O Oak trees 1 Odessa, Tex. 30 Ogallala Formation 18, 23, 24, 25, 27, 29, 45, 46 Ohio River 5 Oil 30, 46, 47, 49 Oklahoma 7 Oligocene Epoch 16, 43 Oregon Trail 2 Ouachita province 5 Outwash plains 35 Ozark Plateaus 5 P Paleocene Epoch 16, 23 Paleozoic Era 21, 30, 45 Palmer Lake 43 Pampa, Tex. 30 Pawnee Buttes 42 Pecos River 7, 25, 45, 46 Pecos Valley 7, 10, 19, 23, 45, 48, 49 Pedernales River 47 Pike, Zebulon iii, 2 Pine Ridge escarpment 7, 29, 40 Pioneers 2 Plains Border Section 19, 48 Platte River 2, 25, 29 Pleistocene Epoch 5 Poison Canyon Formation 16, 23, 24 Powder River 39 Powder River Basin 39 Purgatoire Formation 24 Purgatoire River 25