28. The isolated white spots in the aerial photograph of the previous slide are sand volcanoes formed by hydraulic fracturing. If we were to dig a pit into the center of one of these sand volcanoes, as in the example in this slide, we would see that the feeder dike is thin and planar (and not circular). The morphology of the dike through the fine-grained cap is very important to note for verification of origin, as will be seen later.