Summary and Overview
Tessa McDonald’s Master’s Thesis (working in Jason Briner’s lab at the University at Buffalo) proposed that an ice cap with glaciers covered a significant part of Baranof Island well after the retreat of the continental ice and the opening of Chatham Strait. Direct exposure dating is possible for granitic rocks that have remained exposed to the sky since the ice retreated. Tessa’s work included such dating, and the results are what led (in part) to the proposal that an ice cap and glaciers flowing from it persisted. Another line of evidence is the pattern of presence (or absence) of ash from the Mount Edgecumbe eruptions. If intact layers of ash are present at a location, that is evidence ice had retreated by that time. When ash cannot be found in areas where it would otherwise be expected (given the broader observed pattern of ash fall), this may be suggestive that ice was still present when the eruptions happened (with recognition that other factors may result in a lack of ash, so it’s important to look at overall patterns of topography and ash presence/absence within that context).
Unlike cosmogenic dating methods, looking for ash exposures requires no special equipment, so I’ve taken it upon myself to poke around a bit and make notes on what I find with the hope that someday (in the not too distant future), this effort may contribute to a better understanding of glacial history of this area.
Around Sitka ash exposures are regularly seen along cut banks created in the construction of roads. They are common to see around most of Sitka above about 36 feet elevation (which is the highest sea level made it here post ice age).
With some motivation stirring after seeing Tessa’s MS defense, and some conversation with Jason, I began checking the road along Silver Bay. With it’s proximity to the highest peaks of Baranof Island, and a road along almost the entirety of one side of the bay, it was a logical place to start. I also realized that I did not remember seeing ash along Herring Cove and Beaver Lake trails in the uplands between Herring Cove and Blue Lake. I have begun to look there as well.
I’ve separated my notes by area. I will update as a (re)visit and make observations.
Areas to check:
- Road between Sawmill Creek and Herring Cove
- Uplands between Herring Cove and Blue Lake (including Beaver Lake)
- Green Lake Road
- Medvejie Valley
- Blue Lake Road
- Silver Bay (East side)
- Katlian Bay and Valley
- Nawkwasina Valley
Road Between Sawmill Creek and Herring Cove
Quick notes based on April 24th, 2025 visit, with photos and more detail to come (need to follow up and check pole numbers for ash and till exposures):
First ash exposure not far past power house drive. Appears to be reworked – perhaps during original construction of the road, or during an even older landslide? Lots of ash, but also large rocks.
Further on, a cleaner exposure with what appear to be intact layers showing. Another a bit further along may be similar, back off the road behind an old landfill site.
Beyond there to where the road goes around the point marking the start of Herring Cove the upland side of the road is more rock exposure, including taller cliffs.
Around the corner, exposed bank has no ash. Appears to be glacial till with organic layer growing directly on top. From there alternating cliffs and ravines where streams come down until Herring Cove trailhead area. No ash deposits noted.
Uplands Between Herring Cove and Blue Lake
Quick notes based on April 30th, 2025 visit, with photos and more detail to come (need to follow up and check pole numbers for ash and till exposures):
No obvious ash deposits along Herring Cove trail or the loop around Beaver Lake.
Maybe a little ash mixed in, but if so, reworked, and not much (depth of soil that looked like it might include ash was typically not more than a few inches).
Much of it was on slopes, so maybe active – but other areas around town are similarly sloped and still have plenty of intact ash.
Where bedrock was exposed where the trail goes through boggy areas, there is no evidence of ash between the ash and organic layers.
Can’t fully discount possibility that ash fell and was subsequently washed out or buried (depending on position on slope), but absence of intact ash deposits may be suggestive that ice was present at least in the Beaver Lake upland area during the ashfall events.
Possible granitic erratic that might be feasible candidate for exposure dating as second line of evidence.
Green Lake Road
Tracing the western shore of Silver Bay for nearly its entire length, Green Lake Road offers convenient access to look for deposits of volcanic ash from Mt. Edgecumbe, and see where they are missing. Serving as access to the Green Lake hydropower facility, the road is lined with sequentially numbered utility poles. Pole numbers are included at times as an assist to more easily relocate locations.
On April 16th, 2025, I (Matt G) travelled the road as far as Bear Cove checking for ash exposures.
Along the first section of road I didn’t notice any obvious exposures. However, the vegetation was thick, and overhanging in places. If intact ash exposures are present, finding them may take more careful looking and some shovel/trowel work.
As the road cuts across the point which marks the south boundary of Herring Cove, it climbs up hill. Near the top of this hill was the first obvious deposit I noticed.
Shortly after, I stopped to document a very clear deposit at Pole G44. I climbed up the cut bank and got several photos showing different parts of the exposure, including the nicely clear interface between glacial till and several layers of ash.
Along this upland section, ash appeared regularly in the cut banks. However, I didn’t try to determine if it was largely intact, or reworked.
Near Pole G50, the road drops back down again, and along a lower section here I saw little or no ash. It could be covered by vegetation, but this area may also be below the marine maximum limit (which could have washed any ash away).
The next clear ash deposit I noticed was by Pole G61, where the road went up hill again. Here, and at Pole G63, bedrock was present below the ash, though there may have been a layer of glacial till between which had been covered by ash shedding from the exposed surface above.
Beyond that, ash exposures showed periodically, including the last obvious deposit I noticed shortly before Pole G86 which is on the final downhill approach above the north shoreline of Bear Cove. I could not tell if any of these were intact deposits based on what I could see from the road.
Several years ago, a landslide cut down and across the road just north of the mouth of Medvejie Creek. There was no obvious ash deposits that I noticed showing in the cut bank sides of this landslide.
Just beyond the Medvejie Hatchery there was ash showing on a cut bank which seemed to have a lot of rock mixed in.
This was as far as I went on this initial trip. I’ll add more if/when I get to check between Bear Cove and Green Lake.
Medvejie Valley
No obvious ash deposits are apparent along Medvejie Lake trail. It is clear the valley has been subject to an immense amount of rock fall, so it’s likely that if any ash was deposited in the valley after ice departed, much (all?) would have subsequently been covered by rock.