Creosote bushes start to grow outward as they get older. The center starts to die out and a ring is left. Creosote rings are some of the oldest plants in existence and have been dated to 10,000 years old.

There is more about creosote rings here, with a description of the oldest one found. It is estimated to be 11,700 years old and to have been alive at the end of the last ice age