Mazon Creek is a premier fossil collecting location in Grundy County, Illinois. The site exposes Late Carboniferous fossils where visitors can hunt for plants, insects, and Tully Monster.
Collecting is allowed with local rules/permits (check Pit 11 guidance from IL DNR). Split open siderite concretions to find fossils – concretions range from golf ball to basketball size.
Mazon Creek fossils come from coal mining operations in the Illinois Basin. The Pennsylvanian coal-bearing rocks contain fossil-bearing siderite concretions. Pit 11 is a well-known spoil pile from strip mining operations. The site is world-famous for the Tully Monster (Tullimonstrum gregarium), found nowhere else on Earth, which became Illinois’ state fossil in 1989.
This complete guide covers everything you need to know about fossil collecting at Mazon Creek, including directions to the site, collecting rules and regulations, the types of fossils you can find, and what to expect during your visit. nan provides the nearest services and amenities for visiting collectors.
Location and Directions
You can find Mazon Creek in Grundy County, Illinois.
Directions to Mazon Creek
To reach Mazon Creek: Find Braidwood State Fish & Wildlife Area / Pit 11 (follow IL DNR guidance for access to spoil piles).
When you arrive, the fossil collecting area is located at the Pit 11 spoil piles in the Braidwood area.
What Fossils You’ll Find
Mazon Creek contains Late Carboniferous fossils preserved in siderite concretions in shale.
Plant fossils are the primary fossil assemblage at the site. Insects is also found throughout the collecting area. Tully Monster (Tullimonstrum gregarium) occurs less frequently but has been been discovered at Mazon Creek.
Geologic History
Mazon Creek exposes the Francis Creek Shale, a significant Late Carboniferous geological unit in Grundy County, Illinois.
Millions of years ago, the area that is now Illinois was a river delta system with brackish water conditions that allowed exceptional preservation in ironstone (siderite) concretions. The climate was tropical and humid with extensive swamp forests and river deltas near sea level.
As these ancient organisms died, sediments buried and preserved their remains. Over millions of years, geological processes transformed these sediments into the fossil-bearing rocks visible at Mazon Creek today.
How Mazon Creek Became a Fossil Collecting Site
Mazon Creek originally operated as an industrial site where commercial activities exposed fossil-bearing rock layers. Quarrying, mining, or excavation operations removed overlying sediments and revealed ancient deposits that had been buried for millions of years.
After industrial operations ceased, Mazon Creek transitioned to a location where fossil enthusiasts could legally collect specimens. The legacy of this industrial activity created the accessible rock exposures that make fossil collecting possible today. Many of the world’s most productive fossil sites share this history of industrial operations inadvertently revealing paleontological treasures.
Collecting Rules & Regulations
Is Fossil Collecting Allowed?
Mazon Creek allows fossil collecting for visitors who follow proper procedures. A permit or admission fee is required before collecting begins. Collecting is allowed with local rules/permits (check Pit 11 guidance from IL DNR).
Key Points:
- Permit or admission fee required
- Collectors keep all specimens found
- Staff available for questions and identification
- Reservations may be recommended during peak seasons
The fee helps maintain the site and ensure quality experiences for all visitors.
Tools and Equipment
Recommended Tools:
Fossil collectors should verify which tools are allowed before beginning their search. Some sites provide tools for visitors, while others require collectors to bring their own equipment.
Common Collecting Tools:
- Rock hammers and chisels
- Brushes for cleaning specimens
- Hand screens for processing sediment
- Sturdy containers, bags, or buckets
- Newspaper or bubble wrap for protection
- Spray bottle with water (reveals fossil details)
Safety Guidelines
Essential Safety Guidelines:
Safety should be every collector’s top priority when visiting Mazon Creek.
Personal Protection:
- Wear closed-toe shoes or boots with good traction
- Eye protection is essential when using rock hammers or chisels
- Sun protection: sunscreen, hats, and protective clothing
- Bring plenty of water and stay hydrated
Site Awareness:
- Supervise children at all times
- Watch for unstable rock faces, deep water, steep slopes
- Be aware of poisonous plants and wildlife
- Let someone know your plans before visiting remote sites
- Never collect alone in isolated areas
Sources
Illinois DNR documentation; Mazon Creek fossil literature; Illinois State Museum information
