As bedrock folded under Pangaea's formation, other zones that had melted as result of subductions, cooled. Slowly, molten rock cooled into the granite that marks the V-shaped Deerfield valley.
Then, in what would be opposite the motion that caused formation of the super-continent, 200 million years ago, Pangaea began breaking apart. Stress from the super-continental separation created the Eastern Border fault that runs through New Hampshire, Massachusetts and Connecticut. Stress, from this fault zone led to development of a rift valley where sediment was deposited as shown by the image upper right (developed by Richard Little and Will Sillin). Whereas the block diagram of the rift valley was caused by geological events from 200 MYA and shows modern day river systems, it's worth remembering that these river systems are thousands of years old, not millions. Older granite and metamorphic rock underlies the rift and can be observed in far western and eastern Massachusetts, sediment flowed into the rift. This sediment is younger rock, largely red Arkose (sandstone containing feldspar, an aluminum silicate). In Deerfield, red arkose is readily seen along the Pocumtuck Range.
