antecedent valley[‚ant·ə′sēd·ənt ′val·ē]
a river valley that cuts across an increasing elevation and in terms of geological age is older than that elevation. Antecedent valleys arise when a portion of the earth’s surface on which a river network has formed is uplifted and the rate of the river’s erosion exceeds the rate of the uplifting. The proof of such origin is the unique flexure of the river terraces, which reach a maximum in the axial part of the uplift. Antecedent valleys are narrow and rather deep and have steep slopes.