A Golden Spike in Time
Along the Yoro River in Ichihara City, a quiet cliff face holds the global standard for a major event in Earth's history. This location, formally known as the Chiba composite section, is the Global Boundary Stratotype Section and Point (GSSP) for the Chibanian Age. The International Union of Geological Sciences ratified this designation on January 17, 2020, making it the first GSSP in Japan. A GSSP is a geological "golden spike" that marks the boundary between two ages in the official geologic time scale. The Chibanian age spans from approximately 770,000 to 126,000 years ago.
The rock layers here record the most recent reversal of Earth's magnetic field, an event called the Matuyama-Brunhes reversal. This was the last time the magnetic north and south poles swapped places. The name is for Japanese geophysicist Motonori Matuyama, who first proposed that Earth's field had reversed in the past, and French geophysicist Bernard Brunhes. The geologic section is part of a nearly two-mile-thick deposit of former deep-sea sediments called the Kazusa Group. These layers of silt and clay, once on the ocean floor, were uplifted by tectonic activity. They accumulated at an exceptionally high rate, averaging around 2 meters per thousand years, which created a detailed record.