Not sure which came first, but that sounds very much like "Motel of the Mysteries", a 1979 pseudo-future-archaeology book by David Macaulay.

An archaeologist in the year 4022 walking through the ancient land of "Usa" stumbles across an ancient U-shaped structure with dozens of individual or double-sized tombs. He uses these to piece together the burial customs of that long-lost civilization of Yanks and deduce the meanings and usage of items such as the ceremonial Sacred Urn - found in the Inner Chamber - and the Great Altar towards which the body on the Ceremonial Platform was clearly intended to face in the afterlife (holding the Sacred Communicator in its right hand).