Before we get off the subject of curiosity, I need your help on this. I've Googled any number of times. I've asked those who might know and there's never been a satisfactory answer as to what caused this:
The scene appears on the west side of Hwy 5, a bit north of Crow's Landing and before one goes over Altamont Pass.
Mr. Bob has guessed everything from molehills to dump truck loads. I don't buy any of it.