Oklahoma Rock 01/29/19
The rock shown in the photos below was picked up by Elizabeth and me as we drove through SouthEastern Oklahoma, from Norman to Mineola, probably in 1975. The area was in the region of the Ouachita Mountains, approximately at the Northern boundary of Gulf of Mexico when it was at its greatest northern extension, before retreating southward. The rock was on the side of a small hill, and there were many just like it around the area, all with similar shapes. Probably, the route was from Atoka to Hugo, but at this late date, this location is not absolutely sure.
Had little idea of the significance of the rock before seeing the August 2018 issue of Physics Today (cover below) which had an article on the Megadroughts encountered in the Southwestern part of the US. The magazine photo is of a dried river or lake bed in Spain, but the similarity is useful. Realizing that at one time our part of the North American Continent was covered by water and as the waters receded, the old river or lake beds dried allowing segregation to occur in the mud material. As further cooling occurred, the top began thermal cracking. That pattern is shown in the Alamazora Reservoir in Spain as well as the SE Oklahoma rock. The gradual cooling and drying allowed movement within the crystalographic structure to yield the almost cubic crystal shape seen here.
The presence of the small animal paw prints as well as the snake or worm trails in the top of the rock lower photo (IMG_4119) and at the top end of the ruler in IMG_0138 indicates that the rock was on the surface and with a formable mud layer in the Tertiary period, 2-66 million years ago.
The rock appears to be a plutonic type, one which forms from a central grain during very slow cooling.