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McIntosh Lot Preserve: Ancient history on display

One of a series of articles exploring the natural and human history of Harpswell Heritage Land Trust’s protected land. With each of these articles comes a public event. On May 27 you are invited to learn about the geology of the Giant’s Stairs (click here for details).

By Nancy West

Ed Robinson photo

What comes to mind when you think of the Giant’s Stairs? A “stern and rockbound coast”? That’s a catchphrase used by P.G. Wodehouse about Bertie Wooster: “[He] once got engaged to… Honoria, a ghastly dynamic exhibit who read Nietzsche and had a laugh like waves breaking on a stern and rockbound coast.”

The dynamic exhibit of waves smashing against massive rocks at Giant’s Stairs delights visitors, and the Town of Harpswell’s Giant’s Stairs Trail is a favorite. Harpswell Heritage Land Trust (HHLT) opened a backdoor to the Giant’s Stairs by accepting the gift of the McIntosh Lot Preserve in 1993 from the estate of Adelaide Hall McIntosh. The Preserve turns an out-and-back trail into a loop, with a bonus of parking spaces along Washington Avenue. Plus, the trail through the Preserve allows you to avoid the poison ivy shrubs large enough to swallow small dogs at the north end of the Giant’s Stairs trail.

Ms. McIntosh purchased the land from her aunt, Virginia D. Coyle, the last of four unmarried Coyle sisters. Those four inherited the land from their aunts, Emily and Cornelia Coyle, who purchased the land in 1921. (A year earlier they had purchased the parcel to the south – the one with three cottages).

As elsewhere on Bailey Island, family ties ran deep here. Previously, James S. Rogers had owned the future Preserve. He acquired it from Mary Chahoon, probably a distant cousin. James bought the land when his mother sold the cottages to Ida Waugh. Before that, both properties had been owned by Captain William Henry Sinnett and his wife, Joanna. Why trace ownership backwards? Two reasons: First, the Sinnetts were kind enough, in their deeds of sale, to preserve access to the shore for the public. In 1909, the Sinnetts went all out and gave the strip of land along the shore to the Town of Harpswell. Thanks to the Sinnetts and Adelaide McIntosh, we can walk to the Giant’s Stairs.

The second reason to carry on about land ownership is the meshing of art and geology at the Giant’s Stairs. Frederick Judd Waugh, a marine painter who studied under Thomas Eakins, visited his sister, Ida Waugh, in the cottages and captured the essence of the shore. In this painting, you can almost hear eiders cooing. But Waugh also captured the fabric of the rock. As you walk the Preserve’s path, you step over sheet upon sheet of rock that stand almost vertical. Geologists refer to these parallel sheets as foliation because of their resemblance to pages of a book. (Think: Shakespeare’s folios.) In this case, the sheets began as sediments deposited in a sea 444 to 488 million years ago. We know they were buried deeply because clay-rich sediments metamorphosed into muscovite, garnet, sillimanite, and other minerals that do not form on the ocean floor.

Let’s look at those minerals literally and theoretically. When you get to the shore end of the McIntosh Lot trail, turn right to head south and stop where you can look east across a south-facing cove called Little Harbor (often with eiders). First, notice the shiny light-colored flakes that reflect light. They are muscovite, or white mica. The surface looks as if it’s covered in Saran wrap. Platy mica crystals grow perpendicular to forces of compression–it’s their path of least resistance during a chemical metamorphosis. Mica is the predominant reason the foliation exists. At this spot, you can also find garnets, the little red-brown soccer balls, one to two millimeters in diameter. Garnets are useful to geologists who study rocks like these because a) finding them is like winning a treasure hunt and; b) with careful lab analysis garnets have the potential for revealing the temperature and pressures to which this rock was subjected. Pressure translates into an estimate of how deeply buried the rocks were. The third mineral, sillimanite will be nigh unto impossible for a non-geologist to identify, but it is a goldmine of information in the field for a geologist. Its two siblings, kyanite and andalusite, have identical chemistry, but different structures that depend on temperature and pressure. Sillimanite is the high temperature, low pressure version. With only a magnifier, a geologist who sees the mineral sillimanite knows that this rock was subject to high temperatures while relatively shallowly buried. See graph to the right. For the geologic setting of the ancient Giant’s Stairs, the heat would correspond to a depth of about 25 kilometers.

The graph shows that the temperature must have been at least 500 degrees Celsius.

As you listen to waves smash, what else can you learn? Imagine rolling a ball down one of the flat sheets, the foliation surfaces. What direction would it roll? Yes, it would roll west. Yet, if you were at an outcrop of the same rocks in Phippsburg, the ball would roll east. Those observations indicate a giant fold in the rocks, a huge wrinkle in the crust that is concave downwards–an “anticline.” It bends this rock, called the “Cape Elizabeth formation,” along with rocks above and below it.

Now look at the milky white quartz veins that cut across the foliation of the rocks. Look around the margins of the veins where the Cape Elizabeth formation butts against the quartz. Do you see small folds? The quartz vein was a fluid injected into the Cape Elizabeth formation. When cooled and crystallized, a second episode of folding bent the Cape Elizabeth formation locally against the more rigid quartz.

One other cool feature at the east end of the Preserve’s path are flat, almost horizontal surfaces the size of tables. They are parallel to each other and look smooth and chalky. These surfaces form when minerals recrystallize along faults, where crust has broken and moved. If the ground shook when this occurred, there was an earthquake. These surfaces have the offbeat name: “slickensides.”

This handful of examples demonstrate multiple episodes of folding deep in the crust, injection of a fluid into metamorphosing sediment, and faulting. The folding happened under compression whereas injection happened under tension. It’s not possible to say what forces were at work to create the faulting. This one living room sized area at the end of the Preserve’s trail is a wonderful dynamic exhibit of the Earth, depicted by a talented artist, observable with your eyes, and decipherable with your common sense.

Forge ahead north to the Giant’s Stairs and see what you can figure out for yourself. Look for contrasts in rock types. Even if you don’t know what the rocks are, if they look very different, they are very different and they formed under different circumstances. You might even find a new mineral or two if you bend close to the rock surface, look carefully at the constituent minerals, and, if you have a knife handy, scratch the rock surface to see how hard or soft the minerals are.

If one rock type cuts across another, then it is younger than the rock it cuts. If the rocks are folded, they were buried deeply and compressed – they were under enough stress that the rocks folded. And that has amazed me since seventh grade.

Click here for more information about visiting the McIntosh Lot Preserve and the Giant’s Stairs Trail.

 

References

Curry, Clare A. and Hyndman, Roy D. 17 August 2006. “The thermal structure of subduction zone back arcs.” Journal of Geophysical Research: Solid Earth. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2005JB004024 Captured 3 May 2018.

Hussey, Arthur M. II and Marvinney, Robert G. 2002. Bedrock geology of the Bath 1:100,000 quadrangle, Maine.

Hussey, Arthur M., II. 2015. A Guide to the Geology of Southwestern Maine. Portsmouth, NH. Peter E. Randall, Publisher. 229 pp.

Mogk, Dave. “Gibbs Phase Rule: Where it all Begins.” Science Education Resource Center. https://serc.carleton.edu/research_education/equilibria/phaserule.html Captured 3 May 2018.

Waugh, Frederick J. 1909. East Coast Bailey’s Island. https://www.the-athenaeum.org/art/detail.php?ID=67367 Captured 3 May 2018.