When we re in an airplane or looking at ariel imagery of the earth, we often question why rivers have so many turns, sometimes to the point they seem to defy the laws of gravity itself.
This geological phenomena is called meandering which entails the sinuous watercourse of a river and erodes the out riverbanks over time by alternately depositing eroding sediments to the outside and inside of the stream. A meandering river creates many features in from is source to its destination as well as over time through natural erosion.
Bobbit's Hole is a place where the river meander's close to 90 degrees in its journey east to Falls Lake. Because this is an ancient river situated on what was once much higher and rocky, this particular bend formed a hole through natural processes of river erosion, creating unique features for this spot. Tens of millions years ago, Bobbitt's Hole was formed as a plunge pool because of the rock you see across the river extended to the side you stand on now - forming a waterfall. The soft rock continued to erode through corrasion and the hydraulic actions further eroded the banks causing the bend you see now.
Please read on to learn more about the process of natural erosion and the features they form over time. The lessons being learned here today is how the forces of water along with the topography of the land work together to continually shape the land and direction of the river through the natural erosion.
A river erodes in the following ways:
1) Corrasion (abrasion):
-The river uses its load to grind against the bed and sides
-The action would dislodge the materials and carry them away
-This process operates in 2 ways (verticle;depth;lateral;width)
-Potholes are the product of corrasion
2)Attrition:
-The loosened materials that are being carried away collide against the river sides and bed and against one another
-Over time, they would become smaller and eventually reduced to fine particles called silt
3)Solution (corrosion):
-The solvent action of water dissolves soluble materials and carry them away in solution
4)Hydraulic action:
-The breaking down of rocks
-Removing and dragging rocks from the bed and banks of the river by the force of the running water (like a water jet)
-Usually there are lines of weakness like joints and cracks in the river
-The work of hydraulic action forms plunge pools (small lakes)
EROSIONAL FEATURES
1)Potholes:
-Circular depressions on the river bed
-Formed by corrasion (abrasion)
-Most effective in flood conditions
-Pebbles which are trapped in hollows on the river bed are swirled about in turbulent/ fast flowing water
2)Plunge pools:
-A large depression at the foot of a waterfall
-This depression is deepened by hydraulic action of the plunging water -Condition: likely to form when the water plunges into less resistant rocks
-It may be further enlarged by rock debris swirling about by turbulent water at the base
3)Rapids:
-A series of very short and fast falls
-Condition: an area of alternating bands of resistant and less resistant rocks
This particular section of the Eno River is special. The rock formations here are the product of a very violent geological past and this entire area was once part of the Hillsborough Caldera (click link on for this earthcache in the same area for more information. This land was much more rocky and its gradients and drop offs much more prominent which is the likely cause of this particular feature in the river.
To log this earthcache:
1) Identify the general directional flow of the Eno River.
2) What type of erosional feature is "Bobbitt's Hole" (careful with this one - try not to think in the present, but the past and what geological features of this river may have looked like)
3) Listed the two types of erosion listed above you see that has caused this feature.
4) At the posted coordinates, stand on river bank looking downstream at the feature. What direction is the gradient of the river flowing causing the erosion you see at the feature?
Note - From the parking coordinates, take the Cole Mill Trail (blazed yellow) to Bobbitt's Hole Trail (blazed red) staying along the river. This is a gorgeous hike. Pay attention to the geological features along the way on both sides of the river. This is about a 3.5 miles (give or take) round trip hike on a well maintained trail. But do be careful during wet periods and take heed of signs posted on the trail that divert sections that are fragile. Please also stay on the trail as many areas here are ecologically fragile. Enjoy the spring, summer and fall flowers. And during the winter, take notice of the great rock features exposed on the cliff sides. Also, why swimming is allowed at Bobbitt's Hole, it is discouraged unless you are a strong swimmer. It is measured to be up to 18 feet deep. There are, however, many places to sit, relax, have lunch, take a nap in the shade, put up a hammock, wade above Bobbitt's or down river below it. As always, though not required, please take pictures. I love pictures anywhere along the Eno.
More info on the trails in this setion of the park can be found here http://enotrails.com/cole-mill-area.htm
| I am a proud |
 |