The cross (~10 molecular layers deep) was carved just before measuring the first image.
13 successive AFM images (~1 minute apart)
Each 256 x 256 pixel image is 5 mm wide
and high.
(You may view the movie frame by frame by clicking the right/left arrow keys)
Two things worth noting:
(1) The rate of dissolution is "anisotropic" (different in different directions). Dissolution right and left is much faster that dissolution up and down. In fact the light, top edge of the "horizontal" scratch does not dissolve at all.
(2) Between the 9th and 10th frames the AFM tip happened to drag across the surface from top left to below the center, carving a narrow groove one molecule deep. In the final three frames this groove widens rapidly.