Two identical obelisks of 15th century B.C. Egyptian origin are examined. One was re-erected in New York and one in London during the latter part of the 19th century. The one in London is still in fair condition while the one in New York is badly eroded. The cause of the erosion is not so much the air pollution, but mostly the re-crystallization of salts which were deposited in the stone between 500 B.C. And 16 B.C. The obelisk in London was saved probably by im mediate treatment with hot wax. The environmental history of the obelisks is traced during 3500 years and the consequences of transitions are explained.