The Gypsum Sinkhole is an occurrence formed by the reverse of the process that created Glass Mountain. Here groundwater is dissolving a buried gypsum plug. The cavity left behind has collapsed under the weight of overlying rock layers. This collapse has created a large sinkhole nearly 50 feet (15 m) in diameter and 200 feet (61 m) deep.
Stay away from the edge of the Gypsum Sinkhole. The rocks here are very soft and unstable, and can collapse at any time.
Black boulders strewn across the landscape are remnants of lava flows that capped Boulder and Thousand Lake Mountains about 20 million years ago. Short glacial periods on these peaks broke up the underlying lava. Glacial outwash and mudslides, along with the natural process of erosion, helped move the boulders far from their original location.
The dikes and sills seen in Cathedral Valley formed as recently as 3-6 million years ago. Dikes and sills are the result of molten lava flowing into vertical joints (dikes) or between horizontal layers of sedimentary rocks (sills), then solidifying. Plugs, more massive lava intrusions, and Cathedral Valley. More resistant to erosion than the surrounding layers, the lava outcrops provide a stark and rugged contrast, forming jagged ridges and pointed outcrops.