The nonlinear evolution of the collisional tearing mode is numerically determined for a two-dimensional current sheet configuration whose magnetic field lines are tied at one end to a stationary surface. The configuration is analogous to that occurring in the geomagnetic tail at the start of a substorm. The numerical results suggest that the formation of a near-earth neutral line at substorm onset is due to the asymmetric tearing that occurs because the field lines in the geomagnetic tail are partly line-tied, or anchored, by the earth's ionosphere. The results also suggest that during substorm recovery the neutral line in the tail moves away from the earth at a speed on the order of the speed of the plasma flowing into the neutral line region. Overall, the solution is consistent with proposed reconnection models of the substorm but suggests that the recovery phase may be more complex than previously expected, due to the growth and coalescence of multiple magnetic islands (i.e., plasmoids).