2017/12/20

Grassy Cove, North America's Largest Sinkhole

Grassy Cove Community Center, Cumberland County, Tennessee

Nestled in the southeastern part of Cumberland County, Tennessee lies a quite community known as Grassy Cove. Its flat fertile land is home to scenic farms and farmland which are still owned by the descendants of the first European inhabitants of the area.

Without looking more closely at the landscape, it might escape the casual viewer that one was sitting inside a giant bowl. But with the aid of maps, it becomes readily apparent that Grassy Cove isn't like a regular hollow.

My first rule as a geographer is "Ask a local". Locals know a lot. For example, it's long been known that if it rains too much in Grassy Cove, then the flat farmland turns into a lake. It's also known from the days of yore that discarded corn shucks in the cove will appear in the Sequatchie River a few miles away, a few days later. What's going on here?

Grassy Cove flooding, Cumberland County, Tennessee 2  

Structurally, there's a lot to be said about Grassy Cove; it's not just a simple sinkhole. Its formation is the middle stage of a grand weathering process, with hints of its beginning and end observable along the axis of Sequatchie Valley Anticline, a giant geomorphological feature that spans two states. The Sequatchie Valley Anticline is a remnant of the same compressional forces which created the ridges and valleys in the physiographic province of the same name east of Grassy Cove. The Sequatchie Valley Anticline is a huge structural deformation where the bedrock was folded. Imagine a carpet getting scrunched up; the spots where it bends upwards are anticlines, downwards are sinclines.

Anticline, Sequatchie Valley, Cumberland County, Tennessee


The anticline has been around for a really long time, slowly being weathered away. The erosion process means that it is retreating northward, so features at its northern extent may offer a glimpse into what happened further south longer ago (that is what is happening now in Grassy Cove). And it also lets us see what the future of Grassy Cove is (to be part of the Sequatchie Valley).

Currently though, Grassy Cove is drained completely through a cave system and the water from there emerges at Devilstep Hollow, at the head of the Sequatchie Valley. 

Devilstep Hollow Cave, Head of Sequatchie Unit, Cumberland Trail State Park, Cumberland County, Tennessee 2


As stated, this is the largest sinkhole in North America (which I am aware of). Back in 2013 Tom Dunigan and I crunched some numbers for sinkholes in Tennessee. I made areal and volumetric estimates based on 10m Digital Elevation Models (DEMs). Now, 2.5' DEMs area available, which are 172 times more detailed than the 10m DEMs. I wanted to make a renewed comparison and try to tease out more detailed information.

Map of Grassy Cove, Cumberland County, Tennessee

Below are the updated figures for the region based on the 3DEP lidar.


Region Measure                 Value Unit
Watershed Area 33.17 Kilometers2
Watershed Perimeter 42.15 Kilometers
Sinkhole Area 13.31 Kilometers2
Sinkhole Perimeter 35.71 Kilometers
Sinkhole Major Axis 7.67 Kilometers
Sinkhole Minor Axis 4.36 Kilometers
Sinkhole Axial Declination 30.80 Degrees
Sinkhole Minimum Elevation 467.27 MASL
Sinkhole Maximum Elevation 507.24 MASL
Sinkhole Depth 39.97 Meters


Overlook, Grassy Cove, Cumberland County, Tennessee 1

2017/12/12

Sandstone Boxwork

Sandstone boxwork is a unusual phenomenon that we see on the margins of the Cumberland Plateau.

There are three things that must happen first to produce boxwork.

  • Jointing
  • Mineralization
  • Weathering

Photo credit: Brian Solomon

Jointing


Produced in a parent rock of sandstone, this type of boxwork originates as small joints which run in a box shape. The origins of these joints could be from frost wedging - where water in the rock freezes and cracks the rock, or it could unloading pressure from the rock de-watering, or it could be from some other physical process. The origins of the boxwork joints are still a bit of a mystery to me.


Mineralization


The joints become a place of preferential mineralization. Groundwater loaded with iron minerals moves through the pores of the sandstone. Where it finds a void, like in the joints, it tends to drop minerals out of solution. The rust colored minerals you find on your water heater at the house are chemically quite similar to this boxwork. 


Weathering


The joint surface must first be exposed to weathering, whether by cliff failure, slump, or some other wasting process. The exposed surface's sandstone weathers out first, leaving in place the mineralized boxwork. Because the sandstone around it is removed, the boxwork tends to protrude after its been well developed.



Similar Features 


Boxwork is a type of Liesegang ring. Liesegang rings lack obvious structural control, as in they do not have joint influenced development.

Painted Rock, Painted Rock Cave, Big South Fork NRRA, Fenress County, Tennessee Liesegang rings, Rockcastle conglomerate, Eagle shelter, White County, Tennessee 2 Liesegang rings, Claire Murphy, Fentress Formation, Big South Fork NRRA, Scott Co, TN