There’s an old joke among rock experts. Ask a geologist what’s two plus two, they’ll likely answer “somewhere between three and five and close to four,” said local geologist Cindy Buxton who gave an “Introduction to Haines Geology” via Zoom on Thursday, March 25.
“It’s not because we can’t add,” Buxton said. “It’s because the question isn’t that simple. We know that we can’t (always) be very precise and we can’t always be very accurate so by saying, ‘It’s between three and five and close to four,’ we’re conveying there’s some uncertainty and that’s important to us.”
Buxton’s was the first of several presentations the Takshanuk Watershed Council is hosting as part of its Environmental Science Lecture Series Thursdays at 7 p.m. via Zoom.
Buxton said putting together a geologic picture is like completing a 1,000-piece jigsaw puzzle but with no picture to work from and only having 30 or 40 pieces in the box. Each puzzle piece, a fossil, basalt, fault line or spring, is a clue for putting together the rest of the picture, for understanding a geologic history which spans millions of years. In that time, many puzzle pieces were lost.
“Some of the pieces we need are buried beneath the forest,” Buxton said. “Some are destroyed. Others are really hard to get to.”
In the Chilkat Valley and surrounding areas, clues, littered across the landscape, were left behind that help geologists understand the region’s glacial history. Boulders made of rock unlike the surrounding cliff sides like one at Seduction Point (glacial erratics), scratches in the rock at the top of Mount Riley and Pyramid Island are all features in the landscape leftover from grinding and flowing glaciers as they formed and melted.
Glaciers flow as snow compresses into ice over time. As rock and other debris erode and fall onto the glacier, they become a part of the ice as it grows and flows. When glaciers melt, they can dump mounds of debris, from boulders to silt, in front of the glacier, what’s called a terminal moraine, Buxton said.
“If the front of the glacier has stayed in one place for a long time then it (usually) makes a big pile,” she said. “If it is slowly melting back it just leaves a scattered layer of rocks, or scattered rocks.”
Pyramid Island, the spit of land at Taiyasanka Harbor and the steep hillside behind the homes on Lutak Spur Road are all examples of terminal moraines.
Buxton also explained how the local valleys formed and that the depth of Lynn Canal is similar to the depth beneath the Chilkat River. The difference is that the space under the river was filled in by more sediment as glaciers retreated. Both areas are above fault lines. The Denali Fault, which is directly under the Chilkat River, once divided the Pacific Plate from the North American Plate. Another fault line runs under Lynn Canal. When the Denali Fault was more active, it left a zone of broken rock. Erosion and glaciers scooped out loose material in the fault zones, forming the valleys of Lynn Canal and the Chilkat.
“Faults often end up being valleys,” Buxton said. “In Southeast Alaska, whenever you have a long straight valley, there’s a good chance there’s a fault there.”
Buxton’s full lecture and the accompanying slide show can be viewed at the Takshanuk Watershed Council YouTube page. Her next presentation scheduled for Thursday, April 1 is “Rocks of Haines.” On Thursday, April 8, Erik Stevens will give a presentation titled “Haines Micro-climates: Simplicity and Complexity.”
Zoom links and additional presentations can be found at takshanuk.org.