You are standing on the Cripple Creek Granite, dated between 1.40 and 1.43 billion years old. From here you can see the deeply eroded remains of the domes and lava flows of the Guffey Volcanic Center, today just remnants of once large stratovolcanoes (similar to Mt. St. Helens) that existed here 34 million years ago.
Other mountains of the Thirtynine Mile volcanic field (including Witcher, McIntyre, Castle, and Thirtynine Mile) are comprised of the stratified lava flows (formed from molten lava) and lahars (formed from a cooler flow of mixed debris) that formed on the flanks of the Guffey Volcanic Center.
The lahars filled an ancient valley that was to the left of the modern Four Mile Creek that you see at this site. These lahars filled the valley to a level that would have extended far above your head. Most of these lahars were later eroded away, although remnants can be seen as outcrops (the low hills) south of the Four Mile Creek drainage, and also in the road cut at the other stop on this trip.
When the new drainage was reestablished and eroded through these lahars, it encountered this section of the underlying, harder Cripple Creek granite, formerly a ridge that was later covered by lahars. Trapped in the valley that it had already begun carving through the lahars, the stream was forced to continue eroding its channel through this mile-long section of the granite. This process resulted in the formation of the canyon that you see to the right of this viewpoint.