Location: Turnagain Pass – Mile 68.4
Pull into restrooms on northwest side of highway
Short Description: Students will use observation skill and infrared aerial photography to decipher glacier and moraine development.
Required Equipment: Copies of Infrared Aerial Photograph of area (included)
- Binoculars for students
- Decent footwear for off trail hiking
Safety Consideration: Relatively low risk factors. If students plan to explore area, shoes/boots appropriate for wet muskeg environment recommended
- Pit Toilets on Site
- Low clouds will obscure important mountain visuals
Inquiry Prompts: What’s Going On Here?
There are a number of clues that make this a instructionally rich area. Among these are:
- Wide U– Shaped Valley
- Glacial Moraines throughout valley
- Glaciated valleys to the Southeast (but not to the Northwest)
- Striations on Mountainsides (particularly to the south)
- Debris “cones” below drainages to the north.
There may be other clues but make sure students make the above key observations (both on the ground and from aerial photo. It is helpful to mark moraines and debris cones on aerial photos.)
But alas . . . the story.
The Turnagain Pass Valley illustrates several complex geologic relations. Any self respecting Alaskan student can easily see that the main Turnagain Pass valley (graphically steep and U—shaped) is our run of the mill glacially carved valley. Likewise, the gravel piles on the valley floor are moraines that are the result of glaciation. This much is true. However, after walking through the area, the students will note that the moraines don’t really align with a glacier running the length of the valley. In fact, if students would identify and trace landscape shapes on the aerial photograph another story would emerge.
If viewed carefully, the aerial photo suggests that the moraines may have been pushed from the south (they were) and indeed the remnants of a possible old stream bed against the base of the mountains to the north (it was). . .
The Rest of the Story:
The story of Turnagain Pass is a complex and complicated series of glaciations. The valley was originally dredged out during past glaciations. The glaciations widened the broad valley of Turnagain Pass, which eventually became the thoroughfare for the Seward Highway. Trim lines and striations on the south wall of the valley add further testimony not only to this era but to also the thickness of the ice.
The moraines, however, tell a different story of later glaciation. Some 11-12,000 years ago during the final phases of the Elmendorf stage of the Naptowne glaciation, glaciers developed in the valleys (Tincan, Lyons, and Ingram) to the south. It was these glaciers that carved out these valleys resulting in the moraine debris fields. In the farthest extent of this ice (as evidenced by the infrared enhanced photo) the valley stream was pushed against the mountains to the north creating an isolated stream bed diverted by an ancient wall of ice.
A student might bring up the question, “How come the glaciers came from just one side of the valley? What’s up with that?” Indeed, what IS “up with that?” In order to create a glacier one must have persistent snow pack . . . snow that doesn’t melt during the “off season.” A quick look at the compass reveals which mountain is north facing (ok, north-west facing) versus south facing. Indeed, there are a number of examples on the Kenai Peninsula where glaciers originated from north facing slopes which gets less sunlight than its cross-the-valley south facing cousins.
And One More Thing:
Before hopping back on the bus, take one more look at the aerial map. Make note of the cone shaped features on the north wall of the pass. Each of these originates at the terminus of a drainage. Careful, these are not alluvial fans formed by water (they are much too steep). The cause is the flow of snow…avalanches that annually cascade down these slopes. All the slopes are susceptible to sloughing snow, however the V-shaped drainages magnify the debris material down the chute and to the terminus where it hits the valley floor. The result? A steep cone of course rock debris which builds with each winter cycle.
These debris cones are a handy way to remind students the perils of traveling in the corridors. Travelers (both early and modern) have had to deal with the perils of avalanches. To the present-day traveler it may mean delays when avalanches close the highway or caution when recreating in the back country. However, to earlier travelers although winter offered easier travel through the alders and over marshes, the threat of avalanche was very real. And very deadly. Not far from this valley, at the Lynx Creek mine, six men were buried (five perished) at the turn of the century when a slide ripped through the camp.