Chapter 40 -- Karstology Characterizing the mechanism of cavern accretion as "force" tends to suggest catastrophic attack, not a process of subtle persistence. Publicity for Ohio's Olentangy Indian Caverns illustrates the misconception. Formed millions of years ago by the tremendous force of an underground river cutting through solid limestone rock, the Olentangy Indian Caverns. There was no tremendous event millions of years ago; it's been dissolution at a rate barely discernable, century to century. Another rendition of karst stages, this time in elevation, as opposed to cross-section. Juvenile Youthful Mature Complex Extreme 594 DRAFT 8/8/2013 Updates at http://www.unm.edu/~rheggen/UndergroundRivers.html Chapter 40 -- Karstology It may not be the water, per se, but its withdrawal that initiates catastrophic change in conduit cross-section. The figure illustrates stress lines around natural cavities in limestone. Left: Distribution around water-filled void below water table Right: Distribution around air-filled void after lowering water table. Natural Bridges and Tunnels Natural bridges begin as subterranean conduits, but subsequent collapse has left only a remnant of the original roof. "Men have risked their lives trying to locate the meanderings of this stream, but have been unsuccessful." Virginia's Natural Bridge, 65 meters above today's creek bed. George Washington is said to have surveyed Natural Bridge, though he made no mention it in his journals. More certain is that Thomas Jefferson purchased "the most sublime of nature's works," in his words, from King George III. Herman Melville alluded to the formation in describing Moby Dick, But soon the fore part of him slowly rose from the water; for an instant his whole marbleized body formed a high arch, like Virginia's Natural Bridge. As we noted in chapters dealing with literary fiction, whaling ships have inexplicable propensity to encounter underground seas. And while we're visiting Natural Bridge, here's a children's version, "The River that was Found and Lost," St. Nicholas (1911) by Mary Mapes Dodge. It all depends on how you look at it. Some name it "The River that was Found"; others, "The River that was Lost"; but I observe that all visitors refer to it by one or the other of the two 595 DRAFT 8/8/2013 Updates at http://www.unm.edu/~rheggen/UndergroundRivers.html Chapter 40 -- Karstology names, mostly as "lost," probably because that is the name on the sign that directs the visitor to follow the path through the ravine, for about a quarter of a mile, north of the Natural Bridge of Virginia. Out of a cavern the little "river" gurgles and bubbles, sometimes even with a tinkling sound. It flows clear and cool for a few feet, and then vanishes under the ledge. There is no other trace of the stream in all the surrounding territory. It has no sphinx-like silence as one propounds questions as to its origin and its purpose, but seems almost to laugh, "I'll never tell," when it is asked, "Whence are you coming and whither are you going?" Natural Tunnel in western Virginia, 260 meters long, up to 60 meters wide and 24 meters in height, was carved through a limestone ridge by Stock Creek. The railroad constructed through the tunnel in 1893 still operates, but only to transport coal. When the tracks are clear, Natural Tunnel State Park offers walking tours. William Jennings Bryan called Natural Tunnel the “Eighth Wonder of the World,” a title, we may recall from Chapter 39, also awarded to Old Jumbo well of Iowa, Natural Tunnel $5.00 Wonder of the World The Eighth Natural Tunnel was a subject of Outlines of the Earth's History, A Popular Study in Physiography (1898) by Nathaniel S. Shaler. All the while that subterranean streams are cutting the caverns downward, the open-air rivers into which they discharge are deepening their beds, and thereby preparing for the construction of yet lower stories of caves. These open-air streams commonly flow in steep-sided, narrow valleys, which themselves were caves until the galleries became so wide that they could no longer support the roof. Thus we often find that for a certain distance the roof over a large stream has fallen in, so that the water flows in the open air. Then it will plunge under an arch and course, it may be, for some miles, before it again arrives at a place where the roof has disappeared, or perhaps attains a field occupied by rocks of another character, in which caverns were not formed. At places these old river caverns are abandoned by the streams, which find other courses. They form natural tunnels, which are not infrequently of considerable length. One such in southwestern Virginia has been made useful for a railway passing from one valley to another, thus sparing the expense of a costly excavation. 596 DRAFT 8/8/2013 Updates at http://www.unm.edu/~rheggen/UndergroundRivers.html Chapter 40 -- Karstology Scallops The small scoop-like depressions in karst walls, floors and ceilings are known as "scallops." The depressions, typically a few centimeters in width, have a steep wall on the upstream side and a gentler slope on the downstream side. The figure illustrates the formation of a scallop. Boundary-layer flow separated from the main current splits when it re-engages, the lower portion eddying upstream, an effect at a much larger scale known to whitewater enthusiasts as a "hydraulic." Overshooting flow continues downstream and the process repeats. L Abrasion from the eddy sculpts the scallop. While we refrain from crediting engineering fluid mechanics with omnipotence in all 45,000 maters fluid, quantitative explanation of the process has indeed been hypothesized, The figure to the right, adapted from 40,000 "Deducing Flow Velocity in Cave Conduits from Scallops," NSS Bulletin 36:2 (1974) by Rane Curl, relates a particular Reynolds 35,000 number, Re, to the ratio of scallop length L to conduit diameter D. The figure's shape has theoretical basis; its 30,000 numerical placement is the result of experimentation. Re, Scallop Reynolds Number 25,000 10 100 D/L Take a 2-meter circular conduit with 0.1 meter scallops. D/L of 20 indicates a Re of 32,500. Re ν V L where V is flow velocity, and v is kinematic viscosity, 0.0000013 square meters/second at 10o C. In our example, V is 0.4 meters/second. 597 DRAFT 8/8/2013 Updates at http://www.unm.edu/~rheggen/UndergroundRivers.html Chapter 40 -- Karstology Pop Quiz Let us test our knowledge of karstology. Q: What can we deduce about the geologic history of the cross-section shown to the right? A: As stalactites and stalagmites don't form under water, this cavern subsequently flooded. Q: And what about the future? A: Chapter 81, Mainlining the Sewage, suggests an unfortunate consequence of an oil leak from the red car. Q: Stalagmites or Stalactites in Indiana's Lost River? A: Stalactites. 598 DRAFT 8/8/2013 Updates at http://www.unm.edu/~rheggen/UndergroundRivers.html Chapter 41 -- Sinkholes CHAPTER 41 SINKHOLES Sinkholes are natural depressions in the land surface caused by the ceiling collapse of karst cavities not far below the surface. Sinkholes may range in both diameter and depth from a few to several hundred meters, may vary in form from bowl-shaped to precipitous chasms and may be created gradually or catastrophically. Our interest in sinkholes derives from the fact that sinkholes are almost aways associated with subterranean streams. A subterranean cavern of sinkhole magnitude is the product of water pass-through at a rate sufficient to bear away vast quantities of solute. A stagnant aquifer leaves no caves. It is not uncommon for inflow and outflow conduits to be recognizable immediately after sinkhole collapse, but once the sinkhole attracts local runoff, washed-in soil tends to obscure the hydrologic features. Degraded limestone covered by only a thin horizon of soil can lead to a saucer-shaped depression that develops continuously, but slowly. If the limestone is replaced by cascading particulates too stable to be washed onward, a sinkhole may remain only a few meters in dimension. 599 DRAFT 8/8/2013 Updates at http://www.unm.edu/~rheggen/UndergroundRivers.html Chapter 41 -- Sinkholes Whatever the surficial appearance of a sinkhole, however, there is reasonable certainty of active karst channels below Precipitation Typical hydrologic pathways Runoff Evaporation Spring Journalistic Pursuit As our subterranean journey is as much a passage though social history as it is one of science, we'll note how sinkholes have been described in popular journalism. While the Roman encyclopedists (Chapter 3) were geographically aware of karst, theirs wasn't a scientific curiosity. By the 18th century, however, the emerging natural sciences had become parlor talk in lettered class. In The Natural History of Lancashire, Cheshire, and the Peak in Derbyshire (1700), Charles Leigh noticed that in Derbyshire, Several cavities, which the Inhabitants call Swallows, [and] into these run several Rivulets of Water, but where the water has its Exit is not known... It is from them these Subterraneous Rivers are formed. Derbyshire landscape "Observation on the White Limestone and other Eocene or Older Tertiary Formations of Virginia, South Carolina, and Georgia," Quarterly Journal of the Geologic Society, 1845, One of the characteristic features of the region of tertiary white marl and limestone in South Carolina and Georgia, is the frequent occurrence of lime-sinks, or funnel-shaped cavities, arising from natural tunnels in the subjacent limestone, through some of which subterranean rivers flow. At Wantoot, there is one of these sinks in the limestone, and a spring issues from the rock so much above the temperature of the air during a frost as to send off clouds of steam.