In the Peak District of the United Kingdom, the picturesque village of Castleton nestles at the foot of a limestone outcrop crowned by a medieval castle. Today, hikers flock to the natural beauty of this region, home to the United Kingdom’s first national park. But 800 years ago, the wild moors and wooded gorges were “covered in toxic lead pollution,” says archaeologist Chris Loveluck of the University of Nottingham. “The royal hunting forest near the castle was an industrial landscape.”

Here, farmers mined and smelted so much lead that it left toxic traces in their bodies—and winds blew lead dust onto a glacier 1500 kilometers away in the Swiss Alps. Loveluck and his colleagues say the glacier preserves a detailed record of medieval lead production, especially when analyzed with a new method that can track deposition over a few weeks or even days.

Lead tracks silver production because it is often found in the same ore, and the team found that the far-flung lead pollution was a sensitive barometer of the medieval English economy. As they report in a study published this week in Antiquity, lead spiked when kings took power, minted silver coins, and built cathedrals and castles. Levels plunged when plagues, wars, or other crises slowed mining and the air cleared. “This is extraordinary—lead levels correlate with the transition of kings,” says historian Joanna Story of the University of Leicester, who was not part of the study.

Most people associate lead pollution with the Industrial Revolution, when lead became widely used in paints, pipes, and ceramics. But researchers have long known that the Romans also absorbed high levels of lead as they smelted silver and other ores. Recently, scientists have identified startling spikes of lead deposited in medieval times in Arctic ice cores and in lake sediments in Europe. A study last year suggested most of the pollution came from mines in Germany.