In caves, canyon passages are taller than they are wide. Boone Avenue in western Kentucky's Mammoth Cave system is a beautiful example of a canyon passage. Much of it is only wide enough for single-file walking. Canyon passages form in the vadose zone - above the water table. When a subterranean river flowed through here, it was heading downhill toward the old water table (= phreatic zone). The phreatic-vadose boundary occurs at the northwestern end of Boone Avenue, next to Snowball Dining Room. The paleoriver in Boone Avenue was a tributary that joined a larger river that occupied what is now Cleaveland Avenue, a tubular passage formed in the phreatic zone (= at or below the water table). Snowball Dining Room is part of Cleaveland Avenue. The Cleaveland Avenue-Boone Avenue intersection is at about 550 feet elevation, as are all level C passages in the Mammoth Cave system.

Boone Avenue is developed in the Ste. Genevieve Limestone (upper Middle Mississippian), which consists principally of shallow marine limestones. Minor dolostone is present in parts of the formation. The walls of Boone Avenue have exposures of (in ascending order): upper Fredonia Member, Spar Mountain Member, Karnak Member, and Joppa Member.

Boone Avenue is a multilevel canyon - it has 3 or 4 levels. the height of the passage varies tremendously - the ceiling is sometimes almost out of sight. Relatively little speleothem ("cave formations") is present along this stretch of cave, with the exception of "Mary's Vineyard", which has a nice concentration of coralloids (a.k.a. knobstone; a.k.a. cave popcorn). Near Mary's Vineyard, the entrance to another multilevel canyon passage is encountered - El Ghor. Modern tours do not enter El Ghor.

Several closely-spaced cave passages are present in this section of Mammoth Cave. Boone Avenue and nearby passages are developed along the axis of a subtle syncline.

Well-developed and abundant scallops are present along the walls of Boone Avenue. Scallops are asymmetrical, ridge-like, dissolutional features formed along cave walls by flowing water. They somewhat resemble asymmetrical ripple marks, in that they form in a one-directional current, and the short side of each scallop represents the downstream direction.

Boone Avenue and other level C passages in Mammoth Cave started forming at about 1.92 million years ago, during the Late Pliocene (sensu traditio; = Early Pleistocene, sensu novum). This was a little after the start of a major glacial ice advance. Level C passages formed during development of the Teays River (<a href="https://upload.wikimedia.org/wikipedia/commons/d/d4/Teays_River_watershed;geo2.gif" rel="nofollow">upload.wikimedia.org/wikipedia/commons/d/d4/Teays_River_w...</a>). The Teays was part of a major drainage system that started in Virginia and headed northwest into West Virginia, and Ohio, then westward through northern Indiana and northern Illinois. Subsequent glacial ice advances blocked the Teays and diverted it. The Teays is now mostly buried in glacial sediments. Mammoth Cave's level C phreatic passages were abandoned at about 1.39 million years ago, later in the Pleistocene.

The timing of Mammoth Cave's passage development comes from radiometric dating of cosmogenic nuclides in cave passage sediment fills. Aluminum-267 and beryllium-10 are produced in tiny amounts in quartz sediments at or near Earth's surface. When the sediments are carried underground and deposited in caves, these isotopes decay at different rates and are not replenished. Measuring the Al-26/Be-10 ratio in cave sediments provides the date of sediment burial. This dating method is good up to about 5 million years. See Granger et al. (2001) for details.

Boone Avenue can be examined by visitors on the Grand Avenue Tour (4 miles, 4 hours) and the Wild Cave Tour (6 miles, 6 hours). Tours walk in the upstream direction.

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