New England has been on the passive margin of the North American plate (like the rear end of the car in a head-on collision; the west coast is the active margin, speeding head first towards the Eurasian and Pacifc plates. This is why all the big earthquakes and volcanoes are on that coast). We’ve been the passive margin of the continent ever since Pangea broke apart ~200 million years ago. When Vermont was part of Pangea our landscape was epic, Himalaya-size epic. Giant craggy peaks reaching 20,000’+ up into the clouds.
Without earthquakes from that continental collision to push our mountains ever higher, those big Himalaya-size and shaped mountains began to break down. And slowly over time their impressiveness started to dwindle. While 200 million years did much to soften the terrain, what would make New England a pleasing and mild landscape rather than an impressive craggy montane region was the Quaternary Glaciation. This prolonged period of Ice Ages coming and going a dozen or so times began about 2 million years ago and ended about 25,000 years ago.
The repeated assault of advancing glaciers inexorably ground away at our mountains. And these were big continental glaciers, ice sheets, a mile + thick in Vermont. While valley glaciers tend to forge all sorts of awesome jagged mountain features like horns, aretes, and cirques, continental glaciers (like the Laurentide Ice Sheet that covered Vermont) tend to grind down and round over topographical features. Yes, the Green Mountains are tall, but they’re not nearly as tall as similar mountains in the Blue Ridge and Smoky Mountains that lie south of the terminus of the glacier. Being north of the terminus All of our mountains would have been ‘submerged’ under the ice flowing overhead and received a nice smoothing out.