If you’ve spent much time in the Arb, you’ve probably seen a boulder or two. There are a number of them around the Arb, and they’ve sparked debate among geology students. Whether or not you’ve pondered their origins, these boulders tell an amazing history. Where these boulders came from is somewhat of a mystery. 

Most of them are found in the Arb’s prairies and are made of rock otherwise foreign to the area. Since boulders don’t just appear in the middle of a flat prairie, they must have been brought here. Either they were transported by humans, or they are glacial erratics. A glacial erratic is a rock that was formed in one location, picked up and moved by a glacier and dropped in another location by the glacier as it melts. 

Some argue that they must be erratics, as humans can’t and don’t usually transport a boulder hundreds of miles. Others say that they only appear within the Arb suspiciously near trails, indicating that they were placed there on purpose. The reality is more complicated than we might expect. 

The boulders of the Arb are indeed glacial erratics, but they were also moved by humans. Around 1.1 billion years ago, 500 million years before any of the Arb’s bedrock began to form, and far to the north, igneous magma cooled to form rock, which eventually broke off into boulders. 

At some point during the last million years, massive ice sheets ripped these boulders from their home and carried them as they grew towards the south. In the process, they were rounded from battering against ice and other rocks. Then, when the glaciers melted away, the erratics were left behind. Very recently, in comparison, nearby farmers moved them out of their fields to their current locations, mimicking naturally deposited erratics.

In reality, both conclusions are true. Things are, unsurprisingly, more complicated than we might have imagined.