Welcome to the CacheMore 2023 series! This cache was hidden as part of a friendly geocaching competition to encourage exploring the areas surrounding North Central WV, Southwest PA, and Western MD. To learn more and participate for yourself, please click HERE.
Welcome to Mason-Dixon Historical Park. Known as the most westerly point that Charles Mason and Jeremiah Dixon personally surveyed, this 300-acre park preserves the history of the expedition as well as provides a unique place for people of all ages to explore. Whether it is hiking to the WV / PA marker atop Brown's Hill, or taking a stroll through the fairy garden, this unique park has something for everyone. This earthcache will take you to the park's boundary to a little spot known as Tucker Falls.
This fourteen-foot-high waterfall is about a 1.5-mile round-trip hike on gentle trails with minimal elevation gain. Your journey starts at the park entrance; from there, take the green trail along Dunkard Creek to the "Third Crossing of Dunkard Area" then turn on the orange trail toward the waterfall. Please review the park map pictured above or reference the coordinates in the waypoints to guide you. Sturdy hiking shoes are a must; as you'll learn while completing this earthcache, rainwater runoff can drastically change the conditions of the trail. As a result, all trails have the potential to be wet at times. Lastly, please obey all signage and do not leave marked trails. Tucker Falls is located at the end of the orange trail; proceeding past the marked signs is prohibited and is considered trespassing. All questions for this earthcache can be answered at the posted coordinates and additional waypoints.
The Geology of a Waterfall
The caprock model of waterfall formation states that the river courses over resistant bedrock, erosion happens slowly and is dominated by impacts of water-borne sediment on the rock, while downstream the erosion occurs more rapidly. As the watercourse increases its velocity at the edge of the waterfall, it may pluck material from the riverbed, if the bed is fractured or otherwise more erodible. Hydraulic jets and hydraulic jumps at the toe of a fall can generate large forces to erode the bed, especially when forces are amplified by water-borne sediment. Horseshoe-shaped falls focus the erosion to a central point, also enhancing riverbed change below a waterfall.
A process known as "potholing" involves local erosion of a potentially deep hole in bedrock due to turbulent whirlpools spinning stones around on the bed, drilling it out. Sand and stones carried by the watercourse, therefore, increase erosion capacity. This causes the waterfall to carve deeper into the bed and to recede upstream. Often over time, the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it. The rate of retreat for a waterfall can be as high as 1.5 meters per year.
Often, the rock stratum just below the more resistant shelf will be of a softer type, meaning that undercutting due to splashback will occur here to form a shallow cave-like formation known as a rock shelter under and behind the waterfall. Eventually, the outcropping, more resistant cap rock will collapse under pressure to add blocks of rock to the base of the waterfall. These blocks of rock are then broken down into smaller boulders by attrition as they collide with each other, and they also erode the base of the waterfall by abrasion, creating a deep plunge pool in the gorge downstream.
Streams can become wider and shallower just above waterfalls due to flowing over the rock shelf, and there is usually a deep area just below the waterfall because of the kinetic energy of the water hitting the bottom. However, a study of waterfalls systematics reported that waterfalls can be wider or narrower above or below a falls, so almost anything is possible given the right geological and hydrological setting. Waterfalls normally form in rocky areas due to erosion. After a long period of being fully formed, the water falling off the ledge will retreat, causing a horizontal pit parallel to the waterfall wall. Eventually, as the pit grows deeper, the waterfall collapses to be replaced by a steeply sloping stretch of river bed. In addition to gradual processes such as erosion, earth movement caused by earthquakes or landslides or volcanoes can lead to the formation of waterfalls.
(Reference)
Rock Layering
In geology and related fields, a stratum is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as either bedding surfaces or bedding planes. Typically, a stratum is generally one of a number of parallel layers that lie one upon another to form enormous thicknesses of strata. The bedding surfaces (bedding planes) that separate strata represent episodic breaks in deposition associated either with periodic erosion, cessation of deposition, or some combination of the two. (Reference)
A facies is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formation), and the changes that may occur in those attributes over a geographic area. A facies encompasses all of the characteristics of a rock including its chemical, physical, and biological features that distinguish it from adjacent rock. (Reference)
Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the kind of cleavage feature that develops. Generally, these structures are formed in fine-grained rocks composed of minerals affected by pressure solution. (Reference)
The presence of fabric elements such as the preferred orientation of platy or elongate minerals, compositional layering, grain size variations, etc. determines what type of cleavage forms. Cleavage is categorized as either continuous or spaced. (Reference)
Questions
Once at the posted coordinates, please make some observations based on the information shown above to describe the geological significance of Tucker Falls. Once you are satisfied with your observations, please submit your answers to the CO.
1. Please describe the flow rate of Tucker Falls during your visit. Do you think more or less water once flowed through the canyon to create the geological phenomenon shown here?
2. Describe the facies of the rock both above and below the waterfall. What type of cleavage makes up the lower stratum?
3. Are attrition and abrasion present?
4. How do you think the topography of Brown's Hill and Mason-Dixon Park contributed to the potholing seen at GZ?
5. (Optional) Post a picture of yourself at GZ during your visit.
