Most caves created from dissolved rocks are formed in limestone or dolomite, but Oregon Caves was formed in marble.
Of the more than 3,900 cave systems managed by the NPS, only those in Oregon Caves National Monument and Preserve, Kings Canyon National Park, and Great Basin National Park have marble caves.

The parent rock in which the cave developed was formed about 190 million years ago as limestone that was part of a tectonic plate beneath the Pacific Ocean.
Granitic plutons intruded this part of the ocean crust, the Applegate terrane, about 160 million years ago.
As the oceanic crust carrying the terrane subducted under the North American plate, the terrane accreted onto the North American Plate and the limestone was subjected to heat and pressure that metamorphosed it to marble.
Further tectonic movements eventually lifted the marble to about 4,000 feet (1,200 m) above sea level.
The marble block containing the cave is at least 1,080 feet (330 m) long, 490 feet (150 m) wide, and about 390 feet (120 m) high.

The cave's creation took place long after the marble formed.
As groundwater seeped into cracks in the marble, it eventually dissolved enough rock to expand some of the cracks to the size of tunnels.
Generally, the age of a cave cannot be determined directly because the cave itself is an empty space, but scientists can sometimes determine the age of speleothems or sediments in a cave.
An early 21st-century study of speleothem development in Oregon Caves focused on the past 380,000 years.
Based on the available evidence, the cave is thought to be at least a million years old and "probably not much older than a few million" years.

Marble has a more coarse-grained texture than limestone, but both are made of calcite (CaCO
₃).
Caves often develop when slightly acidic groundwater dissolves calcite along natural fractures in the rock.
A reversal of the dissolving process can create flowstone and dripstone such as stalactites, which hang from cave ceilings like icicles, and stalagmites, cone-shaped masses that form on cave floors, usually directly below stalactites.
These structures form when acidic groundwater with a high concentration of dissolved calcite drips slowly from the ceiling of an air-filled cave, becomes less acidic, and leaves some of its calcite behind as a solid precipitate.
Oregon Caves includes a variety of cave formations created through precipitation of calcite.
Although many of the speleothems in the public sections of the cave have been broken, discolored by human skin oils, or otherwise damaged, the narrow twisting passages of the "show cave" have been largely preserved.

The cave is not pure marble.
Streams have deposited silts and gravels from the surface.
Dikes of diorite, an igneous rock that was part of a pluton, and shales and sandstones, sedimentary rocks interbedded with the marble, are part of the cave.
 
The monument has more than 50 paleontological sites ranging in age from Late Pleistocene to Holocene.
A fossil of a grizzly bear more than 50,000 years old and a jaguar fossil between 40,000 and 20,000 years old have been found in the cave.
Other fossils include amphibians, and rare finds of the mountain beaver and the blue grouse.
The monument's
mammalian fossils, found in non-public sections of the cave, are of national significance.
Most caves created from dissolved rocks are formed in limestone or dolomite, but Oregon Caves was formed in marble. Of the more than 3,900 cave systems managed by the NPS, only those in Oregon Caves National Monument and Preserve, Kings Canyon National Park, and Great Basin National Park have marble caves.
The parent rock in which the cave developed was formed about 190 million years ago as limestone that was part of a tectonic plate beneath the Pacific Ocean. Granitic plutons intruded this part of the ocean crust, the Applegate terrane, about 160 million years ago. As the oceanic crust carrying the terrane subducted under the North American plate, the terrane accreted onto the North American Plate and the limestone was subjected to heat and pressure that metamorphosed it to marble. Further tectonic movements eventually lifted the marble to about 4,000 feet (1,200 m) above sea level. The marble block containing the cave is at least 1,080 feet (330 m) long, 490 feet (150 m) wide, and about 390 feet (120 m) high.
The cave's creation took place long after the marble formed. As groundwater seeped into cracks in the marble, it eventually dissolved enough rock to expand some of the cracks to the size of tunnels. Generally, the age of a cave cannot be determined directly because the cave itself is an empty space, but scientists can sometimes determine the age of speleothems or sediments in a cave. An early 21st-century study of speleothem development in Oregon Caves focused on the past 380,000 years. Based on the available evidence, the cave is thought to be at least a million years old and "probably not much older than a few million" years.
Marble has a more coarse-grained texture than limestone, but both are made of calcite (CaCO
₃). Caves often develop when slightly acidic groundwater dissolves calcite along natural fractures in the rock. A reversal of the dissolving process can create flowstone and dripstone such as stalactites, which hang from cave ceilings like icicles, and stalagmites, cone-shaped masses that form on cave floors, usually directly below stalactites. These structures form when acidic groundwater with a high concentration of dissolved calcite drips slowly from the ceiling of an air-filled cave, becomes less acidic, and leaves some of its calcite behind as a solid precipitate. Oregon Caves includes a variety of cave formations created through precipitation of calcite. Although many of the speleothems in the public sections of the cave have been broken, discolored by human skin oils, or otherwise damaged, the narrow twisting passages of the "show cave" have been largely preserved.
The cave is not pure marble. Streams have deposited silts and gravels from the surface. Dikes of diorite, an igneous rock that was part of a pluton, and shales and sandstones, sedimentary rocks interbedded with the marble, are part of the cave.  
The monument has more than 50 paleontological sites ranging in age from Late Pleistocene to Holocene. A fossil of a grizzly bear more than 50,000 years old and a jaguar fossil between 40,000 and 20,000 years old have been found in the cave. Other fossils include amphibians, and rare finds of the mountain beaver and the blue grouse. The monument's
mammalian fossils, found in non-public sections of the cave, are of national significance.
Oregon Caves National Monument and Preserve