Originally published in The Weekly Packet, July 17, 2014
The Peninsula’s geological past
BHHT geologist reveals secrets of John B. Mountain
Hikers at the top of John B. Mountain in Brooksville, Maine on a June 28, 2014 geology hike sponsored by the Blue Hill Heritage Trust.
by Tevlin Schuetz
On a hot, sunny day 30 people gathered at the base of John B. Mountain in Brooksville to follow geologist George Fields on a hike up the hill.
Fields described how the area (referred to as Castine volcanics) and its geology resulted from collisions between North America and Europe 500 million years ago. During this process, there was continuous volcanic activity estimated to have lasted a couple million years. A substance called rhyolite was ejected as ash during eruptions and accumulated to a depth of over a mile. Rhyolite is igneous and is the “extrusive equivalent to granite,” Fields said.
The existence of a fault line—a rare occurrence in Maine—along John B. Mountain is a feature that makes the Peninsula special, said Field, as he asked those assembled to jump up and down on the fault for a moment to see if it would have any effect. Many tried, but to no avail.
Fields made stops along the way to point out geological features as he told the story of how the Peninsula and surrounding area came to be as they are today.
In addition to the metamorphic events that took place, weathering patterns have changed the rock over many millennia. Rhyolite is somewhat resistant to wind and water erosion, so other rock types embedded in it were worn down first, leaving the hardened ashen material behind in many cases.
At the top of the mountain, Fields identified an example of layering of rhyolite ash fall, called a tuff, which had been repositioned after having cooled and solidified. Its original accumulation was horizontal, Fields said, but then metamorphic processes such as the continued tectonic pressure from Europe and local volcanic events folded the layered deposits into a vertical position, with the angle being nearly perpendicular, at 85 degrees to its original plane of deposit.
Glaciation has affected the area as well. The glacial build up in the area was around five miles thick, Fields said. This was an incredible amount of weight on the earth’s crust; changes are still occurring since the end of the last ice age. “The earth is still rebounding from the weight of glaciation,” Fields said.
Near the end of the hike, Fields pointed out a vein of quartz running through the rhyolite on the top of John B. Mountain. The quartz crystallized from lava flowing into a fissure in the rhyolite, Fields said. The lava came from an adjacent area of volcanic activity at a point in time after the rhyolite accumulation solidified. This type of anomaly can be used as a valuable tool for dating periods of volcanism in a comparative way from one area to the next, he added.
Fields, who is a licensed geologist in New Hampshire and Maine and who joined the Blue Hill Heritage Trust in February as its first associate director, said this was an interesting place with respect to geological characteristics. The different types of granite in relatively close proximity make the area unique. “You can go just a few miles, and you’re in another terrain,” he said.
In an brief introduction before the June 28 hike, BHHT volunteer Laurie White explained that the organization has almost 7,000 acres in stewardship, including the John B. Mountain preserve. To encourage families to go hiking with their kids, BHHT has placed a letterbox containing a unique stamp at each location where trails exist for the public. Kids can collect a stamp from each trail, and once they have eight they can turn them in for a prize at the BHHT office at 258 Mountain Road in Blue Hill.