We present a numerical study of the formation of current sheets in ideal plasmas. First we confirm the development of singular current sheet in a one-dimensional model. In a second step we extend the analysis to two-dimensional equilibria. Here it is found that the resulting structures are quite insensitive to the boundary conditions. For the special case of a magnetotaillike equilibrium it will be shown that the resulting current distribution provides a possibility to understand the onset of a localized anomalous resistivity from a macroscopic point of view. Furthermore, the resulting structures provide an explanation for the dramatic decrease of the thickness of the current sheet in the magnetotail prior to the onset of geomagnetic substorms. ¿ American Geophysical Union 1994