Cave of the Mounds the Interior and the National Park Service. To be considered a NNL, "a site must be one of the best examples of a natural region's characteristic biotic or geologic features." Cave of the Mounds is home to many varieties of speleothems. As with most caves, stalactites and stalagmites are common. Formations found inside the cave include: These formations come in many different brilliant colors such as reds, browns, blues, and grays. The reds and browns are caused by the presence of iron oxide in the formation. Similarly, blues and grays are caused by manganese oxide. Some speleothems are even partially
Black Canyon of the Colorado the Nevada side of the canyon is the Sauna Cave. This cave was drilled by miners working on Hoover Dam while it was being constructed. It is a deep cave with calcium carbonate crystals on the walls. At the end of the cave is a hot spring, which causes temperatures in the cave to reach . One prominent canyon that splits from this canyon south of the Hoover Dam is Boy Scout Canyon. Boy Scout Canyon has water running through it that is warmed from a geothermal source. On the Arizona side of the river, there are several large hot