Bloody Bluff and Luke

July 03, 2006


Provided and copyright by: Rob Sheridan
Summary authors & editors: Rob Sheridan

On April 19th of 1775, as they fled Concord (Massachusetts) harried by sniping 'Minutemen', a group of 'Redcoats' tried to rally and stand by the rocky bluff in Lexington pictured above (photo taken on April 19th of 2006). The fight that ensued went poorly for the British Regulars, who broke, the survivors fleeing back to Boston. The site has been known as “Bloody Bluff” ever since.

About 400 million years earlier, another violent, albeit much slower, encounter occurred at Bloody Bluff. The volcanic arc micro-continent, Avalonia, formed about 565 million years ago off the coast of the continental land mass known as Gondwana, which was at that time near the South Pole. Floating on its tectonic plate, Avalonia drifted north across the Iapetus Ocean over the next 150 million years and was driven into the part of Laurentia that would someday become the northeastern United States and Maritime Canada. This was the second such volcanic arc micro-continent to impact this area (two more would follow), explaining in large part the series of mountain building events, volcanism, magmatic intrusions, and accretionary wedges that make the bedrock geology of eastern Massachusetts so confusing.

About 200 million years later, Avalonia began to rift apart, eventually forming the Atlantic Ocean, explaining the many similarities in the bedrock geology of New England, the British Isles, and Western Europe. This confounding bedrock geology was further obscured by much later cycles of Pleistocene glaciation, but in some localtions it's exposed at the surface. Bloody Bluff is one such place.

Bloody Bluff is part of a southwest to northeast running bedrock fault that marks the junction of the Avalon and Nashoba terranes, the Nasoba terrane being the first volcanic micro-continent to impact this part of Laurentia. After its discovery in the late 19th century, it was named “Bloody Bluff Fault” for the 1775 incident, and represents the line of collision as Avalonia was driven into Laurentia 400 million years ago.

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