the boulders closely. You may be able to distinguish individual grains and notice differences in color and texture.
Quartz: Quartz is commonly the main mineral in sandstone. It is usually clear, translucent, white, or gray, although tiny grains may be difficult to distinguish with the naked eye. Quartz is relatively hard and resistant to weathering.
Feldspar: Feldspar may appear white, cream, pink, or pale gray. It can break down chemically into clay minerals, making it less resistant to weathering than quartz under many surface conditions.
Iron oxides: Iron-bearing minerals and their weathering products can give sandstone reddish-brown, rusty, orange, or yellow colors. Hematite commonly contributes red tones, while other iron oxides and hydroxides can produce yellow and brown staining.
Clay minerals: Clay minerals may form as feldspar and other minerals break down. They are extremely fine-grained and generally cannot be individually identified without magnification. They may contribute to a dull or earthy appearance.
Cementing minerals: Silica, calcite, or iron-bearing compounds can fill spaces between the grains and hold the sandstone together. The type and amount of cement affect the rock’s strength and resistance to weathering.
These are minerals commonly found in sandstone. Their presence and abundance in the Vicksburg boulders have not been independently verified, so use the descriptions as possibilities rather than definitive identifications.
How does weathering affect the boulders?
Weathering breaks down or alters rock at the Earth’s surface. The main processes to consider are physical, chemical, and biological weathering.
Physical weathering
Physical weathering breaks rock into smaller pieces without changing the minerals’ chemical composition.
- Freeze-thaw weathering: Water enters cracks and pores, freezes, and expands. Repeated freezing and thawing can widen cracks and eventually loosen fragments.
- Abrasion: Wind, moving water, and particles of sand or grit can wear down exposed surfaces.
- Temperature changes: Repeated heating and cooling can contribute to stress and the development of cracks, particularly when combined with other weathering processes.
Chemical weathering
Chemical weathering changes the minerals that make up a rock.
- Oxidation: Iron-bearing minerals react with oxygen, often in the presence of water, producing reddish, orange, or brown stains.
- Dissolution: Water can dissolve susceptible minerals or cement. If calcite cement is present, slightly acidic rainwater may gradually weaken the sandstone.
- Hydrolysis: Feldspar reacts with water and can transform into clay minerals, changing the composition and strength of the rock.
Biological weathering
Biological weathering results from the activity of living organisms.
- Plant roots may enter existing cracks and gradually widen them.
- Lichens and mosses can retain moisture against the rock and contribute to chemical alteration.
- Organic acids from organisms and decomposing material may react with certain minerals.
In Pennsylvania, seasonal temperature changes and precipitation can expose rocks to several of these processes. However, the dominant process on any particular boulder depends on its mineral composition, porosity, exposure, and local conditions.
Remember that weathering is not the same as erosion. Weathering breaks down or alters the rock, while erosion transports the resulting particles elsewhere.
Your EarthCache tasks
Observe the sandstone boulders from a safe position in the Vicksburg parking area. Do not break, scratch, or remove any rock material.
Please answer the following questions based on your observations:
1. Describe the sandstone. What colors do you see?
2. Investigate the mineral clues. Based on the descriptions above, which mineral or mineral group do you see.
3. Look for weathering on the boulders which types of weathering do you see?
4. Consider the climate. Which weathering process do you think could affect these boulders most strongly over time: physical, chemical, or biological weathering? Explain your choice using evidence from your observations and knowledge of the local weather
5. include in your log a picture of you at the boulders ( your face doesn't not need to be included,) or a personal item.
Your answers should reflect your own observations rather than simply repeating the information in the description.
Conclusion
The sandstone boulders at the Vicksburg parking area offer a chance to investigate the rock cycle at a small scale. Sandstone forms when sand-sized particles are deposited, buried, compacted, and cemented together. Once exposed at the surface, the rock begins to weather again.
By examining the grains, colors, cracks, and surface textures, you can connect what you see to the geological processes that have shaped rocks over long periods of time.
Please leave the boulders undisturbed and respect other users of the Buffalo Valley Rail Trail.
References
- Pennsylvania Department of Conservation and Natural Resources — Pennsylvania Geological Survey. Information about Pennsylvania geology, geological mapping, and the PaGEODE mapping application.
https://www.pa.gov/agencies/dcnr/programs-and-services/about/bureaus-and-offices/geological-survey
- Pennsylvania Department of Conservation and Natural Resources — Geology of Pennsylvania. Information on Pennsylvania’s landforms and underlying geology.
https://www.pa.gov/agencies/dcnr/conservation/geology
- U.S. Geological Survey — Quartz. Mineral information and properties of quartz.
https://pubs.usgs.gov/sir/2017/5118/elements/Quartz/Qtz_txt.html
- U.S. Geological Survey — Environmental Characteristics of Clays and Clay Mineral Deposits. Background on clay minerals and their geological occurrence.
https://pubs.usgs.gov/info/clays/
- Union County Trail Authority / Visit Central Pennsylvania — Buffalo Valley Rail Trail. Trail location, access points, facilities, and rules.
https://www.visitcentralpa.org/things-to-do/parks-trails-nature/trails/buffalo-valley-rail-