"Then I slapped myself on the head." Why do you think Dr. Erlandson uses this expression when he describes finding the artifacts in the old redwood box?

This quotation comes from Dr. Jon Erlandson's description of how he came to uncover various artifacts on San Nicolas Island in 2009. He had been looking for "ancient campsites" during this particular expedition, and the discovery of the redwood boxes was completely unexpected. The first box Erlandson discovered had a broken side; his instinctive response, seeing that there were artifacts hidden inside the box, was to begin pulling things out:

I picked out a soapstone bead, a brass button, and a carved soapstone effigy. Then I slapped myself on the head, thinking “You idiot, stop pulling things out of the box!”

To slap oneself on the head is generally not a figurative expression in English. Erlandson seems to mean that he literally slapped himself, a gesture which tends to indicate a sudden realization that one's behavior is inappropriate, foolish, or unconsciously silly. In this instance, it does seem that Erlandson intended to slap himself out of the instinctive daze in which he was pulling out artifacts from the box rather than waiting for his colleagues. With archaeological discoveries, it is generally best practice to leave things as much as possible in situ, so that the entire site can be assessed as it was first found. So, in slapping himself on the head, Erlandson was reminding himself that what he was doing, albeit a natural human reaction, was not the best archaeological approach and that he should stop and call his colleagues to the scene.