We report on observations of unstable growth on GaAs(001) surfaces nanopatterned with grooves of varying length/width aspect ratios. For homoepitaxial growth at temperatures near 500 degrees C, we find that ridges build up at the upper long edges of grooves oriented along [110]. No ridges form at the long edges of grooves oriented [110]; instead cusps form at the bottoms of such grooves. Most interestingly, we find that the evolution of ridge heigths during forwth breaks into two distinct brances, with the separation occurring at a groove lengthe of 7.5 +/- 2.5 mu m for growth at 525 degrees C, and at a lengthe which is an order of magnitude smaller than this for growth at 460 degrees C. These observations indicate the presence of very large, magnitude smaller than this for growth at 460 degrees C. These observations indicate the presence of very large, thermally-activated characteristic lengths which governs the evolution of the topography during growth. (C) 2013 Elsevier B.V. All rights reserved.