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Here is a photograph of Pitkin Limestone from the Ozark Mountains near Fox, Arkansas. This exposure displays a classic orthogonal joint set. The joints are the easy-to-see fractures that divide the bedrock into square blocks. Orthogonal means the joints formed at roughly 90 degree angles to each other, hence the resultant square blocks.
Joints are common features in sedimentary and crystalline bedrock, and they form in a variety of patterns in response to the stresses the rock has been subjected to. Essentially, bedrock is being compressed, and the joints form to relieve that pressure. The squeezing and resultant fracturing result from natural processes such as burial, erosion, and plate tectonics.
Joints are important because they convey information about stress-fields that have acted on the rocks in the past. They can also be useful for understanding the flow of fluids through a petroleum reservoir or aquifer when trying to maximize production from an oil or water well.
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Pedestals are a fairly common erosional feature in Arkansas in places where conditions are favorable. They typically form in massive sandstone units due to an increased rate of erosion along the joint set near a bluff line. Joints are vertical fractures within almost all rocks that formed in response to the tectonic stresses they have undergone in the distant past. Joints are most often expressed as sets oriented in rhombohedral patterns. Water can more easily penetrate the rocks along these joints, eventually opening a gap. When this happens along joints parallel to a bluff face, the gap essentially cuts off the incipient pedestal from the influence of groundwater, isolating it from most of the processes of chemical weathering. Once that happens the majority of weathering of the newly formed block of sandstone is done by wind and rain. Because the corners and edges of a rhombohedron have more surface area, weathering is concentrated there, eventually rounding it off to form the typical pedestal shape. In many places, a capstone of more resistant sandstone is present which contributes to the top-heavy pedestal or mushroom shape. Also, the pedestal-forming unit is commonly underlain by shale or silty-shale on which the fully intact pedestal can slowly creep downslope. Some of them end up quite a distance from the bluff where they started. If you would like to view several fine examples of this erosional phenomenon, consider a visit to Pedestal Rocks Natural Area in the Ozark National Forest.