I’ve got another serious post on recharge rates in California’s aquifers in the works, but for now here’s another overthinking of my weekend hiking adventures…
One Friday at the end of September I was sitting at my desk, working through a lunch break, when I was hit by a sudden mad urge to sleep outside in a tent and go hiking somewhere with rocks to clamber on. Given the prospect of a three hour drive to my usual climbing destination (Jackson Crag near Batesville, Arkansas) I decided to see where two hours of driving could take me. I had never heard of Tishomingo State Park before a fortuitous 5 pm Google search, but it took me exactly 63 minutes after I got off work that day to have my car packed and my wheels turned east. So, what kind of rocks are there in Mississippi, the state that I usually associate with pancake-flat drives through cotton fields?
After rolling out of my campsite in the morning, I parked at the trailhead by the Pioneer Cabin, which has a scenic little pond held back by a concrete block dam nearby. I took the CCC Camp trail around the river bend to the swinging bridge, took the Overhang Loop trail (where the best rocks and climbing routes are), and came back to my car via the Bear Creek trail. The CCC Camp trail has plenty of great picnic rocks jutting out into Bear Creek. Overall the hike was five and a half miles, which left me plenty of time in the afternoon to curl up in my hammock and read a large chunk of a novel before heading back to Memphis in time for dinner.
After I got back to work on Monday I tried to look up the detailed geology of the region to see what rocks I had been hiking on, but Google failed me on that front and I put that on the back burner for a bit. However, I got a stroke of good luck a couple weeks later! One of the many upsides of working at a well-established environmental firm is the library of random old books. While doing some filing this week I ran across a slightly ragged copy of the 1940 title “The Upper Cretaceous Deposits” by Lloyd William Stephenson and Watson Hiner Monroe of the Mississippi State Geological Survey. It has thorough and colorful maps of northeastern Mississippi with fine enough resolutions to pick out the park!
It turns out that Tishomingo State Park sits on a curve of poorly consolidated Cretaceous deposits that have more in common with Central Tennessee than with the rest of Mississippi. The oldest Cretaceous deposit in the area is the Tuscaloosa Formation (Fm.) – an irregularly bedded sand with clay, gravel, and some weakly compressed organic matter (lignite). On top of that lies the Eutaw Fm. whose sand and fine chert gravel form taller hills than the Tuscaloosa Fm. Underlain mostly by that Eutaw Fm., Tishomingo county is hilly, the campground is hilly, and my little Honda Civic valiantly screeched and spun its way up the gravel road to the primitive campsites away from the noise of the lakeside RV campsites. However, the toppled blocks of sandstone and layers of shale that define the park are much older – somewhere around 275 million years older. They were put in place during the Devonian era when the first round of sediment was washed off of the Appalachian mountains to the east, and exposed as Bear Creek cut down through the Cretaceous sediments on its way to the Tennessee River in modern times.
Have you missed my MS Paint skills? Here’s the trail map with the geology overlain.
Northeastern Mississippi is underlain by a stack of sediments tilted to the west, so the oldest rocks are exposed to the east and the youngest to the west. This creates diagonal lines of distinct terrains and ecosystems starting with the Tennessee Tombigbee Hills in the east, followed by the Black Belt, Pontotoc Ridge, Flatwoods, and North Central Hills. Those Cretaceous layers lie unconformably on top of Devonian sandstones, limestones, and shales. The Devonian rocks aren’t often exposed in Mississippi, but form the distinctive “O” of the Highland Rim around Nashville where I spend my undergraduate career scouring their limestone components for brachiopods and crinoid stems.
The Devonian sandstones have been fractured by two roughly perpendicular forces that form the rock into striking geometric shapes like giant building blocks.
The shale layers create overhangs like “Jeans’ Wall”, the most popular climbing route in the park. A hard sandstone cap protects the tan shale and mudstone that crumbles when touched and has been worn into short, irregular columns.
Tishomingo is the only outdoor climbing destination in Mississippi, and I was itching to boulder some short routes that looked anywhere from 5.fun to 5.10. However, as my only companion on this trip was a small plush moose (geologist for scale) which is no good for spotting, I had to settle for taking notes on the map and resolving to round up people to come back with.