What a fun place, Musselman Arch in Canyonlands National Park. The arch is composed of the White Rim sandstone a Permian aged wind blown classic sandstone in a near coast environment. Sandstone is fairly well cemented together making a rock unit somewhat resistant to erosion. Right below the White Rim you can see a reddish (isn't almost everything in the Canyonlands red?) brown silt-mud stone formation. This shaley material is poorly cemented making it fairly easily eroded. You can see how the sandstone in the arch is lasting much longer than the missing shale/silt/mud stone found below the arch.
So the White Rim sandstone is acting as a cap rock to the softer Organ Rock found below.
Musselman Arch is found along the White Rim mountain bike trail. You can ride down the Shafer trail to the arch and back in a day, but the 1300 vertical foot climb back up is an interesting way to end the day's ride.