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PAn unstable wetting front breaks up below a thin wetted layer: the induction zone. Many small fingers form initially, but only a few fully develop. To illuminate the process of finger formation, we used a horizontal array of fast-responding microtensiometers in the induction zone of a two-dimensional, fine-over-coarse glass bead porous medium. The induction zone affected only the initial shape of the wetting front; finger size and spacing were governed by autonomous processes. The lateral pressure head gradients in the induction zone adapted to the location of full-grown fingers before these fingers were visible and remained stable afterwards. The water requirement and spacing of the fingers dictated the magnitude of the gradients. The pattern of the lateral pressure head gradients suggests that the fingers cause non-uniform infiltration from the ponded water layer.