Jurassic Armored Mud Balls 101
200 million years ago, a chunk of mud fell into a stream. As it tumbled through the flowing water, it was molded into a round, sticky ball. The mud ball picked up pebbles from the stream bed as it rolled, forming the “armor.”
After coming to a stop, the drying armored mud ball would have been fragile and crumbled away—as most mud balls do—were it not swiftly buried under sediment.
Geologic forces over millions of years hardened the Jurassic Armored Mud Ball (JAMB), and others like it, into the unique rock features only easily observed in our region.
Take a Closer Look
When you visit the Great Falls Discovery Center, how many Jurassic Armored Mud Balls can YOU find in this sandstone rock? If you examine it carefully, you might find up to nine complete and partial JAMBs!
Respect the Armored Mud Balls: Although the JAMBs are hard, they are still fragile and could be damaged by too much touching. Please be gentle!
Geology professor Richard D. Little first found JAMBs in 1970 in stone abutments for a bridge that once connected Gill and Turners Falls just upriver by Barton Cove.
Only Found Here?
It is rare to find JAMBs because special geologic conditions are needed to prevent the mud balls from disintegrating soon after they are made. Even embedded in rock, they are fragile.
The Jurassic Armored Mud Ball display at the Discovery Center was made possible by the Friends of the Great Falls Discovery Center. Consider joining The Friends and becoming a part of our community. Funding for moving this boulder onto the Discovery Center’s property was provided by First Light Power. The work was performed by the Department of Public Works.
Each side of our Jurassic Armored Mud Ball revealed!
Note! If the rock is wet, the JAMBs are hard to see.
There are nine JAMBS in this boulder. Two or three are partial, while the others are very well formed. This is one of the world’s rarest rocks! Enjoy exploring the geologic history in this flood deposit from Jurassic time.
This block weighs about one ton. A quarter coin is used for scale: 1 inch in diameter. We use the four compass points and the top to describe the different sides of the boulder.
South and West Rock Faces
These two faces illustrate the same history.
- Arrows = JAMBs
- Dashed line = Bottom of the flood deposit (note pebbles and AMBs). A sandy stream channel bed is the layer underneath the dashed line.
Note that the graded beds (larger sediment sizes such as pebbles gradually grade to sand) indicate changes in flow velocity as the flood volume dies out. The important geologic principle: The size of the sediment = the flow velocity. Large sediment sizes, such as pebbles, indicate high flow conditions; sand deposits indicate less energy required, and mud indicates still water, as in lakes. Floods typically begin with high flow conditions and grade to sand as the flow diminishes.
In this boulder, look closely and you can see there are two flood pulses here during the same flood event. Note the coarsening of the sediment along with the AMB at the top of the rock. That indicates an increase in flow velocity a short time after the deposition of the earlier AMBs.
North Rock Face
This North rock face lacks JAMBs. The sediment here is smaller in size than other sides and this indicates being just outside the main water flow channel.
East Rock Edge
A Deeper Dive into the Mud of Jurassic Armored Mud Balls!
What do JAMBs tell us about this place?
The area that is now the Connecticut River Watershed once had earthquakes and weather events such as storms and floods that formed braided streams. The region had a lake and related mudflats that were sometimes inundated by rushing water. Fossils preserve a dynamic glimpse at life in the area, but JAMBs preserve an equally dynamic glimpse into the hydrologic and geologic events in the Jurassic era. For more climate and weather insights on mud ball deposition in the Turners Falls formation of the later Triassic and early Jurassic periods, see Stop #3 [make hyperlink] on the Turners Falls Geologic Walking Tour.
More on climate and the making of mud balls
Monsoon rains were common. The dominant depositional environment of the Turners Falls Formation was a lake, which geologists call lacustrine. The mud flats were created by the rifting of the volcanic valley, forming shallow seas where fine sediments settled — not unlike East Africa today. And it was very flat indeed!
As a result of rifting, the valley slowly deepened over time, and lake water became entrapped and saline since the only way for the water to escape was through evaporation, not unlike Death Valley today. Any incoming water from upstream sources flowed rather in very shallow sheets, so discharge never achieved a sufficient volume to form stream channels; channelization would happen during monsoons. Water movement was very, very, very slow, a necessary condition for accumulating huge volumes of mud. When the monsoon came, water would rather rush in and, in the process, channels would form. Then, when the monsoons ended, the valley would return to its “Death Valley” identity.
So what did the JAMBs capture of these hydrologic and geologic events in the Triassic / Jurassic?
- very flat lacustrine terrain;
- large quantities of mud;
- mud baked in the sun to form another sedimentary structure called “mud clasts” (the precursor to mud balls, and much much more common), some of which must have been very large;
- occasional channelization events (such as monsoons) in which the clasts were moved along (entrained) and rolled into a ball.
Why did the JAMBs survive and not get smushed?
A weather event occurred in which flooding waters carried sediment which covered the JAMBs quickly without damaging them. Then, over time, the sediment became rock through pressure and cementation.
Are mud balls fossils?
No. Fossils are evidence of ancient life. Mud balls were never alive.
How did the JAMBs get to the Great Falls Discovery Center?
The boulder that is now at the Great Falls Discovery Center was quarried from downtown Turners Falls c. 1878 to become part of the Red Bridge abutment in the area of what is now Unity Park. The old bridge was long gone by the early 1970s, but the bridge abutments still remained when Richard Little discovered these JAMB formations in the boulders there. In order to have a sample of these rare JAMBs closer to the Great Falls Discovery Center for educational purposes, the Friends Group in 2024 worked to receive a grant, and then in 2025 this boulder arrived!
Pictures below of moving day, 4/7/25, by Shawn Kennedy:
See this video of the big equipment on a big day for the Jurassic era!
Many thanks to all who helped!
A grant from First Light Power provided the base for the JAMBs boulder at GFDC, landscaping, and these geology web pages. The Montague Department of Public Works, Demers Landscaping, DCR staff, Peter Wackernagel, Julia Handschuh and the Friends of Great Falls Discovery Center all contributed to making the GFDC outdoor exhibit and this website possible.
Where else in Franklin County can you find JAMBs?
- Greenfield Community College
- Unity Park – at 56 First Street, Turners Falls. See stop #8 on the Turners Geologic Walking Tour!
- Stop & Shop parking lot in Greenfield, MA.
- Cabot Woods trailhead boulder.
Are these the only lithified armored mud balls in the world?
Maybe. These are definitely the only easily seen lithified armored mud balls in the world. There are about 10 other locations where lithified armored mud balls have been noted in the geological literature. They are all in locations that are remote, hard to find, and probably not seen today due to erosion, soil cover, etc. Those other locations from Greenland to Trinidad, are listed and described on the ArmoredMudBalls.rocks web site. Photo to right: Turners Falls AMB with dime for scale (GCC Geology Path)
Why are the Franklin County JAMBs the best in the world?
+ Best Armor, Color, & Sizes: The dark mudrock of the ball contrasts with the “puddingstone” conglomerate of the old stream deposited sand and pebbles. The balls are very easy to see. Also, these AMBs’ armor have a diverse mix of colorful streambed pebbles eroded from nearby Jurassic mountains. Some other lithified AMBs from ocean environments are just sand-coated and the color of the ball matches the surrounding rock, making them hard to see.
The great size range of the Franklin County examples, from basketball to golf balls, is unlike any other geological location. + Geological “stratigraphy”: The Franklin County AMBs are found sedimentary strata of two different geological time periods from two different quarries and rock outcrops — Jurassic (Turners Falls Sandstone*, where most of the AMBs are from) and also about 500,000 years earlier, Triassic Period (Sugarloaf Arkose*).
It is extremely rare to have AMBs formed and preserved. It is exceedingly rare to have AMBs formed over such a large time period in the same spot on earth. And, even more amazing: after the formation of the Triassic Sugarloaf Arkose AMBs (found in Greenfield and Deerfield) a 200’ thick lava flow (Deerfield Basalt) covered the region, followed by a Jurassic lake. Then, in the Jurassic Turners Falls Sandstone, AMBs rolled into the old Connecticut Valley once again. And, of course, in both cases, in different towns and geologic ages, these rare forms had to be exposed and discovered!
Why are lithified armored mud balls so rare?
- Lake beds are needed to deposit mud**. This usually is related to wetter climate conditions.
- Dry climate conditions*** and/or land uplift are needed for the mud layer(s) to become dry and hard, followed by stream erosions, typically due to flash floods common in dry climates. The hard mud chunks fall into the stream. They get round, sticky, and roll along picking up pebbles from the stream bed. That is the armor.
- Quick burial as the flood subsides, and with geological time, lithification — turning to stone.
- Uplift and erosion to expose the rock with AMBs
- A quarry is needed to remove those specific sedimentary layers with the AMBs.
- Those quarried blocks have to be placed so that someone can see the round balls.
- Now, someone has to discover, identify, publicize, and save the rare AMBs.
You can also help! Contact your State Legislators. Sign the petition on the ArmoredMudBalls.rocks.
*Turners Falls Sandstone and Sugarloaf Arkose are official geology-mapped names of the rock formations. A “formation” is a geological term (noun) for a mapped rock unit. “Arkose” is a type of sandstone that was deposited close to mountain source areas and so is mostly composed of weaker minerals, like feldspar.
** Some AMBs form from ocean and coastal mud deposits, but not in the Jurassic Connecticut Valley.
*** Dry climates are needed because wetter climates have longer continuous stream transport that destroys AMBs. Dry climates and flash floods make quick burial of AMBs a possibility. Mud balls do not survive very long during stream transport.
Please note: unlithified, “recent” armored mud balls are occasionally found in a number of places. They are very rare, but being so unusual and interesting, there are many photos online. The Factory Butte, UT examples as seen in the cartoon were discovered by noted artist Will Sillin in 2015. A revisit several years later revealed none had survived.