Hell Creek Formation includes an isolated section that was designated a monument on which date?
Hell Creek Formation:  "Pompey's Pillar" at the Pompeys Pillar National Monument is a small isolated section of the Hell Creek Formation. Pompeys Pillar National Monument:  Designated a National Monument on January 17, 2001, and managed by the U.S. Bureau of Land Management, it consists of only 51 acre , making it one of the smallest National Monuments in the U.S.
January 17, 2001