This image is homage to my hometown. As a child, I explored the Green Mountains with great curiosity – questioning what lies beneath the soil and wondering, ‘how did these mountains form? Why is a ridge located here and a valley located over there?” Now, decades later, I return in search of answers. This image shows 4¼ miles in depth below Shrewsbury in Central Vermont looking north – northeast.
The majority of the darker metamorphic rock in this image is gneiss that makes up the Green Mountains. These rocks first formed on the order of 1,400 to 1,300 million years ago as part of the ancient mountain building event (Grenville Orogeny). The large Grenville mountain chain eroded for hundreds of millions of years and the lapetus Ocean formed off the eastern edge of North America.
The lighter dolostone and quartzite rock layers near the bottom and along the left side of the image were originally deposited in the lapetus Ocean basin as marine sediments. Eventually the ocean began to close and these sediments were compressed, uplifted, and folded during the formation of the Appalachian Mountains in the Taconic Orogeny between approximately 500 and 440 million years ago.
Slices of crust were pushed up and to the left causing the older Green Mountain rock to be placed on top of the younger former marine sediments as the tectonic plates collided with the eastern edge of North America. The lapetus Ocean closed completely as tectonic plates connected to form one supercontinent named Pangaea.
Pangaea eventually broke apart opening the Atlantic Ocean. Pull-apart tectonic forces caused many fractures and the movement of magma from deep in the earth up thro ugh these fractures to the surface along eastern North America.
The vertical conduit of igneous rocks, syenite and essexite, on the right side of the image was formed as magma flowed up through fractures in the Green Mountain rock. This igneous intrusion named, “Cuttingsville Stock,” formed 100 million years ago.
The geologic structure for this image was extracted from “Bedrock Geologic Map of Vermont” by Ratcliffe et al, 2011.
