 Raynolds divided his expedition , sending a smaller detachment under Maynadier to explore the Tongue River , a major tributary of the Yellowstone River . Two of Maynadier 's party , James D. Hutton and Zephyr Recontre , the expedition 's Sioux interpreter , took a side trip to locate and investigate an isolated rock formation that had been seen from great distance by a previous expedition in 1857 . Hutton was the first person of European descent to reach this rock formation in northeastern Wyoming , later known as Devils Tower ; Raynolds never elaborated on this event , mentioning it only in passing . By September 2 , 1859 , Raynolds 's detachment had followed the Yellowstone River to the confluence with the Bighorn River in south @-@ central Montana . The two parties under Raynolds and Maynadier reunited on October 12 , 1859 and wintered at Deer Creek Station , on the Platte River in central Wyoming . 
