The city of Le Mans, and its surrounding region, is renowned well beyond the French border by its famous endurance motor races and perhaps also by its delicious rillettes (shredded pork pâté). The area is also appreciated by historians for its artistic and historical past, such as its long link with the Plantagenet lineage (Louis II, King of England and the father of Richard I, the Lionheart, was born in Le Mans!), or as being the country of the Aulerques Cenomans, the celtic tribe from which Le Mans derives its name. For geologists, this is a familiar toponym that inspired Alcide d’Orbigny to create the name Cenomanian, the earliest stage of the Upper Cretaceous and one of the fifty or so French stratotypes identified during the 19th and 20th centuries. This book proposes to describe the rich history of this geological stage. The Cenomanian is today an official stage of the International Geological Timescale; it is the basal stage of the Upper Cretaceous, spanning 6.6 million years. Its boundaries are clearly defined and dated by a Global Boundary Stratotype Section and Point (GSSP), which for the Albian-Cenomanian boundary is dated to 100.5 Ma, while that for the Cenomanian-Turonian is dated to 93.9 Ma.