Hinesburg’s colonial charter was granted in1762, which may seem like a long time ago; and prior to European settlement native peoples probably fished and hunted on the Hinesburg landscape for thousands of years. However, the time-span of the town’s entire cultural narrative is just a blink of an eye in comparison with the history of its rocks.
The geology of Hinesburg dates back more than 500 million years, to a period called the Cambrian, when the position and arrangement of the earth’s continents were much different than it is today. At that time, there was no Champlain Valley or Green Mountain range yet. Hinesburg (and much of the land we today call Vermont) was located just south of the equator, near the edge of an ancient ocean called the Iapetus. Into this ocean basin eroded great quantities of sediment which gathered on the bottom of the sea. Close to shore coarse grained sands were deposited; farther out in deeper water accumulated finer silts and clay. Countless shells of tiny marine organisms also collected on the ocean floor. Eventually the sediments and the shells compressed into sedimentary rocks. Then something wild occurred: over the course of the next 100 million years, continents collided in an event called the Taconic Orogeny and the core of the Green Mountains was formed. The heat and pressure of this impact raised the rocks of the sea floor and metamorphosed them into new rock types called quartzite, marble, and schist. In addition, several rock layers, or strata, faulted and broke apart, resulting in older rocks being thrust up on top of younger rocks. Subsequent continental collisions further transformed and elevated the Green Mountains.
By 200 million years ago most of these chaotic land-forming events had ceased, and the wind, rain, and ice went to work eroding what were once enormous mountains greater than the modern Himalayas down to the familiar peaks we observe today. Large blocks of rock also sank relative to their surroundings, which resulted in the creation of the Champlain Valley.
The rock formations we see in Hinesburg are representative of these geologic events. On the eastern side of town in the hills are predominantly metamorphosed non-carbonate rocks like quartzite, phyllite, and schist. In contrast, the western side of town is characterized by younger, more easily-erodable carbonate rocks (containing lime) such as dolostones and marble. Running down the center of town between these two sides, parallel to the course of Route 116, is the world-renowned Hinesburg Thrust Fault, which extends from Bristol north into Canada. This unique feature marks the contact point where metamorphosed phyllites were pushed four miles westward up over top the younger carbonate rocks during the Taconic Orogeny. A great place to see the fault is at an outcrop of rock up behind CVU High School. You can literally put your hands on two adjacent rocks that span more than 100 million years of geologic history!
Although Hinesburg’s bedrock is ancient, the rocks underlying the landscape have played prominent roles in shaping the overlying patterns of plants, animals, and human settlement, both in the past and still today. Without the steep, rocky terrain in the eastern hills there would be no rocky mountain streams, nor the legacy of woodland stone walls, jagged rock ledges, or Mechanicsville’s historical mill industry. Likewise, the carbonate rocks of the low-lying valley areas on the western side of town help give rise to sweet soils that stimulate plant growth. Though geologic forces often occur on a timescale barely perceptible to humans, they continue to mold the landscape fabric of our daily lives.
Hinesburg Thrust Fault
Bedrock Cross-Section of Hinesburg and Thrust Fault
Outcrop of Non-Carbonate Schist on CVU Road
Enriched Limestone on the Bissonnette Property
Bedrock Map of Hinesburg