Hidden within the remote Siskiyou Mountains of southwest Oregon are a number of unique and interesting marble caves. The most impressive of these was discovered in 1874, and in 1909 its superlative value earned it a designation as Oregon Caves National Monument. With an expansion in 2014, this park now encompasses over seven square miles (over 18 square km) of rugged and beautiful mountains, forests, and caves.
As the name implies the caves are the main attraction here. Like many underground caverns these are solution caves, formed by slightly acidic groundwater gradually dissolving and transporting the mineral calcite (calcium carbonate…CaCO3) found in limestone. This rock is formed over millions of years in marine environments. As animals bearing calcite shells die and decompose on the sea floor, their resistant shells accumulate and harden into limestone. When tectonic activity lifts these rock beds above the ocean surface and groundwater works its way in, caves can eventually form.
Oregon Caves is unique in that the dissolution has taken place in the metamorphic form of limestone known as marble. For millions of years two tectonic plates have slowly collided and ground together. Under this extreme force, existing limestone has been subjected to heat and pressure that has gradually altered its structure. Of the thousands of caves administered by the National Park Service, Oregon Caves is one of only three that are composed of this metamorphic rock.
Due to the complexity and hazards of these caves, they can only be explored on a guided tour. From March through October The National Park Service offers a variety of tours from mild to adventurous, although all are somewhat strenuous. While this park is in the middle of nowhere, prepare for a bit of a wait during peak summer months.
Like any ranger-led hike I’ve been on in a national park, the guide on this visit was both entertaining and informative. After a brief overview of the area geology and some warnings about cave exploration, we were on our way.
As a first-time visitor I went on the General Cave Tour, a 90-minute trek through the highlights of this cave system. Like most caves low ceilings, tight spots, and a lot of steps up and down were typical.
Not far into the cave there’s a sort of “emergency escape” for anyone who, at that point, has realized they’ve gotten themselves in over their head.
This cave features a lot of dripstone (speleothems) formed from the dissolution, transport, and then precipitation and recrystallization of calcite. Common features like stalactites, stalagmites, and columns can all be found here.
Cave popcorn adorns many ceilings:
Flowstone is also common:
In some places the flowstone is really impressive:
We also got to see an igneous dike that had cut through the marble. At some point magma from deep within the earth found its way up through a fracture in the parent rock, eventually cooling and leaving behind this dark band of igneous rock.
Back on the surface the ranger guide continued entertaining and informing, taking questions and providing some excellent insight before turning everyone loose.
In addition to the caves this park features many miles of hiking trails through the Siskiyou Mountains, providing for some scenic views and encounters with interesting plants and animals.
Near the entrance there are also two historic buildings that should be explained. The first is the Chalet that was constructed in 1923 to provide a visitor center for guests. Although nearly 100 years old this building still fills that role.
1934 saw the opening of a quaint hotel known as the Chateau, and this rustic structure continues to lodge guests today. It’s a relatively intriguing man-made feature within the dense mountains and forest, and notable enough to earn a place as a National Historic Landmark.
Although Oregon Caves is well off the beaten path, if you’re ever in northwest California or southwest Oregon, it’s definitely worth seeing for a day or two. It’s a bit of a drive from anywhere, but even the drive is worthwhile.