The Oregon Caves National Monument and Preserve protects approximately 4 miles of underground marble passageways, the area in which these caverns are contained, and all waterways which feed into the cave. When visiting the caves, you are free to hike any of the 6 trails contained within the monument and preserve, view historic buildings, and/or take a ranger-led interpretive walk through the cave. This cache is placed at the top of Cliff Nature Trail. If you wish to log this cache, you can either hike the entire loop starting at the visitor center or, if you choose to go on a cave tour, can start the hike at the cave exit.
The geologic story of Oregon Caves National Monument and Preserve began 250 million years ago in the ocean. To understand the geologic history of the Siskiyou Mountains however, first we must understand some geology basics. The Earth is split up into many segments called tectonic plates, and the interaction of these plates is what creates most landforms. There are two types of tectonic plates, oceanic and continental, which are differentiated based on composition. Because oceanic plates have a higher density than continental plates, when these two plate types interact, the oceanic plate gets subducted (or pushed) under the continental plate. This process pulls dense material beneath the continent, and scrapes/pushes off lighter materials (kind of like a bulldozer) against the continent into an accretionary wedge. When materials are subducted, they are subjected to heat and pressure which fundamentally changes the composition and/or structure of the parent material, called metamorphism. Imagine taking cake batter and putting it into the oven to cook; the structure of the material changes after subjected to heat. In addition, depending on the conditions, volcanic island arcs can exist on the oceanic plate which when colliding with the continental plate also get accreted (‘glued’) onto the existing continent.
In the vicinity of Oregon Caves 250 million years ago a volcanic arc existed just off the then western edge of the North American plate. At the base of this arc, algal colonies created conditions in which calcite could grow. Calcite (CaCO3) is a rock-forming mineral and is very common throughout the world. This calcite eventually compacted into a sedimentary rock called limestone. The limestone was riding on an oceanic plate (Pacific plate), which was on a collision course with a continental plate (North American plate). Approximately 170 million years ago, this limestone got subducted and metamorphosed into marble. Later, more terranes got accreted onto the western edge of the North American plate; which went on to make up the rest of the Siskiyou Mountain rocks. Uplift and erosion pushed the marble and surrounding rock up from deep beneath the surface. All throughout these processes, hairline cracks were created in the rocks.
About 150 million years ago a large body of magma (molten rock) pushed upwards into the cracks in the marble. This created a suite of diorite (an igneous rock) intrusions, which are visible in the cave. Eventually, erosion and continued uplift exposed the marble to the present-day surface.
Carbonic acid also took advantage of these pre-existing cracks to start forming the cave. Marble is susceptible to dissolution, or chemical erosion, when exposed to acidic solutions. Water mixes with carbon dioxide (forming carbonic acid) on the surface and then infiltrates into the marble through the pre-existing cracks. Once the marble and carbonic acid interact with each other, dissolution occurs and the cave begins to form. The primary cave at Oregon Caves National Monument and Preserve has been forming for approximately 1.7 million years.
Here, you can see several mountain ranges converging into the Illinois Valley. Each one of these ranges has its own suite of geologic complexities, which make up the story of the Klamath Mountains and ultimately the cave. In addition, there’s a unique relationship between the landforms and the plant life. Because of the rolling valleys and peaks, various species of plants are confined to isolated pockets between the 2 landforms. Although isolated, these plant species have the capability of migrating from one place to another if the environmental stressors are high enough.
In order to log your find, please answer the following questions:
- Do caves experience the seasons?
- What are the 4 features you can see in or on the marble rock?
- What are the Siskyou Mountains composed of?
Although not required to log the cache, if you’d like please post a photo of you and/or your GPS at the overlook!
Disclaimer: This cache has been placed with the explicit permission of NPS.