Introduction

The extent to which chemical dissolution and mechanical abrasion each contribute to the erosion of cave passages in limestone is an open question. In cave riverbeds that are mixed alluvial and limestone bedrock, we sometimes see clearly scalloped bedrock. The uniquely soluble properties of limestone imply that these scallops are the result of chemical dissolution. However, because we see silt, sand, and gravel deposits that shift in size and location, we infer that there may also be physical abrasion from sediment impacts on the scalloped bedrock surface. In this study we installed micro-erosion stations to measure the rate of erosion of the scalloped floor in Roaring River to learn more about the erosional processes that contribute to cave passage enlargement.

To investigate the time scales involved in karst limestone erosion, we performed repeat structure-from-motion (SfM) scans and established micro-erosion monitoring (MEM) stations on a sculpted bedrock surface of the St. Louis Limestone that experiences frequent submersion. The field area for this micro-erosion study is located past the end of Silliman Avenue, near the shore of Roaring River just upstream from its confluence with Echo River on the Main Cave map sheet of the Mammoth Cave System. This passage has a cross section of about 50 m² and is submerged approximately 60% of the time (fig. 1). The ceiling and walls are in the St. Louis limestone with light streaks in the otherwise dark ceiling that we interpret as tool marks (fig. 2). The floor of this stretch of Roaring River passage is heavily scalloped and the exposed streambed surface area at the field site is approximately 20% bedrock, 20% limestone breakdown blocks, and 60% alluviated by siliciclastic sediments of the thalweg and channel facies (see Bosch and White, 2018, for descriptions of siliciclastic facies in caves), although these percentages shift from one flood event to the next.

Methods

Hanna (1966) first described the usage of micro-erosion meters in cave settings. His approach has been implemented in several studies (e.g., Luritzen, 1986; Cucchi et al., 1987; Stephenson and Kirk, 1996; Muhammad and Beng, 2002; Allred, 2004; Gabrovšek, 2007, 2008; Furlani et al., 2008, Sanna et al., 2015). Gabrovšek (2009) and Stephenson and Finlayson (2009) used MEM in conjunction with pins set in bedrock in cave erosion studies and reported repeat precision to within 2.5 μm.

We are using a variation on that technique, with a precision depth micrometer held by an MEM plate designed for this project and custom machined by Matt Spetz at the University of Cincinnati Innovation Hub. The top side of the plate has one pocket to fit the depth micrometer. The bottom side has three small pockets to fit the pins that get installed at each field site. These are located 120° from one another. Halfway in between each of the pin pockets is a hole that goes through the thickness of the plate. The threaded rods that are installed at the monitoring site go through the holes, and wing nuts are used to secure the plate firmly into place before taking a measurement with the micrometer (fig. 3, 4).

In addition to measuring a single point at each monitoring station, we are performing repeat SfM scanning. As described by Gómez-Gutiérrez et al. (2014) and Smith and Vericat (2015), SfM scans repeated of the same surface with geolocation control can be used to measure the differences in surface position over time. Direct comparison of SfM and MEM measurements of the same site taken at the same times makes this study unique.

In the cave, site selection criteria included picking the fewest sites possible to represent the different hydrologic and sedimentological conditions that may occur in Roaring River (fig. 5, 6, 7). We chose two sites that are very close to the water when it is at base level (RRMEM1 and RRMEM3). These are likely to be submerged more frequently that the two downstream sites. Additionally, the scallops at RRMEM3 had pea-sized gravel in them. Sites RRMEM2 and RRMEM4 are less than 100 m downstream from 1 and 3, on a bedrock ledge about 1 m above the floor of the passage. RRMEM2 had a small amount of silt on it and in its scallops prior to bedrock washing and pin installation. RRMEM4 is in a location where a several-centimeter-thick layer of silt was removed prior to pin installation. At each station, RRMEM 1 through RRMEM4, we collected baseline data including a precise location survey (fig. 9) from known cartographic points in the cave using established cave survey techniques (Dasher, 2011), photographic images for SfM reconstruction of the surface, and the distance to the bedrock using the MEM. For each MEM site, we took photographs for SfM documentation before pin installation. We then drilled holes and installed the pins and threaded rods using marine-grade epoxy. We seated the template on the rods during installation to ensure that the tops of the rods would coincide with a level plane. The template remained in the cave while the epoxy cured.

Since this field location is in the bed of a baselevel river, the erosion monitoring sites are frequently submerged. This makes it an ideal place to study erosion rates in a cave, but it also introduces a challenge with field site access. In an average year, water levels are low enough to reliably permit safe access intermittently from July through November. To account for the flooding hazard, we closely monitor USGS stream gages on the Green River at the Green River Ferry in Mammoth Cave National Park, and upstream at Dennison’s Ferry in Munfordville, Kentucky (fig. 1). Since the Green River defines baselevel for its in-cave tributary, Roaring River, these two gages indicate directly whether the erosion monitoring sites will be submerged. Furthermore, the approach via Silliman Avenue is safely above the flood zone, and when the water is high, Roaring River can be observed visually from this passage before researchers begin hiking down the sand bank toward river level.

Results

We have collected about one year’s worth of MEM and SfM data. The differences in the bedrock surface that we measured (Tables 1, 2, and 3) are within the range of human measurement error and do not yet represent a sufficient data set to calculate long-term erosion rates. The preliminary data we present here is a baseline for a longitudinal study that we hope others will want to contribute to.

Future Work

Data for this project is available at the online data repository, Pangaea, in association with Bosch’s Open Researcher and Contributor ID (ORCID), 0000-0002-8682-1816 (Bosch et al., 2019; Bosch et al., 2020). Specific in-cave station locations, SfM images, and MEM measurements will continue to be stored there and available for open access indefinitely. Investigators interested in collaboration to continue this long-term project are encouraged to contact the authors.

Acknowledgements

This study is being conducted in cooperation with Mammoth Cave National Park under National Park Service Scientific Research and Collecting Permits # MACA-2017-SCI-0019 and # MACA-2017-SCI-0020. Many thanks to Rick Toomey, Rick Olson, Barclay Trimble, Tim Pinion, Kurt Helf, for supporting this project at MACA, as well as Zachary Bosch-Bird, Tyler Bosch-Bird, Chris Sheehan, Gerek Patrick, Ron Manning, Bryce Belanger, Hannah Lieffring, Alec Matheus, Arthur Spoelman, Bill Spoelman, Seth Spoelman, and Heather Levy for all of your hard work in the cave.

A big thank you to Chris Anderson for filming my explanation of this project for the YouTube series, Science Around Cincy. The video, “Measuring Cave Erosion,” can be viewed at https://www.youtube.com/watch?v=196_F9C6vgc&t=10s.

REFERENCES

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