Perhaps hundreds of years after the construction of the mounds in the eastern half of the State--possibly from one to four hundred years ago--some tribe or tribes, probably the Sioux or certain of the village-building Indians, were putting together the turtle effigies frequently encountered on the hills west of the Missouri, and constructing the more widespread and better-known boulder-ring effigies. The purpose of these crude outlines on the prairie is not definitely known. Because the turtle plays a prominent part in medicine ceremonies of the Mandan Indians, some think the turtle effigies were made to win the favor of certain spirits. Others claim they were made to point the weary Indian to good water--a theory which may also apply to a number of the cairns occasionally seen piled on the tops of high hills. Other cairns are ceremonial or commemorative. Boulder rings, which sometimes appear in large numbers but more often present only one or two specimens in a given location, were once thought to be tipi rings. The fact that many of them appear on the sides and tops of hills has discredited this assumption, however. Veneration of the so-called sacred stones of the State probably began in the effigy-building period, but the origin of the very interesting writing rocks (_see Side Tours 3B, 4A, 8A, and 8C_) is undoubtedly far more ancient. The significance of the markings on these rocks has not yet been determined. THE COMING OF THE NORTH DAKOTA INDIAN TRIBES About the time the earlier turtle effigies were made--perhaps 200 years ago--in permanent villages of earth lodges in the valley of the Missouri dwelt a most interesting group of people, raising many cultivated plants, building fortified towns, and in general living a rather ordered existence. These were the Mandan, as far as is definitely known the first of the historic tribes to enter the State. Their exact origin is not clear. Certain of their traditions claim that they long ago lived in the East near a great body of water--most authorities suggest the East Coast or Gulf of Mexico. At any rate, many generations before the coming of the whites, the Mandan--probably crowded by other tribes--began to wander westward. Apparently their long trek finally brought them and their wives and children to the junction of the White River with the Missouri in what is now South Dakota. Grass-grown sites of their old villages along the benchland of the river show how these people, in quest of a new and more satisfactory home, moved northward in successive migrations until in time they arrived at the mouth of the Heart River in the neighborhood of present Mandan and Bismarck. Here they probably remained for generations, carrying on a settled agricultural life. They were visited by the Verendryes in 1738 (_see Tour 8_), at which time they had six large, well-fortified villages. Estimates of their number at this time have ranged from 2,500 to 15,000.