25 December 2014
The long hike to Helen Lake (Banff National Park, Alberta) is worth it, not only for the mass wasting and glacial geomorphology, but also for the stromatolites.
Just another day in the office…
As a “Christmas gift,” enjoy these images… some of the loveliest stromatolites I’ve ever seen:
24 December 2014
This is Helen Lake.
It’s a lovely little tarn. I hiked there last summer with Aaron Barth, to do some GigaPanning of stromatolites. But we’ll save those images for another day. Right now, I want to focus on the geomorphology of this valley.
A wider view of Helen Lake’s setting can be seen in this GigaPan:
One thing that I hope will catch your eye in that GigaPan is the big pink googly eye on the cliffs at left (south) in the background. This is a rock fall deposit:
Here’s a GigaPan of the rock fall:
The rockfall shows up so well because fresh Gog quartzite is very light in color: white or pink. But when it’s been exposed for a while, it tends to host lichens, and they come in green and gray varieties, as seen here:
Super; So that’s straightforward and understandable. We can see the source area of the slide on the cliffs above. We can see a little nubbin of the darker underlying sedimentary rock poking through in the middle. Case closed. But what’s this thing in the middle distance??
Wait, Callan. What “thing” are you talking about?
At first glance, you might be tempted to classify this as a lateral moraine, since it’s a linear feature along the side of the valley, made out of bouldery sediment. However, compare it to this example of a lateral moraine, from the Athabasca Glacier:
You can see that till, the poorly sorted sediment that makes up a true lateral moraine, is dominated by lots of finer-grained stuff in addition to the big boulders. The big guys “float” in a finer-grained matrix. With the Helen Lake example, however, it all seems to be big stuff – the character of the sediment looks distinct from the till I would expect a moraine to be constructed from. It looks more like a talus slope, made of bouldery colluvium, except that it’s clearly offset from the base of the cliffs. Maddenigly, like a lateral moraine, it follows the valley upslope…
I’m tempted to call this a protalus rampart. If I understand the term correctly, it implies partial glacier melt-back, followed by an extended period of time when snow and ice remain in the deepest “corner” of the valley. Spalling of rock from the cliffs above triggers rock fall, but the resultant boulders land on the snowy slope, sliding like blocky sleds downhill to pile up at the toe of the snow slope. Later, the snow and ice melt away, removing the “ramp,” and isolating the line of talus from the source cliffs. What do you think? Have I got that right?
…Or is something else going on here?
23 December 2014
Two years ago, I posted on some interesting structures my students and I saw at Consolation Lakes, near Moraine Lake in Banff National Park. They were little concretions, “oncoids” roughly speaking, and may have indicated (thanks Howard!) that the boulders were sourced to the Peyto Formation, a Cambrian carbonate within the Gog group:
The purpose of today’s post is to confirm that these structures are still there two years later, and to post a few more photos of them:
Note the negative weathering the oncoids show relative to the surrounding matrix.
I also found a couple of boulders at a new location, in the lateral moraine of the Athabasca Glacier that had the same features, though less oxidized…
Note the ooids (upper right) and radial cracks in the “oncoid” here:
22 December 2014
Want to see something cool?
Itty bitty stromatolites… like baby’s fingers!
There’s a big weathered-out stylolite at the base of this stromatolite-bearing layer, too.
These elfin stromatolites are part of the boulder in the lower left (foreground) of this GigaPan, taken at the Icefields Center parking area in Jasper National Park, Alberta:
19 December 2014
Let’s journey to the Cretaceous today, to see sandstones, shales, and even some coal strata that have been folded during the eastward thrusting that built the Canadian Rockies.
Here’s the same fold, in context, shot in GigaPan on a different day, from a different angle. Can you match it up?
Ben Gadd showed me (and my field class) this site last summer. It’s a little west of Canmore. There’s a map on the GigaPan page if you want to find it yourself. Thanks Ben!
18 December 2014
For those inclined toward trace fossils…
…This is from Banff National Park, Alberta, Canada. I saw it on the trail to Consolation Lakes from Moraine Lake. I do not recall rock type – could be dolostone, could be Gog quartzite. It’s float (loose; not in situ), but I infer the photographed surface is the underside of the bedding plane; I’d be fine being totally wrong about that, though. There are two things I would like explained/identified here: (1) the prominent, arcuate ridges (which I infer to be trace fossils of some kind) and (2) the finer features, packed in sets, sometimes apparently emanating perpendicularly from the larger arcs, but also present where there are no “arcs.” They look like rodent “gnawing” marks to my untrained eye.
Anyone want to take a stab at identifying these features, and interpreting them for the sake of a poor geoblogger’s education?
17 December 2014
That’s the view from Woodstock Tower, on the crest of Three Top Mountain, looking east/northeast across the Little Fort Valley and through Mine Gap (a water gap), across the main Fort Valley and then Massanutten Mountain itself, with the Page Valley separating Massanutten’s ridge line from the horizon-forming Blue Ridge.
16 December 2014
Back in 2011, when we were still living in D.C., Lily and I made a hiking trip out to Buzzard Rocks. It was a destination. Now that I live out here in the Fort Valley, I see Buzzard Rocks all the time, and I love it. It’s such a cool feature – a spot on the crest of the hill where you can see the slabby expression of steeply-dipping beds of Massanutten Sandstone.
Here’s the view from the road:
(Were you able to find the rock climber in that shot?)
From atop the crest of Buzzard Rocks, you can walk along this “mohawk of quartz arenite”…
Here’s a look at a sample of the Massanutten Sandstone:
At Buzzard Rocks, it is internally cross-bedded, as seen here:
Buzzard Rocks needs three things in order to exist: (1) deposition of a lot of quartz sand during a time of passive margin sedimentation on the east coast [this was during the Silurian, between the dirty clastic influence of the Taconian and Acadian Orogenies], (2) deformation (tilting/folding) associated with the final phase of Appalachian mountain-building, and (3) differential erosion of the sedimentary strata. Those layers stratigraphically below the Massanutten Formation (graywacke, shale, limestone) have been etched away to make the Shenandoah / Page valleys, and those above it (limestone, shale) have been etched away to make the Fort Valley. In between, the hard, chemically-stable Massanutten stands proud of the adjacent recessive rocks, and makes the crest of the ridge.
Nowhere is this story more plainly stated than at Buzzard Rocks.
Yesterday, I hiked 1.5 miles up the Signal Knob trail to a higher vantage point, and shot this GigaPan from the trail:
You can see the Blue Ridge in the distance there, across the development of Front Royal (see if you can find the Randolph Macon Military Academy’s campus in the shot). You can also see the incised meanders of Passage Creek exposed in the lower foreground, and the slopes of talus leading toward them from Buzzard Rocks.
12 December 2014
Previously I’ve featured a road cut at the intersection of the Trans-Canada Highway and the Icefields Parkway as the Friday fold.
Here are some other look at cool features to be seen at this outcrop, including folds, but also including ripple marks, cross-bedding, graded beds, cleavage, and boudinage.
First, the general setting, showing bedding (both horizontal and moderately dipping) and cleavage (upright, subvertical):
Here’s a fold!
Zooming in on the most folded part:
A close look at one bed, exposed in cross-section…
…at the top are pyrite crystals:
Here’s a striking example of a graded bed:
Another primary structure that can be seen here is ripple marks, exposed on the bedding plane where it coincides with cleavage:
Boudinage deforms the stiffer, sandier-protolith layers (now quartzite transected by quartz veins):
Finally, here are a suite of GigaPan images that Aaron Barth and I shot at the site for additional exploration by you. Check out these features in dynamic context, and revel at the ensuing rush of insight:
Happy Friday, everyone.