Welcome to this EarthCache!

If you’re new to Earthcaching, there’s no physical container to find.
Your observations and answers are your proof of visit!

This Earthcache is here to help you learn differential weathering.

The prisoner is ancient. The sentence is geological.

At first glance, the Old Joliet Prison appears to be a story about people. But the stone surrounding you tells a much longer story, one measured not in years or decades, but in hundreds of millions of years.

Long before there were prison walls, guards, or inmates, this part of Illinois lay beneath a warm, shallow tropical sea during the Silurian Period, approximately 420 million years ago. The sea teemed with marine life. Shells, skeletal fragments, carbonate muds, and other sediments accumulated on the sea floor layer after layer over millions of years.

Eventually these sediments were buried, compacted, and cemented into limestone.

Later, magnesium-rich groundwater slowly moved through the rock. During this process, known as dolomitization, some of the calcium in the limestone was replaced by magnesium, transforming much of the rock into dolomite. The resulting stone became one of northeastern Illinois' most important building materials.

Hundreds of millions of years after its formation, the ancient seafloor was exposed at the surface and quarried by humans. When construction of the prison began in 1858, prisoners helped quarry the local stone used to build the prison walls only a short distance from where you are standing today. The stone blocks surrounding you were once part of an ancient marine environment and now form one of Illinois' most recognizable historic landmarks.

But the geologic story did not end when the blocks were cut and placed.

The rock is still changing.

Your Mission is to investigate the stone—not the inmates!

1. Identify the rock

Find a block that shows obvious evidence of weathering. Describe the features you observe.

2. It's a Mystery...become a 'Stone Detective' 

Compare a heavily weathered block with a less-weathered block nearby.
What differences do you notice?

3. The Final Verdict

• Describe three weathering features visible in the stone.
• Estimate the depth of the largest weathering pit.
• Explain what evidence shows that some portions of the stone weathered faster than others.

To complete this Earthcache, take a photograph at the site showing yourself and the historic stonework that you investigated — OR, if you prefer not to show your face, a photo of a personal item or a piece of paper with your geo-nick may be used (per current Geocaching guidelines) .

Weathering Written in Stone

Although these walls appear solid and permanent, they are slowly being altered by the same natural processes that shape cliffs, canyons, and exposed bedrock across Earth.

Geologists call this process weathering.

Although these walls appear solid and permanent, they are still being altered by natural geologic processes. Weathering occurs when rock exposed at Earth's surface is broken down by physical processes, chemical reactions, or a combination of both.

The stone blocks of the Old Joliet Prison provide an outdoor laboratory where these changes can be observed firsthand. Look closely at the walls. Some blocks remain relatively smooth, while others display pits, grooves, hollows, rough textures, and recessed surfaces. These features were not created when the stone was quarried. Instead, they developed during more than 150 years of exposure to rain, snow, temperature changes, wind, and pollution.

These differences are examples of differential weathering, a process in which some portions of a rock weather more rapidly than others. Although the blocks may appear uniform from a distance, no rock is perfectly identical throughout. Small variations in mineral composition, crystal structure, porosity, fractures, and moisture retention can make some areas more resistant to weathering than neighboring areas.

Over decades and centuries, these subtle differences become visible. Areas that weather more quickly develop pits and depressions, while more resistant portions remain raised and relatively smooth. The result is the uneven surface texture visible on many of the prison walls today.

By studying these patterns, geologists can learn how different rock properties influence the rate and style of weathering over time.

Physical Weathering

One important weathering process in Illinois is freeze-thaw weathering, also called frost wedging.

Water enters tiny cracks, pores, and openings in the rock. When temperatures drop below freezing, the water expands as it turns to ice. That expansion creates pressure within the stone. As the cycle repeats year after year, small fragments loosen and detach from the rock surface.

Physical weathering gradually enlarges cracks and contributes to the rough textures seen on many of the prison blocks.

Chemical Weathering

The stone is also affected by chemical weathering.

Rainwater absorbs carbon dioxide from the atmosphere and soil, producing weak carbonic acid. Although this acid is very mild, it can slowly dissolve carbonate rocks such as limestone and dolomite.

As rainwater repeatedly moves across the stone surface, tiny amounts of rock dissolve. Because some areas are more resistant than others, shallow depressions and irregular surfaces may develop.

The result is a rock surface that records decades of interaction between stone, water, air, and climate.

To log this Earthcache

Please send me an email/message with the GC code and name of this EC with the answers to each question. No Armchair caching, you must visit the site. You may go ahead and log after sending me your answers in the message. Be sure to include the required picture with your log. Please DO NOT post the answers in your log. Thank you for visiting!

Resources

Guide to the Geology of the Joliet Area
NPS -- weathering
JolietMuseum.org