Friday, November 27, 2015

The Geology of Old Man's Cave

The geology of Ohio is incredibly fascinating, and incredibly varied. One of the most famous geological sites in Ohio is Old Man's Cave in Hocking County. Old Man's Cave is one of the many gorges located within Hocking Hills State Park, one of the most-visited parks in Ohio. Many people think of Ohio as a totally-flat state, but that couldn't be farther from the truth. Although approximately 2/3 of Ohio was glaciated and subsequently flattened in geologically recent times (as recent as 10,000 years ago), the other 1/3 was never glaciated. This region is part of the Allegheny Plateau. The Allegheny Plateau used to be a broad, flat plain that was uplifted before experiencing millions of years of erosion by flowing water (creeks, rivers, etc.). This flowing water cut down into the flat plain, creating the hills that eastern Ohio is known for. This post will essentially be a basic tour of the geology of Old Man's Cave starting from the very beginning of the gorge to the end of the gorge. Note, this is going to be a pretty long post!

Map courtesy of the Ohio Department of Natural Resources. (Link)
Old Man's Cave is located within the highly dissected Allegheny Plateau, but there are some other factors at play that have created the unique Hocking Hills Region. To understand why Old Man's Cave formed, we must first go over some basics. The gorges, waterfalls, and so on of the Hocking Hills occur almost completely within a section of bedrock named Blackhand Sandstone. Understanding this Blackhand Sandstone is completely necessary to understanding Old Man's Cave. Blackhand Sandstone is made up of sand particles which were laid down about 330 million to 345 million years ago during the early Mississippian Period. Now, it used to be thought that this sand was deposited in an ancient delta; however, recent studies have suggested another depositional environment. This 2006 study by Matchen and Kammer puts forth the hypothesis that the sand of the Blackhand Sandstone was actually deposited in an ancient braided stream. Matchen and Kammer put forth a very convincing argument, and I will be sticking with their interpretation. A braided stream is a stream with multiple channels that weave in and out of each other, with shifting sand and other sediment bars in between these channels. These types of streams occur when there is a high amount of sediment in the water. You can see an example of a modern braided stream at this link. If you look at the photo above, you can see the extent of Blackhand Sandstone in Ohio. You can also see that it takes the shape of a wide river floodplain, yet another clue to its braided stream history. The stream flowed south to north, and when it got to the extreme northern section it emptied into a shallow sea.

Now that we have the context down, we can move onto another important factor. Pictured above is a joint. A joint is a type of fracture, or a break in a rock. These are similar to faults, but occur on a much smaller scale, and typically movement doesn't occur along a joint as it does along a fault. A joint is a random break in the rocks which occur when the bedrock faces some sort of stress, such as a compressional force. When a joint forms, it creates a weak area in the rock, which in turn can be more easily eroded by water. This feature is very important when it comes to the formation of Old Man's Cave, as we'll see in a minute.

The parking lot for Old Man's Cave lies within a flat valley. This valley was carved out by Old Man's Creek, the creek which ultimately runs through Old Man's Cave. This creek eroded its way through what is called the Logan Formation. The Logan Formation is a set of relatively soft rock layers, and so the creek was able to erode it easily and carve out a broad valley with no cliffs. Eventually it got down to the Cuyahoga Formation. This is the bedrock group which contains the Blackhand Sandstone. Blackhand Sandstone is much, much harder than the Logan Formation rocks, and so it is harder to erode. Once Old Man's Creek got down to this layer, it couldn't easily erode the sandstone. However, it eventually found a large master joint in the Blackhand Sandstone. The water seeped into this joint and over millions of years slowly widened the joint. This widening joint eventually became the gorge. Looking at the picture above, the rock exposed is the top part of Blackhand Sandstone. This small waterfall is the beginning of the Old Man's Cave Gorge.

Upper Falls
Just a few yards downstream Old Man's Creek cascades over another set of rocks, creating Upper Falls. Right below Upper Falls is a plunge pool. You can see the outline of the plunge pool by looking for the deep turquoise coloration in the water (which signifies deeper water). A plunge pool is a feature that forms below a waterfall. The falling water in a waterfall has a lot of energy, and as this water hits the rocks below it scours out a depression. The eroded sand from the plunge pool is then deposited on the bank of the pool. The creek follows one side of the cliffs and continues on its way down the gorge.

Devil's Bathtub Old Man's Cave

Just a dozen yards downstream, the creek encounters another interesting feature. Pictured above is The Devil's Bathtub, which is actually a pothole. Basically, there was a weak area that the creek found and eroded. Sand and pebble particles in the water fell into this tiny eroded section and the current made them swirl around and around. This swirling action carved a bigger and bigger hole, forming what we now call The Devil's Bathtub. 

Devil's Bathtub Old Man's Cave
This pothole filled up and overflowed, eventually carving out a small notch in the top of the pothole. When the creek has enough water flow (which it does for most of the year, save the dry parts of summer), the water drains from this notch into the tight channel pictured above.

Old Man's Cave

The creek then gently meanders its way down the gorge, like in the section pictured above. Here we can also see many Eastern Hemlock trees. This species, along with other plant species such as Canada Yew and Yellow Birch, are species normally found farther north in cooler climates. During the last glaciation event, Ohio was much, much cooler than it is today. Plant communities characteristic of cooler locations (such as Canada) moved southward in order to survive. As the glaciers receded, most of these plants in Ohio died out and temperate forest plants came back from their Gulf Coast refugia and recolonized the area. However, some of these northerly species remained in a few select areas of Ohio. Deep gorges like this one create a much cooler microclimate, which allows these northerly plants to persist. If you've ever been to Old Man's Cave on a hot summer day, you probably know how nice and cool it is within the gorge.

Sandstone cross-bedding
As always, you can click on the photo to enlarge it!

As we approach the middle of the gorge, I'd like to take a quick tangent and talk a little bit more about some features of Blackhand Sandstone. It's a complex rock formation with many interesting features, such as cross-bedding. Cross-bedding is layering at an angle to the main bedding plane. Essentially, sedimentary rocks have one horizontal layer after another. Cross-bedding is an inclined layering which reflects certain sediment features that were inclined. As I mentioned before, Blackhand Sandstone was originally sand deposited in a braided stream. Braided streams have characteristic sand bars in between the water channels (example). These sand bars were created when the water flow pushed sand up a slight incline before the sand settled down on the downslope of that incline. A diagram at this process can be seen at this link. This results in cross-bedded sand bars. These sand bars were preserved throughout the rock bed, creating very exceptional examples of cross-bedding in the gorge. To recognize these cross-beds, look at the cliffsides and look for angled layers. If you look at the photo above, you can see a part of a cliffside featuring normal bedding along with cross-bedding. On the left side is what it looks like, while on the right I have outlined the different sections along with the direction of incline in order to make the cross-bedding more apparent.

Hocking Hills Conglomerate

Another feature of the Blackhand Sandstone is conglomerate bands. Conglomerate is a type of sedimentary rock that is composed of gravel-sized sediments. In the case of Blackhand Sandstone, these gravel-sized particles consist primarily of quartz. You can see multiple bands of conglomerate running through the sandstone in the photo above. This gravel is typically laid down in the center of a stream channel. Water in the center of a channel is moving the fastest; as a result of this, small particles remain suspended in the water. Larger particles, such as gravel, weigh enough to settle out of the water and deposit on the bed of the stream. Essentially, the faster water flows, the larger the particles that are deposited. Since Blackhand Sandstone was originally deposited as a braided stream with channels and sand bars of assorted sizes, the resulting sandstone consists of varying bands of cross-beds and conglomerate bands along with normal sandstone bedding. These conglomerate bands can be seen throughout the gorge in various locations, so keep an eye out!

Honeycomb Weathering

The previous two features of Blackhand Sandstone had to deal with its depositional environment. This next feature has to do with the subsequent erosion of the Blackhand Sandstone. Pictured above is what is known as honeycomb weathering. Blackhand Sandstone is very porous, meaning water particles can easily seep through the rock. Throughout the sandstone are trace deposits of hard iron compounds. Seeping groundwater has deposited some of these iron oxide particles in pore spaces between sand grains, and that iron then makes the sandstone highly-cemented and very hard to erode. As more groundwater seeps through and erodes the sandstone, you end up getting this honeycomb pattern of cavities (less iron and more easily eroded) and rib-like projections (more iron and less easily eroded). It makes for very attractive sandstone. Anyway, back to the gorge!

Old Man's Cave Waterfalls

Old Man's Cave Gorge can be broken up into 3 regions. There's the Upper Gorge, Middle Gorge, and the Lower Gorge. We're now entering the Middle Gorge. This region begins with a long, gentle cascade of Old Man's Creek down bare rock, a rather abrupt change from the gentle, sandy meander of the creek in the Upper Gorge. These cascades are one of my favorite parts of the gorge!

Old Man's Cave
The cascades eventually lead to a small waterfall called Middle Falls, pictured in the bottom right above. More impressive is the incredibly large recess cave located above it. This recess cave is the "cave" of Old Man's Cave. This recess cave is nearly 200 feet long, 50 feet high, and 75 feet deep! Recess caves form throughout the Hocking Hills region, and they all form due to the same feature of Blackhand Sandstone. Blackhand Sandstone is broken up into 3 separate zones; there is the upper zone, the middle zone, and the lower zone. The highly-cemented upper zone is very resistant to erosion, as is the lower zone; however, the middle zone consists of weakly-cemented particles and highly cross-bedded layers. This makes it relatively soft and easy to erode. As I mentioned before, the sandstone is porous and allows water to seep through it. The upper layer doesn't erode much as groundwater moves through it, but the middle zone easily erodes on the other hand, and as a result this tends to form these large recess caves like the one above. The resistant upper zone forms the roof, while the "cave" part moves farther and farther in as it erodes away.

The Sphinx Old Man's Cave

After the creek cascades over Middle Falls, it comes into contact with the highly-resistant lower zone of Blackhand Sandstone. As a result, it has meandered over this layer while eroding more and more of the middle zone because it couldn't erode down into the lower zone. The result was a gentle widening of the gorge. Another big characteristic of this stretch of the gorge are dozens of large slump blocks. These boulders are actually made up of the resistant upper zone sandstone. As the middle zone eroded, the upper zone remained and formed the roof, like in the previous photo. Eventually, the weight of certain parts would become too much, and this stress would cause a large slump block to break off and fall to the floor below. As this process occurred over millions of years, more and more slump blocks began littering the gorge floor. 

Lower Falls Old Man's Cave
Old Man's Creek continued to flow on top of the resistant lower zone until it found yet another joint to exploit. More erosion occurred, and Lower Falls was born. It is here at the bottom of this recess cave that the Blackhand Sandstone members ends and runs into the so-named Fairfield Shale, a much softer rock. The plunge pool visible was created through the erosion of the Fairfield Shale. From here, Old Man's Creek travels a broad valley until it meets up with Queer Creek. Queer Creek is the stream which created nearby Cedar Falls, yet another section of the Hocking Hills State Park.

Overall, the Gorge Trail at Old Man's Cave is about half a mile long. Over the course of that half mile, the gorge cuts through the entire thickness of Blackhand Sandstone. The thickness of Blackhand Sandstone varies geographically, but in this specific area it's about 130 feet thick, meaning from the very first tiny waterfall to the plunge pool of Lower Falls is about a 130 foot drop. I'm currently on a massive geology kick, so expect to see more geology-themed posts over this winter! Hopefully you enjoyed this long post, and thanks for reading!