Fricke's Cave - Alluvial Fan and Erosion
Welcome to Fricke's Cave in Bogue Chitto State Park. It is not actually a cave but a grotto in which rocks sitting on top of softer sediments created unusual erosional artifacts that resembled stalactites in a cave. Today however, most of these features have disappeared because of erosion and human vandalism. Other erosional features are present which is what this earthcache will focus on.
An alluvial fan is a fan or cone shaped deposit of sediment built up by flowing water. This flow comes from a single point source at the apex, or start of the fan. An alluvial fan is usually found where a canyon or restricted channel opens out onto a flat plain.
Example of a large alluvial fan
Fricke's Cave is overlain by a weather resistant sandstone layer that contains many pebble to cobble sized stones although it is not a true conglomerate. The layer under this is composed of sand and clay that is loosely consolidated and easily eroded. When water finds a way through the top layer of rock it begins to erode the softer layer until there is nothing left to support the top layer. The result is Mass Wasting where large pieces of the top layer break off mostly intact. This is what created the formations that gave Fricke's Cave its name.
The loose lower sediments are washed down in rain runoff from the top of the grotto and in some cases they create very nice alluvial fans such as the one you will see at the posted coordinates. Alluvial fans happen on all scales from tens of feet to kilometers, the largest fans are normally in arid climates where there is little vegetation to hold back erosion. In a humid climate like South Louisiana the fans tend to be smaller and are affected by the steepness of the gradient where they are formed.
To Log this Earthcache please email me the following and do not post the answers in your log on GC.com:
Part 1: (Part 1 Reference Waypoint)
1. At the posted coordinates for Part 1 you will see a sign and be standing near a fence with an overlook of the gorge. Tell me your impressions of the geologic features you can see from this vantage point.
2. Using your GPS take an elevation reading. If you have a device that doesn't take elevation you will have an extra task at Part 2.
Part 2: (Posted Coordinates)
1. What is your elevation at the dead end of the boardwalk? Take this reading and subtract it from the elevation in Part 1 for a Cliff Height. If you have a device that does not measure elevation please estimate the height of the cliff you see in front of you.
2. Estimate the distance between the location in Part 1 and the location in Part 2, you may be able to plot the two sets of coordinates with various apps to get a precise answer to this question.
3. Calculate the Slope angle of the alluvial fan as follows: [ Height / Distance ] * 100 = ____ °
4. Lastly, look at the alluvial fan closely. Feel some of the soil near the start of the fan, then feel some of the soil where the fan ends on the board walk. Describe to me differences you notice. Is one more coarse? More fine? What color is it? How does it taste? I'm kidding about the last one.... or am I? Sand you can feel on your teeth while clay is very smooth.
Optional: Post a photo of your experience at Fricke's Cave, please do not post photos that directly show the alluvial fan but any other photos are welcome and please remember to practice CITO!