A construction crew excavating for an office building in London's financial district has exposed the foundation of an 1,800-year-old building constructed by Romans when they ruled England. Archeologists have determined that it is the remains of the 6-foot-thick north wall of the Basilica, the Roman city hall.

The insurance company erecting the building has agreed to defer construction until October while archeologists study the site.

"Three-quarters of the complex has been destroyed by development over the centuries. So this is our last chance to investigate one of London's most valuable and historic sites," said Brian Hobley, chief urban archeologist at the Museum of London. Historians have known the general location of the Basilica for many years, but this is the first opportunity for scientific excavation.

The Romans conquered Britain in 43 A.D. on the orders of Emperor Claudius I and abandoned it in 407. The Basilica housed city administrators, law courts, an assembly hall, the treasury and shrines.

Preliminary study has revealed flaws in the construction -- large cracks in the base of the wall and signs that the building's original floor had broken up and begun to subside. A second floor was constructed and, when it broke up, a third was added.