Evidence for the buried crater

Very large impacts do not leave a deep hole like the one in Arizona. Instead, they fracture the rock deep into the ground over a wide area, leaving a shallow crater that is often raised in the middle. Because the rocks of Panther Mountain show no signs of being disturbed by an impact, the scientists (Yngvar Isachsen, Stephen Wright, and Frank Revetta) think the crater is still buried beneath the mountain. Why, then, is the circular feature visible at all?

The ring valley is caused by two streams, the Esopus and Woodland Creeks, each of which traces out part of the circle. The scientists think the streams have been influenced by small cracks, called joints, that form in sedimentary rock. The scientists have found that, around the edge of the circle, these joints tend to be closer together than elsewhere in the region. In the upper part of the Esopus Creek, you can see these closely-spaced joints in the bedrock near the stream. In lower reaches of the stream, the bedrock is buried beneath glacial deposits. However, the scientists used a seizmograph (a sensitive instrument used to study how vibrations travel through the ground, also used to study earthquakes) to infer that there are a lot of joints in other parts of the circle as well. The joints make it easier for streams to erode away the rocks, so the streams tend to follow the edge of the circle.

The scientists don't know for certain that the increased number of joints was caused by a buried impact structure. However, they found evidence that there is a zone of shattered rock deep beneath the mountain. Specifically, they found that the Earth's gravity is slightly weaker above the mountain than was expected. This suggests that the rock beneath the mountain has been disturbed, making it less dense. This zone of disturbed rock, in turn, could account for the larger number of joints found along the edge of the circle. As the fractured rock material settled, the overlying sedimentary rocks dropped down a little too, and the joints formed around the edge of the circle to relieve the stress. The scientists point out that more research is necessary to be sure that an impact crater lies beneath the circular feature.