It is possible that this trade with the coast became customary only after the horse was introduced. There was a considerable amount of trade between the Yakima and the Thompson River and other tribes of British Columbia which was carried on chiefly through the Okanogan.[442] Lewis[443] states that the Walla Walla who lived to the south of the Yakima at least in later times visited as far north as the Thompson River region, and that certain Sahaptin tribes seem to have moved northward and westward and forced back the Salish tribes which at the time of Lewis and Clark's visit were on the north bank of the Columbia and on its tributaries.[444] These tribes were particularly the Klickitat and the Yakima, an assumption which Lewis states is supported by the definite assertions of the natives themselves. A number of old men positively assured Dr. Suckley that they had pushed their way into the country formerly occupied by the Salish.[445] The Klickitat, although living in a well wooded region on the southern slopes of Mt. Adams and Mt. St. Helens are thought to have been driven by the Cayuse from their earlier home which was further east and south. Later, they went further west into the Cowlitz Valley.[446] This may account for the circular pit surrounded by an embankment which I saw near Rochester in Thurston County and interpreted as the remains of a semi-subterranean winter house site. Lewis also states that the Yakima probably lived on the Columbia near the mouth of the river which now bears their name, and are in fact so located by Cox who places them on the north and east side of the Columbia. The pressure of neighboring tribes caused by the coming of the white race no doubt facilitated the adoption of new cultural details. [435] Lewis, p. 193. [436] Lewis, p. 193. [437] Lewis and Clark, IV, p. 286; Ross, (b), p. 117. [438] Gatschet, p. 93. [439] Wilkes, IV, p. 394. [440] Ross, (a), p. 290; (b), I, p. 44. [441] Gibbs, (a), p. 408. [442] Cf. Teit, (a), p. 258. [443] Lewis, pp. 194-5. [444] Lewis and Clark, VI, pp. 115 and 119; Mooney, pp. 734-736. [445] Gibbs, (b), p. 224. [446] Swan, p. 323. As late as 1854, the Palus, a tribe living further east on the Paloose River regarded themselves as a portion of the Yakima and the head chief of the Yakima as their chief.[447] The general similarity of the Walla Walla language to that of the Klickitat and Yakima rather than to that of the Nez Perce is mentioned by Lewis. [447] Stevens, XII, p. 200, Pacific R. R. Rept., Pt. I.