Complex sentence: The Great Bridge over the Charles River connected Cambridge, Massachusetts, to Little Cambridge, which was the name for Allston-Brighton before it separated from Cambridge in 1807, first becoming the town of Brighton and finally joining the city of Boston in 1874.
Split into simpler sentences: The Great Bridge over the Charles River connected Cambridge, Massachusetts, to what is now known as Allston, Boston, Massachusetts. Allston was originally know as the Little Cambridge district of Cambridge, which became the independent town of Brighton in 1807, and finally joined the city of Boston in 1874.