Explore Geology
geology fieldnotes title

Oregon Caves

National Monument


cover of park brochure

park geology subheading
photo taken inside the cave of speleothems
Oregon Caves National Monument, Oregon

Moonmilk, dripstone, boxwork, cave "ghosts". Strange names for a strange place that seems out-of-this-world. Oregon Caves, though, is clearly of this world, being linked directly to powerful forces shaping the Earth inside and out. Violent geologic events spanning millions of years have created a cave nestled within all six of the world’s major rock types.

The World Below
Creation of the cave began 220 million years ago when continent and ocean rock sideswiped, tearing open an ocean basin. Sea creatures lived and died in the basin, their remains forming calcite-rich muds that hardened into limestone and silica-rich muck that solidified as chert. Unable to sink out of the way, the ocean rock smashed into the continent head-on, obliterating the basin. Land debris and rock melted and crystallized as a lightweight, granite-like rock that "fried" and baked limestone into marble. As it rose, the molten mass broke and pushed aside rock along faults, slowly lifting marble into mountains above dense ocean rock.

Another collision tore the Siskiyous from the Sierras and twirled them to point east and west. In faults and between angled rock layers, water, acidified by carbon dioxide, followed weaknesses to dissolve out a crazy mix of tilted rooms and twisting passages. Rock sliding under the Cascades melted and rose forming volcanoes that dumped ash into the cave. By about 10,000 years ago, erosion had opened a cave entrance. The resulting loss of carbon dioxide in the cave air allowed seeping water to lose its acid as well and to deposit limestone and cemented gravels, completing the group of six major rock types.

block diagram The block diagrams show two phases in a geological process that have taken more than 200 million years. The top diagram shows the ocean basin torn open by side-swiping continental and oceanic rock.

The bottom diagram shows the uplifted marble mountains- the Siskiyous – torn into their east-west orientation. The colors depict the different types of rock: gray denotes basalt, pink is the quartz diorite, and white is the marble in which the cave formed.

cave geology

  1. Molten, plutonic matter oozed from inside the Earth, filling faults and solidifying into this quartz diorite dike.
  2. When the quartz diorite dike intruded, the great heat of the molten mass "fried" the adjacent calcite, changing it - metamorphosing it – into a contact metamorphic rock.
  3. Heat from tremendous bodies of molten plutonic matter "baked" limestone into marble, making it a regional metamorphic rock.
  4. Volcanic ash from distant eruptions filtered into the cave and was deposited in fine horizontal layers.
  5. Fragment sedimentary rock is created from old, worn-down rock fragments, which are cemented together by calcite.
  6. Chemical sedimentary rock is formed from rock materials dissolved by water and deposited in fine-grained layers, gradually hardening into rock.

Oregon Caves may surprise you. Small in size, it is rich in diversity. That richness can be found both underground amidst narrow, winding passageways and above ground where old growth forest harbors a fantastic array of animals and plants found nowhere else. You will discover a land rich in conifers, wildflowers, birds, and amphibians. An active marble cave and underground stream reveal the inside of one of the world’s most diverse geologic realms.

Exploring Underground
A tour through Oregon Caves is an adventure in geology and underground life. All six of the world’s major rock types and a myriad of calcite formations decorate the cave. You will find striking parachute like flowstone at Paradise Lost and what appear to be giant ribs as you squeeze through the Passageway of the Whale. Tiny rimstone dams resemble miniature waves on the sea. Minute mushrooms grow on the massive root of a Douglas-fir. Other cave creatures are often secretive but you may see daddy longlegs, crickets, moths, and bats.

It is the drip, drip, drip of water, though, that decorates the cave, building the bizarre and eerie sculptures. How the water moves – seeping, dripping, flowing, - and how many crystals come out of the individual drops of water dictate the shape and size of formations made of calcite, the same mineral found in chalk, cement, and eggshells. As easily as formations are created, they can also be dissolved. Rising warm air condenses on the cold ceiling. Acidified by carbon dioxide in the cave, this water dissolves formations. "Cave ghosts", nubbins of former stalactites, are all that remain.

Visiting the Cave
The temptation to reach out and touch can be overwhelming, but formations break easily and oils in your skin will discolor them. Look, but do not touch. The cave is cool, wet, and slippery in places. Cave temperatures remain in the 40s° F year round. Wear rubber-soled shoes and warm clothing. Waiting times for a tour can reach 90 minutes in the summer.

Cave Formations
Some formations provide information about the cave. Water evaporated by air flowing in from outside leaves a residue of bumpy cave popcorn. Just as hikers use moss on the north side of trees, cavers use popcorn as a compass to find new passages or, when lost, their way out. Moonmilk is made of tiny calcite crystals but has the look and feel of cottage cheese. From early times it was a folk medicine smeared on livestock to heal wounds. Because it cured infections almost overnight, people called it "gnome’s milk", a seeming gift from the nether world. But some folklore is true moonmilk is created by the same type of bacteria used to make today’s antibiotics.

Discovery, Development and Rebirth
As his last match flickered out, 24-year-old hunter Elijah Davidson found himself in the total blackness of the cave. Davidson was chasing after his dog Bruno, who in turn was pursuing a bear. One following the other, the dog and bear entered a dark hole high on the mountainside. Davidson stopped at the mysterious dark entrance. He could see nothing, but an agonizing howl pulled him into the cave to save his dog. Now the matches were gone and Davidson was in total darkness. Fortunately, he was able to wade down a gurgling, ice-cold stream and find his way back into daylight. Bruno soon followed. It was 1874.

Later, other brave souls explored deeper into the cave, returning home to tell of its great beauty and mystery. In 1907, a party of influential men, including Joaquin Miller, the "poet of the Sierras", visited the cave. Charmed by it, Miller wrote of the "Marble Halls of Oregon." The ensuing publicity alerted federal officials to the possibility of preserving the cave. In 1909 President William Howard Taft proclaimed a tract of 480 acres as Oregon Caves National Monument. In 1922 an automobile road reached the park, and 12 years later a six-story hotel, the Chateau, was constructed. The very same year, 1934, Oregon Caves National Monument was transferred from the Forest Service to the National Park Service, which still administers it.

Workers blasted tunnels and widened passages in the cave during the 1930s. They put waste rocks in side passages, covering many limestone formations. Changes in air flow patterns altered the growth of formations and caused greater swings in temperature. Freezing water now cracked rock layers. Lights in the cave promoted the growth of algae, which turned portions of the cave green and dissolved some formations. Smoke from torches and lint coming off visitors' clothing blackened other portions.

Since 1985 the National Park Service has removed more than a thousand tons of rubble in its efforts to restore the cave. Transformers, asphalt trails, and cabins were removed to prevent sewage or oils from leaking into the cave from the surface. Thousands of formations buried under rubble were uncovered. Crystal clear water once again cascades over white marble. Some broken formations have been repaired with epoxy and powdered marble. Airlocks have restored natural cave winds by blocking airflow in artificial tunnels. Spraying with bleach keeps the algae under control. The new lighting and trail system will reduce evaporation and unnatural foods, which have attracted surface insects and driven out native species. Not everything has been or can be restored. For example, the dissolution and formation of cave decorations are in delicate balance with the amount of carbon dioxide in the air and water. A global increase of this gas in the atmosphere, caused largely by deforestation and burning of fossil fuels, is affecting this balance. Still, one can now see a renewed cave, a valuable benchmark against which we can measure human impacts, now and in year to come.

park maps subheading
Available maps for Oregon Caves National Monument: For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

A geology photo album has not been prepared for this park.


books, videos, cds subheading



Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6



geologic research subheading





selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading


updated on 01/04/2005  I   http://www.nature.nps.gov/Geology/parks/orca/index.cfm   I  Email: Webmaster
This site is best viewed in Internet Explorer 6.0 or Netscape 7.0