The sedimentary rock in the picture above is a sandstone with pebble molds. If the pebbles were present, this rock would be considered a conglomerate. Conglomerates consist of 2 mm or larger rounded fragments of rock, or clasts, surrounded by finer-grained sediment which geologists call “matrix”. The clasts in the rock above were pebble sized, 2-64 mm, and the matrix is sand sized.
Even though many of the clasts have been removed by erosion, we can tell that they were primarily shale pebbles. The sandy matrix was more resistant to erosion than the softer shale pebbles, so we are left with cavities where the pebbles were (pebble molds) on the rock’s surface. This creates an interesting optical illusion. Did you see the cavities as pebbles or as molds when you first looked at the picture?
This type of conglomerate is deposited by energetic and dynamic water, such as is found in rivers and waves. During higher flow periods, only large clasts are deposited. When flow is lower, finer-grained sediment settles in between the larger clasts.
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.