We present a detailed cross-sectional scanning-tunneling microscopy investigation of threefold stacked InAs quantum dots in a GaAs matrix grown by metal-organic chemical vapor deposition. The dots are vertically aligned and show a layer-dependent size. Detailed images with atomic resolution indicate that the dots consist of pure InAs with a shape well described by prisms with {110} and additional {111} side faces as well as a (001) top face. The wetting layer is found to be inhomogeneous, while no In diffusion into the overgrowing GaAs layer is observed. These findings demonstrate that growth in the present sample is more related to kinetics than to equilibrium conditions. Furthermore it is demonstrated that the image contrast is strongly influenced by a cleavage-induced outward relaxation of the strained quantum dots.