Archaeological digs in 1951 unearthed the Ziyang Man fossil. This finding proved that human settlers occupied this area about thirty-five thousand years ago. A county-level administrative organization was established in 135 B.C. during the Han Dynasty (206 B.B. - 220 A.D.). Ziyang gained its name for its location north of the Zishui River (the present Tuojiang River). The city lies in the center of the Sichuan Basin with higher terrain in the west and lower terrain in the east. Ziyang's topography is ninety percent hilly and ten percent alluvial plain. The Tuojiang River flows southeast through the city. While Ziyang's weather avoids extremes, it still has four definitive seasons. Summers are warm but the heat is not oppressive.