A barcode for Earth's history
In the Bottaccione Gorge outside Gubbio, a 400-meter-thick exposure of pinkish limestone records a dramatic history of Earth. This rock formation, the Scaglia Rossa, was deposited layer by layer on the floor of the ancient Tethys Ocean from the late Cretaceous to the early Eocene. The layers are composed of the calcium carbonate shells of countless planktonic foraminifera and coccoliths that rained down for millions of years.
As these sediments slowly accumulated, they trapped tiny, iron-rich magnetic minerals like magnetite. These minerals aligned with Earth's magnetic field, acting like microscopic compass needles. When the sediment solidified into rock, it locked in a permanent record of the magnetic field's orientation at the time of deposition. The Gubbio section provides a continuous, high-fidelity log of the planet's magnetic field, showing dozens of geomagnetic reversals over more than 50 million years. This uninterrupted sequence allowed geologists to create the first detailed Geomagnetic Polarity Time Scale (GPTS), a master "barcode" to date and correlate rocks worldwide.
The dinosaur-killing clay
While studying these magnetic reversals in the 1970s, geologist Walter Alvarez identified a peculiar 1-centimeter-thick layer of clay. This thin seam sits precisely at the boundary between the Cretaceous and Paleogene periods, 66 million years ago, the moment of the great mass extinction that wiped out the dinosaurs. Below the clay, foraminifera fossils are large and diverse. Directly above it, they are small and scarce, a sign of a catastrophic event.
Intrigued, Walter Alvarez and his father, Nobel laureate physicist Luis Alvarez, analyzed the clay with colleagues Frank Asaro and Helen Michel. They discovered it contained a concentration of the element iridium about 30 times higher than in the surrounding limestone. Iridium is extremely rare in Earth's crust but much more abundant in asteroids. This iridium spike was the first direct evidence for the Alvarez hypothesis: that the impact of a massive asteroid triggered the mass extinction. The discovery of the Chicxulub impact crater in Mexico later confirmed the theory. The Gubbio gorge, therefore, holds the evidence for two foundational geological concepts in one continuous rock face.