A trove of more than 4,000 bronze and silver coins dating back to ancient Rome, uncovered this summer in the orchard of a fruit and vegetable farmer, has been described as one of the biggest treasures of this kind found in Switzerland.

The huge hoard of coins, buried about 1,700 years ago and weighing 15kg (33lb), was discovered in Ueken, in Switzerland’s northern canton of Aargau, after the farmer spotted some shimmering green coins on a molehill in his cherry orchard.

He guessed the coins were Roman, following an archaeological discovery a few months earlier, of remains of an early Roman settlement in the nearby town of Frick. He contacted the regional archaeological service , which later labelled it one of the largest such finds for Switzerland.

On Thursday the archaeological service announced that after months of digs, 4,166 coins had been found at the site, most in excellent condition.

The coins’ imprints remained legible, and an expert dated the money to the period stretching from the reign of Emperor Aurelian (270-275) to the rule of Maximian (286-305), the most recent coins made in 294.

“The orchard where the coins were found was never built on. It is land that has always been farmed,” said Georg Matter, an archaeologist, explaining how the treasure could have laid undiscovered for so long.

The coins’ excellent condition indicated that the owner systematically stashed them away shortly after they were made, the archaeologists said. For some reason that person had buried them shortly after 294 and never retrieved them. Some of the coins, made mainly of bronze but with a 5% silver content (an unusually high amount), were buried in small leather pouches.

The archaeologists said it was impossible to determine the original value of the money due to rampant inflation at the time, but said they would have been worth at least a year or two of wages. How much the coins were worth today was beside the point, Matter said. He said the farmer would be likely to get a finder’s fee “but the objects found belong to the public, in accordance with Swiss law”.

The Ueken treasure will go on display at the Vindonissa Museum, in Brugg, Aargau.