Clark and his party traveled by horse along the north side of the Yellowstone River until July 24, 1806, when nine members began floating in two lashed-together canoes made from cottonwoods found along the river. Sergeant Pryor and Privates Hall, Shannon, and Windsor went overland with the party's remaining horses, but after those horses were lost, the four men walked to the Yellowstone River near Pompeys Pillar, built bull boats from buffalo bull hides stretched over lashed branch frames, floated down the Yellowstone and into the Missouri, and rejoined Clark's group on August 8. The reunited men joined Lewis' party on August 12, south and east of present-day Williston, North Dakota. During this stretch of the journey, Clark recorded the fatigue and sore feet of the horses, the theft of 24 lost horses indicated by Indian signs, the start of the canoe voyage on July 24, a council lodge on an island, abundant wildlife along the river, and his arrival on July 25 at a remarkable rock he called Pompys Tower, now known as Pompeys Pillar, where he marked his name and the date. That signature remains visible there today as the only remaining on-site physical evidence along the entire route of the Corps of Discovery's 8,000-mile journey. On July 27, after passing the Big Horn, Clark wrote that he took his leave of the tremendous chain of Rocky Mountains white with snow.