From the American Journal of Science: In November 1829, a 30-cubic-foot block of marble was raised from a quarry northwest of Philadelphia and taken to a Norristown sawmill to be cut into slabs.
“One was taken off about three feet wide and about six feet long, and in the body of the marble, exposed by the cutting, was immediately discovered an indentation, about one and a half inches long and about five eighths of an inch wide, in which were the two raised characters” (left).
“Fortunately, several of the most respectable gentlemen residing in Norristown were called upon to witness this remarkable phenomenon, without whose testimony it might have been difficult, if not impossible, to have satisfied the public, that an imposition had not been practised by cutting the indentation and carving the letters after the slab was cut off.”
No explanation is offered. The block had been raised from a depth of 60-70 feet in the quarry.