In a remote Uzbek gorge, a bronze plaque marks the exact layer of rock defining the start of the Emsian Age, 408 million years ago.
A global standard in stone
In the Zinzilban gorge, a ravine cutting through the southwestern spurs of the Zarafshan Mountains, a small marker identifies a location of global significance. This is a Global Boundary Stratotype Section and Point (GSSP), one of over 80 such sites worldwide that act as official reference points for the geologic time scale. This specific GSSP, established in 1979 and ratified by the International Union of Geological Sciences in 1997, defines the precise start of the Emsian Age of the Devonian Period.
The marker, often called a "golden spike," is fixed to a layer of dark micritic limestone 35 centimeters above the top of the Madmon Formation. This exact point was chosen based on the first appearance of a specific fossil: the conodont Polygnathus kitabicus. Conodonts are the microscopic, tooth-like fossil remains of an extinct eel-shaped vertebrate, and their rapid evolution and wide distribution make them ideal for dating rock layers. The transition from its ancestor, Polygnathus pireneae, to P. kitabicus provides the signal for geologists to identify this time boundary in rocks anywhere on Earth.
The Kitab Reserve was selected for this honor because its rock layers are remarkably complete and undisturbed by major tectonic events. The strata, originally horizontal sediments on an ancient seabed, have been tilted almost vertically, allowing researchers to walk across millions of years of continuous geological history in a relatively short distance.
A Devonian seabed's diary
The rocks of the Kitab Geological Reserve were once sediments on the floor of a warm, shallow sea that was part of the Paleo-Tethys Ocean. The layers contain an abundant and diverse collection of fossils that document the marine ecosystem of the Early Devonian, approximately 408 million years ago. Beyond the conodonts, the limestone cliffs hold beautifully preserved corals, brachiopods, trilobites, and dacryoconarids (small, conical shelled organisms).
Soviet-era geologists began systematically documenting the fossils here in the 1970s, laying the groundwork for the site's protection and international recognition. The fossils show that the Emsian age was a time of significant evolutionary activity. This interval in Earth's history saw the continued diversification of fish, especially jawed vertebrates, and the expansion of the first complex plants across the land. The rock record at Zinzilban provides a high-resolution chronicle of marine life just before these major evolutionary developments.
Today, the reserve covers 3,938 hectares and protects the unique geological formations and a mountainous semi-steppe ecosystem. It is administered by Uzbekistan's State Committee on Geology and Mineral Resources and includes a small museum displaying some of the fossils and minerals found in the area.
💡Fun Facts
The term "golden spike" is figurative; the actual GSSP markers are typically bronze plaques or other durable monuments.
The index fossil, *Polygnathus kitabicus*, was named for the Kitab Reserve where it was first identified as the definitive marker for the Emsian boundary.
The rock layers at the Zinzilban gorge are tilted to an almost vertical position, creating a "stone book" where geologists can read chapters of Earth's history.
The reserve hosts 168 species of vertebrates, including the Central Asian cobra and the occasional Tien Shan brown bear.
The reserve is a natural area. Access to the GSSP and other geological sites may require arranging a guided tour with the reserve administration, which has an office in the nearby city of Shakhrisabz.
Admission
Access was previously limited to scientists but opened to tourism in 2019; fees or permits may be required for guided visits. The reserve includes a small hotel and cottages for visitors.
Accessibility
The terrain consists of remote, mountainous gorges. Reaching the GSSP requires hiking on uneven dirt paths and is not wheelchair accessible.
Kitab Geological ReserveKashkadarya Region, Uzbekistan