Northern Convergence: The Mystery Photo Explained

Thanks for all of your conjectures about yesterday's mystery photo! The Earth is such a fascinating place, and there are so many different kinds of strange patterns that have different origins. Let's review some of the guesses (and they were very good, by the way):
a. Sinkholes and karst topography
b. Bison wallows
c. Ground squirrel or prairie dog workings
d. Periglacial patterned ground
e. Dried marshes, evaporative processes
f. Pithouses

Take a look at the depressions from ground level. The rocks are gravel and sand, so karst is not likely (sinkholes most often result from the collapse of caverns in areas underlain by limestone). Something like a buffalo wallow makes some sense, but these are very concentrated, and the region is not really good buffalo habitat (too much mountain slope and not enough prairie). Rodents like prairie dogs or gophers tend to produce mounds (so called 'mima mounds') rather than hollows. The location of the site on a high river terrace tends to rule out the idea of swamps or evaporative processes.

Of the natural processes that could make this phenomena, periglacial processes make the most sense. Freezing and thawing of soils often result in rings and polygons that might resemble what we saw in the picture. The region underwent glaciation in the Pleistocene epoch and continues to have extremely cold temperatures in winter. It makes some sense, but then why are the pits filled with so many artifacts?

So yes, these pits are made by humans. We were paying a visit to the lands of the Xwisten people, a First Nations group whose traditional lands were along the Fraser and Bridge Rivers near Lillooet, British Columbia. This river terrace surface contains the remains of dozens of pithouses.

The former village along the Bridge River is the site of ongoing archaeological excavations by researchers and students of the University of Montana as well as local members of the Xwisten tribe.

We were being led on a tour of the archaeological site by several members of the Xwisten Bridge River Band who had been involved in some of the excavations. The setting was dramatic, with high mountain peaks on all sides.

As is the case with most archaeological digs these days, only a small portion of the site is being studied in detail. There is an expectation that future technology will be able to better analyze the site, so much of the village remains undisturbed for the time being. Only one of the pithouses was exposed when we visited.

The village site was abandoned over a century ago, although many Xwisten live in a modern village just a few hundred yards away. Evidence from the site investigations show nearly continuous occupation of the site beginning about 1,800 years ago. Many of the sites were used over and over, with some of the pits containing 12 levels of occupation.

A few years ago the Xwisten decided to build a pithouse on the site using the knowledge of their elders. One of our guides noted that what took their ancestors several weeks to construct took them more like three years even though they had modern tools. Entry was usually through a square hole in the room, although some may have had side entrances. The ladders, as you can see below were notched logs. With well insulated ceilings the pithouse seemed like it would be roomy and warm in winter conditions.

The Xwisten subsisted in part on salmon, and part of the demonstration was the preparation of salmon for eating and preservation. Our tour concluded with a salmon dinner. The sun was getting low, and we had another two hours of highway ahead of us. We drove into the night arriving at our hotel in Kamloops sometime after ten o'clock. It was a long, but fascinating day. We had left the Coastal Belt and were now making our way through the Intermontane Belt. We'll pick up the story in the next post!

To AF, the commenter in the last post who lives in Lillooet, let me know if I got anything wrong! Our visit was very quick, and I'm just a geologist, not an anthropologist!

Northern Convergence: Moving Pictures of the Canadian Wilderness, from Whistler to Lillooet

I PROMISE to reveal the origin of yesterday's mystery picture SOON! In the very next post, in fact, barring any major earthquakes in Northern California, or other major geologic distractions. It's just that I value continuity and order...I wrote about our journey up the Sea to the Sky Highway yesterday as well, and I complained mildly about the rainstorm and overcast conditions that prevented us from seeing the mountains of the Coast Belt. I was looking at my photographs, and from Whistler to our next stop at Lillooet I didn't take any pictures. I was driving after all, and we weren't able to stop, due to a tight schedule. There was a good 120 kilometers of what I recall was a pretty spectacular drive through the Coast Mountains and the Cadwellder Range.

What I forgot is that Mrs. Geotripper was next to me snapping picture after picture as we sped around curves and over passes. I went over to her photo files and found a wonderful record of "moving pictures". Moving pictures can mean "movie", or "emotionally striking", or "taken from a moving car". In this case the term today refers to the third meaning, with a smattering of the second. The drive was just stunning.

The Coastal Belt mountains, as described in the previous posts, are composed largely of Mesozoic granitic plutons that intruded into older metamorphic rocks of exotic terranes, tracts of crust that were shuffled about the Pacific Ocean and ultimately slammed into the west coast of North America ("Slammed" in the geologic sense, of course. The terranes were moving a few inches a year).

As we passed beyond Whistler and Pemberton, we emerged into the "rain shadow" side of the mountains, and the storm that had caused us so much frustration in the morning had begun to break up. Like a feather boa dancer, the clouds revealed little bits of the high mountain ridges, and then just a little bit more. We started to see patches of blue sky, and then the long-forgotten sun appeared.

Evidence of glaciation was everywhere, from the horns, aretes, and cirques of the high peaks to the deep U-shaped valleys. As we moved east, the canyons became deeper and more rugged. I'm hard put to recall a more dramatic highway (except for later on the same trip!).

For sixty miles or more we followed the highway, seeing almost no other travelers, and not a single settlement or other evidence of human activity. It was the kind of road that looked blank on the maps during the trip planning process, but which just begged for a stop here and there to look at the rocks. But we had an appointment down the road. We drove on.

The high ridges of Mt. Brew became visible, and we switchbacked down into a dramatic valley containing Cayoosh Creek and Seton Lake. In the distance we could see the valley was blocked by the massive Fountain Ridge. It's sort of unusual for a glacial valley to make a ninety degree turn unless something really forced it to do so. In this case it was a major fault line, the Yalakom fault, which forced the original rivers to change direction, and the glaciers followed suit. The fault was active in late Cretaceous and early Paleogene time, around 65 million years ago. It must have startled more than a few dinosaurs...

There were a few more sharp turns beneath hulking masses of rock, and we rolled into the deep valley of the Fraser and the small village of Lillooet. The town has room to exist only because of terraces that lie high above the river. These terraces are the result of sediments backing up behind gigantic landslides into the Fraser River a few thousand years ago.

We arrived at our appointment, which just happened to be the place where our mystery photo was taken! More on that in the next post.

And thank you, Mrs. Geotripper, for the wonderful pictures! Pretty good for a photographer who was trapped in a speeding car...