In an effort to connect with and learn from expert naturalists, I am participating in a year-long natural history course: Hitchcock Center for the Environment’s Nature All Year Program.  Divided into monthly classes, the course will cover topics ranging from vernal pool ecology to plant identification to mammal tracking. This month’s class focused on the geologic history of the Connecticut River Valley led by Nikki Hill. Although it’s hard to imagine given the relatively short human lifespan, over geologic time the Earth’s surface and the rocks that comprise it have undergone dramatic changes, moved great distances, and have iteratively been submerged by water and exposed.

The Holyoke Mountain range in the Hadley/Amherst area (one of the main topics of Sunday’s discussion) is a traprock mountain range, meaning it is formed from an elevated landform composed of a dark-colored, extrusive volcanic rock – in this case basalt.  As a fairly hard rock, this basalt ridge remained, while much of the more easily erodible sedimentary rock comprising much of the Connecticut River Valley was eroded away.  Some of this stability is likely due to the fact that basalt is often high in iron.  This iron also results in the characteristically dark-colored surface of basalt, which weathers to shades of red when exposed to the air. The basalt in the current Holyoke Mountain range is the result of volcanic activity during the Triassic and Jurassic periods. As lava cools during the formation of basalt, the resulting rock formations often form into roughly hexagonal columns or tubes.

But why hexagons? First, lava in any particular flow cools at different rates depending on its location within the flow. When a portion of the lava flow begins to cool from the inside, it contracts, and fractures as it does so. If cooling rates are fairly uniform, it contracts and fractures fairly evenly, leading to tall, well-developed, generally hexagonal basalt columns. Although cracking tends to begin with 90° angles, junctions of three propagating crack faces tend to rearrange such that the initial right angles between them start to approach 120°, thus producing the hexagonal shape common to so many columnar basalt formations.

As part of Sunday’s class, we went for a walk in Joseph Allen Skinner State Park to explore some unique geologic features first hand. The most striking feature encountered on our walk was an impressive basalt outcrop named Titan’s Piazza. Here, because the lava was still flowing slowing as it cooled, the resulting formation resembles a rock waterfall. A quick glance at the close-up photos shows the reddish color the iron-rich basalt has taken on through weathering. The outer layers of the bottom portion this formation have broken off over time, leaving a flaked, bulging look to the exposed end of each of the columns (as well as a fairly difficult walk up a steep hill covered with loose shards of rock).