Aspect ratio 4 Different densities 4 Evolution equations 4 Low Reynolds number 4 Lubrication 4 Lubrication approximations 4 Lubrication theory 4 Mountain building 4 Perturbation method 4 Perturbation techniques 4 Reynolds number 4 Viscosity 4 Viscous flow 4 Viscous fluid layers 4 Flow of Fluids 2 Fluid Mechanics--Mathematical Models 2 Gravitational Effects 2 Lubrication--Theory 2 Mathematical Techniques--Differential Equations 2 Phase-Plane Formalism 2 Scaling Laws 2 Viscous Gravity Currents 2