It is Dakota Age Sandstone estimated to be 130 million years old.
To validate your find :
You need to post in your log a picture of the Sandstone WITH YOUR USERNAME VISIBLE, CLEAR AND READABLE written on the medium of your choice (paper, hand, phone, t-shirt, etc...).
Pictures taken with only yourself, your GPS or your Logo/Avatar are not accepted as well as photoshopped/edited pictures with your username inserted afterward.
Failure to follow these instructions will lead to your log deleted.
Thanks to DBCHeadoc & Regular Girl for letting me adopt this virtual cache.