Following the Taconic orogeny was the Acadian orogeny. The Acadian orogeny took place about 360 million years ago when a landmass called Avalonia smacked into the North American continent and began to subduct. During this time of subduction, magma plutons were generated deep within the earth’s crust. Some really neat structures were formed from this event, one being the lamprophyre dikes which are still seen today cutting across the Potomac River at Great Falls National Park. Here is a picture of what the dikes on the Virginia side looks like today. Notice how deep the grooves are? This is because the mafic minerals that make up the lamprophyre are relatively unstable compared to its surrounding rock. Potassium and Argon dating has dated these to be about 360 million years old. Hey…that appears to be around the same age as the Acadian orogeny…could there be a connection?
Here are the dikes on the Maryland side of the Potomac River. Standing on the edge of the river, it’s not hard to see that these two have a pronounced lack of alignment. Controversy on the reasoning behind the offset is still in question. We will input all of the data we have collected and come up with a hypothesis later….check back.
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