Art: Temple on the Thames

In a grey, blitz-battered part of London, between the Thames and St. Paul's Cathedral, is a rubble-littered hole where a 14-story office building will soon rise. Since that part of London stands on many layers of history, Archaeologist William Grimes of the London Museum got permission to dig a trench to see what lay deeper down.

First he uncovered a wall of Roman age; then the foundations of an ancient building came slowly to light. About 60 ft. long and 22 ft. wide, it looked a good deal like an early Christian church, with a...