File:Gypsum snowballs (Snowball Dining Room, Mammoth Cave, Kentucky, USA) 10 (38016992355).jpg

Cleaveland Avenue is a long tubular passage in western Kentucky's Mammoth Cave, the longest cave on Earth. As of fall 2017, 412 miles of passages have been explored and mapped. Tubular passages are wider than they are tall, and are phreatic in origin - they form at or below the water table. In this case, the water table has long since lowered and the passage is currently dry. Local base level is the Green River. All water-bearing passages in the Mammoth Cave system drain toward the Green River, along which water emerges at springs.

When first discovered, Cleaveland Avenue had abundant, complex, large gypsum structures - some the size of celery stalks. Naturally-detached speleothem littered the floor. The passage has been intensely vandalized by thousands of guides and tourists over the decades. Moderately impressive gypsum speleothem still remains, such as gypsum coatings, crusts, flowers, and helictites. The original color of most of the gypsum speleothem was bright white. Almost all of it is now stained or highlighted with dark gray coloration - this is from decades of lantern smoke. Some gypsum is orangish-brown in color, due to minor iron oxide impurity.

The sulfur in the gypsum is derived from pyrite in an overlying unit. Downward-percolating water oxidized the pyrite (FeS2 - iron sulfide - "fool's gold") and produced sulfuric acid (H2SO4). The acid dissolved part of the limestone, which liberated calcium (Ca). In the presence of water, the calcium and the sulfate (SO4) produced gypsum.

The speleothem shown here are gypsum snowballs - irregularly hemispherical masses of gypsum. When first discovered, this site was likened to a place where boys had just engaged in a snowball fight. The gypsum was originally white. Some of the smoke staining was removed from these snowballs during a long, careful cleaning project conducted by the park. Pristine, bright white gypsum snowballs are present in non-vandalized parts of the Mammoth Cave system (e.g., Turner Avenue - see: <a href="https://www.flickr.com/photos/jsjgeology/8321107667">www.flickr.com/photos/jsjgeology/8321107667</a>).

Cleaveland Avenue is at Level C in the Mammoth Cave system. Level C passages started forming about 1.9 million years ago. A subterranean river used to flow through the passage.