 We model the elasticity of the cerebral cortex as a layered material with bending and elastic energy along the layers and between them in both planar and polar geometries, subjected to axons pulling from the underlying white matter. Above a critical threshold force, periodic undulations emerge, leading to folds in the cortex. We identify analytically the critical force and wavelength of the undulations, which are physiologically relevant values. Our model is a revised version of the axonal tension model for cortex folding that takes into account the layered structure of the cortex and draws a connection with another competing model for cortex folding. We study the relationship between brain size and the critical force and wavelength in the polar geometry to understand why small mice brains exhibit no folds while larger human brains do. Finally, we estimate the bending rigidity constant for the cortex based on the critical wavelength. This article was authored by O. V. Manuhina, David Mayet, and J. M. Shorts.