File:Giant canyon passage (Main Cave, Mammoth Cave, Kentucky, USA) 6 (38076299212).jpg

Western Kentucky's Mammoth Cave is the longest cave system on Earth, with 412 miles known and mapped as of fall 2017. The name does not refer to the early discovery of fossil mastodon or mammoth bones here. Rather, the name refers to the immense size of many rooms and passages. The system is famous for having giant canyon passages. Canyons are taller than they are wide. Giant canyons often have a boxy or rectangular cross-section because they are significantly filled with sediments and breakdown - often more than 50% filled. At one site (at least), about 80 feet worth of fill is present.

Shown above is Main Cave, the main trunk in Mammoth Cave Ridge. The original passage stretched for many miles, but it has become erosively segregated into segments. Upstream, in the southeastern parts of Mammoth Cave Ridge, Main Cave is called Grand Avenue, or Kentucky Avenue. A short segment called Sandstone Avenue, near the Carmichael Entrance, is also part of this main trunk. Breakdown blocks both ends of Sandstone Avenue. Just downstream from Sandstone Avenue is the Violet City entrance, which is in a sinkhole. From Violet City ~northward, there is a continuous, uninterrupted passage called Main Cave. This continues near the Historic Entrance as Broadway Avenue and Audubon Avenue. A large breakdown pile underneath a surface sinkhole blocks Audubon Avenue - further downstream segments of the passage have been eroded away.

Dim lighting makes photography difficult. The view seen here is looking in the old upstream direction. A subterranean river used to flow through here.

Main Cave is at level B in the Mammoth Cave system, which is the 2nd-oldest and the 2nd-elevationally highest set of passages. Level B passages started forming during the Pliocene, 2 to 4 million years ago.

The ceiling and walls of Main Cave consist of marine limestones of the upper Ste. Genevieve Limestone (upper Middle Mississippian).