Interpreting Geological History - Compton Peak vs. Limber Lost Trail

After one of the longest hikes up hill I have ever been on with my structural geology class I was taken aback by one of the coolest' primary structure I have ever seen, columnar jointing. If you have never seen what this looks like I highly recommend it because it is a reminder at how beautiful nature is even without trying. The map below illustrates the locations of both Limberlost Trail and Compton Peak. These magnificent structures formed during the break up of Rodinia roughly 750 million years ago. Chronologically these are above the Swift Run Formation.

The green arrow with coordinates represents the Limberlost Trail and the basalt columns. Image from Google Earth.

The green arrow with associated coordinates is the location of the Compton Peak basalt columns. Image from Google Earth.

As noted in previous discussions during a rifting period basalt flows to the surface and cools relatively fast and as the basalt losses heat it also starts to contract along the flow surface. Arrest lines are indicative of this and run perpendicular to the surface, which looks stacking of tabular structures. As a result of this cooling process these basalt columns tend to have a polygonal shape with angles that equal a total of 120 degrees. 

The angles of the basalt columns on Limberlost Trail are not equal to 120 degrees, however and must have gone through some type of metamorphic event as noted in the picture below.

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In contrast the basalt columns at Compton Peak showed very little if any deformation. In order to determine this measurements of angles as well as the strike and dip of the joint surfaces were taken.   The data revealed that, in fact, there had been little to no deformation at the Compton Peak location which is interesting since it is only 1.4 miles from the Limberlost Trail site. Most of the angles measured by the class at Compton Peak were at or very close to 120 degrees. If both of these basalt columns were formed during the same geological period, which we are assuming they were, than the Limberlost Trail basalt's underwent deformation relative to Compton Peak. This is very interesting because, again, the distance between these two outcrops is minimal especially on a geological scale.

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