A fluorescent dye tracer was injected into the pycnocline on the Oregon shelf at a depth of 9-10 m. It spread rapidly cross-shelf as two distinct layers, one above the other in the water column, split by interleaving dye-free water. The vertical scale of these layers, and associated density steps, was 1-2 m, and the horizontal extent of interleaving exceeded 1.6 km after an inertial period. The upper dye layer was sharply peaked and embedded in a strong vertical density gradient. The lower layer was slab-like and associated with weak stratification. Both layers were inclined slightly in density space. It is proposed that internal wave-induced mixing and the lateral collapse of mixing patches were important mechanisms. Analogies can be drawn between these dye structures and frequently-observed thin planktonic layers. By approximating the dye dispersion as a Fickian process, estimated isopycnal and diapycnal eddy diffusivities of κ x = 4.1 m2 s-1 and κ z = 1.4 × 10-5 m2 s-1 are obtained.