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Liesagang Weathering
The swirled concentric layering of yellow-brown and brown colors is called liesagang. It is a weathering phenomena. The gray color in the center, where the penny is, is the fresh, unweathered color of the rock. The suface we are looking at has been exposed by breaking the rock. Liesagang weathering is a very common phenomena.
Weathering proceeds from the outside surface of a rock toward the inside. Because rocks are often broken into blocks along joints, ground water percolates along the joints causing weathering to proceed inward from the joints. During chemical weathering soluble ferrous (Fe+2) iron is released and carried by the water into the pores of the rock opened by weathering. Later the iron precipitates as insoluble ferric iron (Fe+3) forming the liesagang concentric layers. In time even the gray area inside will weather, and turn yellow-brown, before the rock disintegrates completely.
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