Seeing the Devil's side made me curious enough to want a learn a bit about geology:
The sides of the slide are hard, weather-resistant limestone layers about 40 feet high, 25 feet apart, and several hundred feet in length. In between these two hard layers is a shaly limestone that is slightly different in composition from the outer limestone layers.
This middle layer is softer, which makes it more susceptible to weathering and erosion, thus forming the chute of the slide.
This site is a tilted remnant of sediments deposited in a sea that occupied Utah’s distant geologic past. Approximately 170 to 180 million years ago, a shallow sea originating from the north spread south and east over areas of what are now Montana, Wyoming, and Utah. This sea extended as far east as the present-day Colorado River and south into northern Arizona.
Over millions of years, massive amounts of sediment accumulated and eventually formed layers of limestone and sandstone. In northern Utah, these rocks are known as the Twin Creek Formation and are approximately 2700 feet thick.
About 75 million years ago, folding and faulting during a mountain- building episode tilted the Twin Creek rock layers to a near-vertical position. Subsequent erosion has exposed the near-vertical rock layers and created Devils Slide.
I'm not convinced that the site got the number of years right, but it does explain why there are so many rock structures that are at 45 degree angles from the ground.
And why there are seashells embedded in the rock!