South Burlington

Bedrock Geology

Five-hundred million years ago, the land that is now South Burlington was beneath a shallow, tropical ocean. Thousands of streams etched the surrounding land, pulling huge amounts of sediments with them to the sea. Layers of sediment miles thick piled up in the oceans, the product of the wearing down of mountains miles high. These sediments were compressed to form the various types of bedrock that underlie South Burlington. The youngest bedrock in South Burlington is interbedded limestone, calcareous sandstone, and quartzite called the Bascom Formation, covering about the eastern half of the town. Outcrops near its eastern boundary are metamporphosed as a result of the Hinesburg Thrust that occurred during the uplift of the Green Mountains. An example is depicted here, where I-89 passes under the Hwy 116 bridge:

The exposed bedrock in South Burlington becomes progressively older and older from east to west. As the map shows, the bedrock formations are oriented in thin north-south trending bands. This pattern is a visible reminder that the big squeezes that uplifted the mountains of Vermont came from what is now east (what was then south). The oldest formation — the Monkton Quartzite, is metamorphosed sandstone, laid down over 500 million years ago. This is most easily seen at Red Rocks Park.