Substantial research on legged robotics focuses on the design and morphology of leg structures. In this context, the advantages of passive compliance for reliability and simplicity have long been recognized. For example, composite leg designs with a half-circular profile used on later versions of the RHex hexapod were found to support a rich set of dynamic behaviors. However, the complex geometry and compliance properties of these legs have been difficult to model, preventing the use of dynamics imulations. In this paper, we present a simple dynamic model for this leg design for a planar monopod, taking into account both the kinematics of rolling contact and the nonlinear compliance of the geometry. We show through simulations that realistic predictions for system trajectories can be generated and leave experimental validation of this model for future work.