Heading south on Hwy. 63 through the Central Plateau, one will quickly notice that the natural heritage of historic prairies, savannas and open oak woodlands no longer exists. In the least dissected region of the Ozarks, an area perched on carbonate rock with exceedingly dry soils, most of the landscape has been converted to agriculture. Just before reaching the Arkansas border, however, turn onto Hwy. W to visit a spectacular geologic formation that makes the rather featureless, monotonous drive worth it.
A short walk through a nice little patch of dry (very dry) chert woodlands reveals an enormous collapsed cave system with vertical walls measuring 130 feet high. All but one small 250 ft.-wide span of dolomite collapsed into the cave roughly 10,000 years ago, creating an enormous chasm with a small natural bridge spanning the width. The resulting canyon is roughly a mile long with two forks, the convergence of other collapsed caves. One fork represents Bussell Branch, an ancient creek that thousands of years ago flowed on top of the then-concealed cave system. The historic creek bed once continued south into Arkansas; the original drainage can be seen on topographic maps and in aerial photographs. Bussell Branch drains 24 square miles of surrounding land and waterways. The second fork in the canyon drains 2 square miles, making the entire watershed for the canyon 26 square miles. During heavy rain events, the chasm fills with water, creating a muddied lake that persists for weeks or months.
Water drains slowly out of the chasm, percolating through a small portal, ending up at nearby Mammoth Spring in Arkansas. Last week, armed with flashlights and muddied boots, we hiked the distance of the chasm, through the natural bridge to the sinkhole. It's actually a rather large opening which, under normal circumstances, would likely drain water quickly. But in the early part of the 20th century, rain events following a massive tornado blow down sent thousands of trees and debris into the chasm, subsequently blocking the cave opening. The blockage remains today, but early explorers recount visits into the cave, providing evidence of a large lake:
The ceiling dipped so we were not able to stand straight, and the guide had never gone farther; but to his surprise here was a light boat which I am ready to admit he displayed no eagerness to appropriate to his own use, and swimming about it, close to shore, were numerous, small, eyeless fish, pure white and perfectly fearless; the first I have ever seen....Cave Regions of the Ozarks and Black Hills, Luella Agnes Owen (1898)
In the 1990s, officials sent a probing device and camera through the debris into the cave. It was determined infeasible to remove the debris, with the direct impacts to Mammoth Spring considered. The slow percolation process of the chasm provided by the blockage helps make Mammoth Spring the second largest spring in the Ozarks, producing 9 million gallons of water each hour (Big Spring in Van Buren, Missouri is the largest in the Ozark Highlands, producing on average 278 millions of gallons of water daily). With the blockage removed, an enormous torrent of water would undoubtedly flood the adjacent Arkansas community. Of course, no one knows if the cave fish still live in the cave or if the blockage destroyed the natural water flow to the detriment of the population.
Annual plants like polygonums, clearweed and nettles dominate the floor of the chasm. With water so persistent after rain events, it comes as no surprise that hardly any woody plants thrive down there. Rangy grape vines spill over the sides of the steep slopes, covering many of the geologic features during the growing season. The woods surrounding the chasm are rich with woodland species, despite the lack of fire management. Like Mammoth Spring, the deep canyon outside of Thayer is a designated National Natural Landmark.