Approximately 130 million years before the dinosaurs walked the earth (360 million years from now), in a time known as the Late Devonian period… a meteor struck the shallow water (30-60 ft deep) of the Chattanooga Sea and created a crater that was around 2 miles in diameter and 650 feet deep with a central uplift from rock being lifted upwards of 1500 ft above it’s normal position.
After the crater formed, it’s rim was likely above the water, but it was soon breached and the black, silty, mud of the shallow sea gradually filled in the crater, protecting it and it’s central uplift from significant erosion.
Over the next 360 million years, the landscape changed significantly and the area eventually became the land that we see today, known as Tennessee. A first generation karst landscape resulted from regional uplift due to the formation of the Nashville Dome where slightly acidic groundwater leaked through cracks and crevices in the limestone, gradually dissolving it and creating passages and caverns. Once the meteor impact occurred, additional cracks became present from the upheaval of material creating a second generation karst landscape. The region as a whole averages one cave per square mile, but the impact site target rocks have 5.5 times the concentration of caves.
The impact site in general, and it’s cave systems, attracted the attention of the National Aeronautics and Space Administration (NASA) who performed an intense investigation leading up to the first American Moon landing. They wanted to study and compare it to help identify impact craters that occured in a surface liquid on other Solar System bodies (Mars and Titan as examples). NASA figured that information gained here could assist in predicting the locations of caves on other planets that could be used to offer protection to human explorers from UV radiation, solar flares, and high energy cosmic particles.
Of particular interest is the Hawkins Impact Cave which is the only known cave in the world that developed in the central uplift of a complex crater. It was discovered by it’s landowner in 1989. It has two large rooms (Mars and Upper) that were formed by the dissolution and collapse at the intersection of several major faults.
It’s historical marker reads:
Over 360 million years ago, an asteroid or comet struck this area at more than 10 miles/second forming a crater 2.5 miles across and, in seconds, ejecting 0.3 cubic mile of rock. Deformed limestone beds seen in adjacent cliffs were violently uplifted to form a shattered central peak. This crater, now buried beneath younger rocks, was used by NASAlUSGS in training Apollo astronauts and represents an important class of craters found on other planets and satellites in our solar system.
Irregular rock formations can be seen throughout the area that resulted from the upheaval of material from the meteor impact.
A sample from this site was donated to the Middle Tennessee Museum of Natural History by the landowner of the Hawkins Impact Cave.
I encourage everyone to check out this incredible local resource that was put together by Paleontologist Alan Brown and consider giving a donation in support on this #givingtuesday where the Bill & Melinda Gates Foundation is providing up to $2 million in donations to nonprofits (matched via Facebook) starting on Nov 28 at 8 AM ET. You can donate here:
The cave entrance itself is small and requires a belly crawl of approximately 10 feet before opening up to a small chamber where you can sit upright. A wooden, sign is present that informs you that you are in the only known cave that resides in the central uplift of a impact crater, though dirt and moisture have weathered it to a point where it is difficult to decipher it’s text.
|Short Entrance Crawl That Opens Into A Small Room With Sitting Space|
One more short crawl and you encounter a passages that angles at approximately 45 degrees before opening into the Mars room previously described.
From the Mars room, the cave branches in a “Y” with the left path taking you up a high, steep climb that is possible without rope, though hazardous.
|Ascent Up The Left Passage From The Mars Room|
Continuing on through this passage, the incline levels out a bit and opens to a dome where it’s evident that the groundwater from above that is responsible for the formation of the cave still seeps in along with various other material like the deer leg bone that we found present.
|Dome At The Top Of The Left Passage Off Of The Mars Room|
|Deer Leg Bone|
The occasional hibernating bat can be found though on multiple trips over a couple years there has never been evidence of great numbers a colony.
Making your way back down from the left passageway back into the Mars room, rimstone is evident so caution is needed to cross over to the right side passageway.
|Example of rimstone found in the Mars room|
The right passage is a much more gradual slope upwards, through a wet, gravel like material with various obstacles along the route.
Near the end, a broken ladder can be found that leads to a climb up (that the ladder would be needed for) that results in a dead end.
Please note that caving can be a dangerous activity for the inexperienced. If you have an interest in exploring caves, check out a local grotto from the National Speleological Society website so you can connect with experienced cavers in your area who will show you the ropes. Also, remember that caves can be on either private property or government land so please always ensure that you either have permission from the landowner or the proper permits obtained before visiting a cave. While there, remember to…
Take nothing but pictures.
Leave nothing but footprints.
Kill nothing but time.