The basic physical mechanisms governing the daytime evolution of up-valley winds in mountain valleys are investigated using a series of numerical simulations of thermally driven flow over idealized three-dimensional topography. The three-dimensional topography is composed of two, two-dimensional topographies: a slope connecting a plain with a plateau, and a valley with a horizontal floor. The present two-dimensional simulations agree with results of previous investigations. In particular, with respect to the valley case, the heated side walls require a compensating subsidence in the valley core which brings potentially warmer air from the stable free atmosphere into the valley core. In the context of the three-dimensional valley-plain simulations, we find that the subsidence heating in the valley core is the main contributor to the valley-plain temperature contrast along the valley axis and, under the hydrostatic approximation, the pressure difference which accelerates the up-valley wind. We show how this mechanism (subsidence in the valley produces the valley-plain temperature contrast) improves on the current textbook description based on bulk thermodynamic arguments.