Zipping through Geologic Time

Over the course of a 3-hour trip, while standing on platforms high above the valley floor, zipping from tree to tree and dangling from ropes to reach the ground here and there, you may not realize how much geology you’re experiencing. The Berkshire Mountains are not high as mountains go, maxing out at about 3,500 feet, but they’ve been around over half a billion years and they hold a lot of geologic history.  

Zipping along the top of the course, you’re travelling from northwest to southeast across a rocky ridgeline.  Beneath you, hidden in places by a covering of leaves and a thin layer of soil is a metamorphic rock known to geologists as the Hawley Formation.  It’s a layered rock known as a schist that is composed mostly of micas, quartz and feldspar and was formed about 450 million years ago when Africa collided with North America.  The shock waves have mostly subsided at this point.  It was formed by intense heat and pressure from the collision of the two continents, so it tends to be layered like a pack of cards standing on edge and it’s often oriented northwest/southeast.  

After you’re lowered to the ground at the Knoll Oak, where our water break takes place, check out the rock outcrop just next to the picnic table. This rounded silvery/grey rock is one of many examples of this schist (locally known as the Hawley Formation) which was most recently ground smooth under the weight of the last glaciation about 22,000 years ago.  If you look carefully at some of the rock outcrops along the course, you can see striations where the glacier dragged along the bedrock, leaving grooves that trend generally north/south in the direction of movement.  

When you hike from the picnic table down to the start of Two-Step zip, look at the rocks on your left side. Some have visible garnets (a red, roundish mineral) that stand out like raisins in oatmeal and help geologists unravel the history of the rock. Some of these garnets show well defined faces that show the crystal structure of the mineral.

A lot of people come zip lining to feel young again. Knowing that the rocks beneath your feet are nearly half a billion years old has to help with that feeling!