Exploring the Geology of the Fells with Prof. Jack Ridge

A guest post by Stephen Engel

It is easy to wander through the Fells without thinking about when or how the rocks beneath your feet formed. Rocks in the Fells are 900-580 million years old and formed adjacent to what is today West Africa. This ancient landscape had many volcanic eruptions of lava and ash, and large bodies of granitic magma crystallized in the subsurface. The rocks were later separated from Africa to form a microcontinent called Avalonia. Avalonia eventually collided with North America about 430 million years ago and now forms southeastern New England and part of Newfoundland. Since that time, the Boston landscape has been heavily eroded, exposing rocks once deep in the crust.

Beginning about 2.5 million years ago the area was covered by large ice sheets during successive ice ages. Glacial ice left the Fells for the last time about 17,000 years ago, leaving behind large glacially transported boulders and glacial scratches and grooves.

All photos by Prof. Jack Ridge

Recent investigation of the Fells’ geology has brought the details of its rich pre-history to light. Beginning in 2007, Prof. Jack Ridge in the Earth and Ocean Sciences Department at Tufts has made geologic maps of the Fells as part of his research. Because of abundant rock exposures in the Fells it is perfect for detailed geologic investigations. It really is the low hanging fruit!

Ridge’s research is now accessible to the public online in the form of geologic maps. A bedrock map shows the area’s many rock formations and their relative ages. Another map shows the surficial geology that depicts glacial features and deposits above the bedrock surface. Each map has detailed explanations. “Jack” has also prepared a 7-part, self-guided tour of the Skyline Trail with other tours planned for east of Spot Pond. Each part of the tour is 2-4 miles long and can be completed in a few hours. The public is invited to investigate the downloadable maps and tours on “The Geology of the Middlesex Fells” web site at: https://sites.tufts.edu/fellsgeology/. Jack hopes the web site will serve as a resource for teachers and those interested in the natural history of the Fells.

Steve Engel is a Friends of the Fells board member and resident of Winchester.