The model equation for the mixed layer proposed by Niiler (1975) combines the Kraus-Turner turbulent erosion prescription with the Pollard-Rhines-Thompson treatment of induced shear-flow deepening and limiting by Coriolis forces. We show here that both are special cases which emerge asymptotically from the model equation. Numerical solutions show the dynamics to pass through four distinct regimes, in the case of wind-mixing of an initially resting fluid.