Tag Archives: Field Trips

Capitol Rock Close-Up

Close-up outcrop photograph of Capitol Rock, viewed from the north-northeast.  Image credit: Bill Mitchell (CC-BY).
Close-up outcrop photograph of Capitol Rock, viewed from the north-northeast. Image credit: Bill Mitchell (CC-BY).

Two years ago, I came tantalizingly close to Capitol Rock, an outcrop in southeastern Montana (45.572189, -104.087964) just a few miles over the border from Camp Crook, SD. Unfortunately, I did not have an opportunity at that time to explore the outcrop from any closer than about a quarter mile, but I did find the Ekalaka Quadrangle 30’x60′ (pdf) geologic map.

Recently, I was out in the area again, and this time made sure to have time to take some pictures and see some of what was to be seen. Let’s start with the quarter-mile view, which is roughly equivalent to what I saw last year.

Wide view of Capitol Rock from the east.  Image credit: Bill Mitchell (CC-BY).
Wide view of Capitol Rock from the east. Image credit: Bill Mitchell (CC-BY).

Capitol Rock has three major parts to it: an easily eroded base, a laminated sandstone middle, and a massive sandstone top. A handy turn-out from the forest service road leads right to the base of the outcrop.

The easily eroded base is made of fine, chalky, white sediment sediment, and it remains in horizontal orientation. In several places, this unit is at least superficially porous. Surprisingly, there are occasional chert clasts in the otherwise fine sediments—I’m not quite sure how those would have been deposited or formed here.

Basal unit of Capitol Rock.  Foot for scale.  Image credit: Bill Mitchell (CC-BY).
Basal unit of Capitol Rock. Foot for scale. Image credit: Bill Mitchell (CC-BY).
Cherty clast in the basal sediments at Capitol Rock.  Foot for scale.  Image credit: Bill Mitchell (CC-BY).
Chert clast embedded in the basal sediments at Capitol Rock. Foot for scale. Image credit: Bill Mitchell (CC-BY).

Above the basal unit is a somewhat more resistant, coarser-grained set of beds. These strata are finely bedded, and have a tendency toward spheroidal weathering. Occasionally interbedded with the spheroidal beds are 1–3 cm thick, well-cemented strata of a white or pink color [discoloration?].

Spheroidal weathering of finely-laminated strata.  Hand for scale.  Image credit: Bill Mitchell (CC-BY).
Spheroidal weathering of finely-laminated strata. Hand for scale. Image credit: Bill Mitchell (CC-BY).
Laminations in the unit displaying spheroidal weathering.  Hand for scale.  Image credit: Bill Mitchell (CC-BY).
Laminations in the unit displaying spheroidal weathering. Hand for scale. Image credit: Bill Mitchell (CC-BY).
Non-spheroidal bed 1–3 cm thick and slightly orange-pink in coloration, within the spheroidal beds at Capitol Rock.  Hand for scale.  Image credit: Bill Mitchell (CC-BY).
Non-spheroidal bed 1–3 cm thick and slightly orange-pink in coloration, within the spheroidal beds at Capitol Rock. Hand for scale. Image credit: Bill Mitchell (CC-BY).

The spheroidally-weathered unit also seems to have one or more channels within it.

Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below).  Possible channel cut at right.  Outcrop height in image is ~10 m.  Image credit: Bill Mitchell (CC-BY).
Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below). Possible channel cut at right. Outcrop height in image is ~10 m. Image credit: Bill Mitchell (CC-BY).
Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below).  Possible channel cut at right has been annotated.  Outcrop height in image is ~10 m.  Image credit: Bill Mitchell (CC-BY).
Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below). Possible channel cut at right has been annotated. Outcrop height in image is ~10 m. Image credit: Bill Mitchell (CC-BY).

The upper unit at Capitol Rock has more massive sandstone (see wide view above). I didn’t notice many channels in this unit, although I didn’t get very close. A butte just to the north of Capitol Rock provided a good photograph (below).

Massive unit of Capitol Rock, seen in the butte immediately to the north of Capitol Rock.  Cliff is ~30–40 m tall.  Image credit: Bill Mitchell (CC-BY).
Massive unit of Capitol Rock, seen in the butte immediately to the north of Capitol Rock. Cliff is ~30–40 m tall. Image credit: Bill Mitchell (CC-BY).

Although I have those observations, I don’t have much for interpretation of them. The depositional environment seems to be relatively low-energy (give or take the chert clasts), evidenced by the flat strata, fine grain sizes, and relatively few cross-beds. Changes in the rock types would suggest changes in the sediment sources or the depositional environment (or both). There may be post-deposition alteration effects as well, such as cementation of the spheroidally-weathering strata.

View SSE from the butte just north of Capitol Rock.  Truck for scale in pull-out near Capitol Rock.  Image credit: Bill Mitchell (CC-BY).
View SSE from the butte just north of Capitol Rock. Truck for scale in pull-out near Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Capitol Rock is an interesting outcrop, and if you’re in the area, I’d recommend a stop. The rocks are interesting, there are US Forest Service campgrounds nearby, and the view is quite nice. These units can probably be correlated to the Slim Buttes in South Dakota (~45 miles east).

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Nares Glacier to Mount Drygalski Panorama

Panorama from Nares Glacier (left) to Mt. Drygalski (right) from the Atlas Cove camp.  This view spans from ESE through SSW, and is roughly 85 megapixels at full size.  Image credit: Bill Mitchell (CC-BY, hosted on flickr).
Panorama from Nares Glacier (left) to Mt. Drygalski (right) from the Atlas Cove camp. This view spans from ESE through SSW, and is roughly 85 megapixels at full size (28 MB). Image credit: Bill Mitchell (CC-BY, hosted on flickr).

One year ago, I was on Heard Island. Over the course of the expedition I took more than 6000 photos. Although I took three images with the Gigapan (Big Ben, the Azorella Peninsula, and—my favorite—Windy City), I also took photos for stitching together manually using my own camera.

I have been slow in stitching these pictures together, but with the one-year anniversary of their production coming around, I decided it was time to finish one or two of them. This is the first, and I hope I’ll find time to finish more. Putting it together, I was amazed that this is still a relatively wide-angle compared to what I had available: 70 mm on a 70–200 mm lens. The detail came out well, as is evident at full-size. The glaciers, moraines, and hills are all more than a kilometer distant over the “nullarbor”, a broad, flat, volcanic-sand plain.

Toward the left half of the image are some penguins for scale. They look like king penguins, putting their height around 1 meter. I count at least 31 penguins in the entire image.

I think I spot some of the relatively rare basement limestones cropping out at the very left edge of the image, and their appearance is consistent with a dip of 25–35° to the south.[1] A closer view (200 mm focal length) shows them more clearly.

Location of limestone, with annotation.  Image credit: Bill Mitchell (CC-BY).
Location of limestone, with annotation. Image credit: Bill Mitchell (CC-BY).
Full-zoom on the limestone outcrop.  Bedding is clearly visible, dipping south.  Image credit: Bill Mitchell (CC-BY).
Full-zoom on the limestone outcrop. Bedding is clearly visible, dipping south. Image credit: Bill Mitchell (CC-BY).

[1] Quilty, P. G. & Wheller, G. 2000; Heard Island and The McDonald Islands: a Window into the Kerguelen Plateau. Papers and Proceedings of the Royal Society of Tasmania. 133 (2), 1–12.

Heard Island’s Most Spectacular Outcrop

Head-on view of a block of Drygalski Formation (mixed volcanics and glacial sediments, here glacial sediments with volcanic clasts).  Image credit: Bill Mitchell (CC-BY).
Head-on view of a block of Drygalski Formation (mixed volcanics and glacial sediments, here glacial sediments with volcanic clasts). 53° 01.927′ S, 73° 23.704′ E. Image credit: Bill Mitchell (CC-BY).

Heard Island is home to a spectacular outcrop. It’s the coolest outcrop I’ve ever seen, besting the Bishop Tuff tablelands, the potholes along the St. Croix River at Taylor’s Falls, Zion Canyon, The Badlands, and various outcrops in Yellowstone and Grand Teton. Admittedly this outcrop doesn’t intrinsically have the scale of many of the others just mentioned—it’s a roughly car-sized block—but the power that went into creating it and the effect it created is truly amazing.

On its face (see above), it looks quite pedestrian: a block of lithified glacial till with clasts of vesicular basalt reaching up to grapefruit size. However, it’s important to consider it from a different perspective.

Side view of a block of Drygalski Formation.  From this view, it is much easier to see this is a ventifact (carved by the wind).  There is a pile of sand on leeward (left) side. Image credit: Bill Mitchell (CC-BY).
Side view of a block of Drygalski Formation. From this view, it is much easier to see this is a ventifact (carved by the wind). There is a pile of sand on leeward (left) side.
Image credit: Bill Mitchell (CC-BY).

When viewed from the side, a pile of sand in on the leeward (left, east) side of the block is evident. Additionally, the basaltic clasts of the rock face seem to be protecting the softer, tan-colored glacial matrix from the sand-blasting.

Here’s a close-up from an oblique angle:

Close-up, oblique view of the outcrop face.  Here the differential weathering (resistant grey clasts, weak tan matrix) is very apparent.  Spires of matrix are left to the leeward of the clasts, and are roughly horizontal. Image credit: Bill Mitchell (CC-BY).
Close-up, oblique view of the outcrop face. Here the differential weathering (resistant grey clasts, weak tan matrix) is very apparent. Spires of matrix are left to the leeward of the clasts, and are roughly horizontal.
Image credit: Bill Mitchell (CC-BY).

In the oblique view, the volcanic clasts making up the face of the outcrop are seen sheltering the matrix directly to the leeward from mechanical erosion. To tie all of these views together, I took a short video (embedded below).

This outcrop is located on the edge of a volcanic sand plain roughly 1.5×1.5 km. Strong westerly winds are present most of the time (9 m/s is average, measured at a site nearby).[1] In fact, the audio which accompanies the video is mostly wind noise, though there’s a little unintelligible chatter with my field partner, Carlos. Winds when the recording was made were “moderate” (for Heard Island) and from the west, exactly the kind of winds that shaped this outcrop. At the time of the recording, the winds weren’t strong enough to kick up much sand, nor were ice pellets falling from the sky, but on a gustier, stormier day, the face of this outcrop will take a beating.

Looking toward the ventifact outcrop from Windy City, Heard Island.  Although the outcrop itself is hidden behind the small reddish rise at center, this image illustrates the expanse of vegetation-free sand plain. Image credit: Bill Mitchell (CC-BY).
Looking toward the ventifact outcrop from Windy City, Heard Island. Although the outcrop itself is hidden behind the small reddish rise at center, this image illustrates the expanse of vegetation-free sand plain.
Image credit: Bill Mitchell (CC-BY).

In my travels and geo-adventures, I’ve seen differential weathering and ventifacts (outcrops shaped by wind), but never so strikingly combined as at this outcrop on Heard Island. That’s why I can confidently say it’s the coolest outcrop I’ve seen on Heard Island or anywhere else in the world.

[1] Thost, D., Allison, I. “The climate of Heard Island” in Heard Island: Southern Ocean Sentinel, ed by K. Green and E. Woehler. Surrey Beatty & Sons, Chipping Norton 2005, p. 52-68.

Pictures from the Field

Standing just outside the tents at Atlas Cove, Heard Island, on a clear evening.  Note that there is no incandescence from lava on Mawson Peak.  Image credit: Adam Brown.
Standing just outside the tents at Atlas Cove, Heard Island, on a clear evening. Note that there is no incandescence from lava on Mawson Peak. Image credit: Adam Brown.

A few days have gone by, and they have been busy! We’ve been fortunate in that when the weather has been poor, the radio propagation has been good. A fair bit of windy, drizzly weather has been present this week, and we have managed to make more than 50,000 contacts with stations all around the world.

Unfortunately, the weather has meant I haven’t had the opportunity to take more gigapans. I am prepared for wet weather, and this morning I went a few hundred meters across the lake which had formed in front of camp (ankle deep) to Wharf Point, the point inside Atlas Cove. There on the cobbles lining the beach I did a stationary count of the birds in the hummocks nearby, on the water, and along the beach. It took about 10 minutes, and I managed to get the list recorded in a weatherproof notebook for upload later. Getting out of the tent and away from things for a while was a welcome change.

Inside the operating tent are many tables with radio equipment.  We have six stations set up, two of which are outside the frame to the left.  The galley is just barely showing on the right, and I'm standing in the front door.  The sleeping tent is through a little hallway.  From left to right, by leftmost extent of the head, we have Adam, Dave Lloyd, Jim, Vadym, Ken, Arliss, and Hans-Peter.  Image credit: Bill Mitchell (CC-BY)
Inside the operating tent are many tables with radio equipment. We have six stations set up, two of which are outside the frame to the left. The galley is just barely showing on the right, and I’m standing in the front door. The sleeping tent is through a little hallway. From left to right, by leftmost extent of the head, we have Adam, Dave Lloyd, Jim, Vadym, Ken, Arliss, and Hans-Peter. Image credit: Bill Mitchell (CC-BY).
The sleeping tent, which sleeps 14.  Although there are windows, they are kept shuttered all day.  It's a good place to sleep, but not particularly warm.  Image credit: Bill Mitchell (CC-BY).
The sleeping tent, which sleeps 14. Although there are windows, they are kept shuttered all day. It’s a good place to sleep, but not particularly warm. Image credit: Bill Mitchell (CC-BY).

One thing which has been abundantly clear on this expedition is that if you want to do something that depends on the weather, be prepared to do it. The weather can shift very rapidly (especially if it’s permissive weather), so “I’ll just wait until later” often won’t cut it. If you see Big Ben and want a photograph of it, get your camera and shoot. There may not be another chance. This evening I didn’t immediately take a picture when there was a clear, starry sky. I at least saw the starry sky, but did not get the photograph. With only a bit more than a week to go, I hope I can still get that picture.

In the afternoon a few days ago, the weather cleared enough to get a view of Mawson Peak atop Big Ben. I quickly grabbed the camera, put on the telephoto lens, and got a few pictures of the summit. Indeed, there was a small plume indicating (at minimum) hydrothermal activity or venting, but possibly a small active lava flow.

Mawson Peak with a small plume indicating volcanic activity.  Image credit: Bill Mitchell (CC-BY).
Mawson Peak with a small plume indicating volcanic activity. Image credit: Bill Mitchell (CC-BY).

Heard Island’s mood changes with the weather, and the effect that has on the landscape can be quite striking. The picture at the top of this post and the one immediately below are taken in pretty similar places looking in similar directions. What a difference the weather makes!

Antenna Lake, Atlas Cove, Heard Island.  Rain fell fast enough to flood much of the low-lying volcanic sand plain near our camp.  We were glad not to have camped there, and the antennas still worked.  It looks quite other-worldly, with the dark, broken lava flows and fog concealing the mountain.  Image credit: Bill Mitchell (CC-BY).
Antenna Lake, Atlas Cove, Heard Island. Rain fell fast enough to flood much of the low-lying volcanic sand plain near our camp. We were glad not to have camped there, and the antennas still worked. It looks quite other-worldly, with the dark, broken lava flows and fog concealing the mountain. Image credit: Bill Mitchell (CC-BY).
Camp seen on a rainy, dreary day typical of Heard Island.  Image credit: Bill Mitchell (CC-BY).
Camp seen on a rainy, dreary day typical of Heard Island. Image credit: Bill Mitchell (CC-BY).

Finally, the king penguins make tracks as they walk around on the wet sandy ground.

King penguin tracks in the sand of the nullarbor, Heard Island.  Each track is roughly 8 cm in length.  Image credit: Bill Mitchell (CC-BY).
King penguin tracks in the sand of the nullarbor, Heard Island. Each track is roughly 8 cm in length. Image credit: Bill Mitchell (CC-BY).

Split Rock Anorthosite

Looking SW from Split Rock Point (a large anorthosite block).  Note the gentle dip of the rocks toward Lake Superior.  Image credit: Bill Mitchell (CC-BY).
Looking SW from Split Rock Point (a large anorthosite block). Note the gentle dip of the rocks toward Lake Superior. Image credit: Bill Mitchell (CC-BY).

This summer, I took a field trip up to Split Rock State Park in northern Minnesota, along the north shore of Lake Superior. While I wrote a little bit about the trip, there is still a bit more to be said and shown.

Part of what makes Split Rock interesting, besides a picturesque lighthouse which I didn’t take many pictures of, is the large blocks of anorthosite. Anorthosite is a rock formed primarily of the mineral anorthite, which is a calcium-rich feldspar, and the mineral zircon—used in U/Pb dating—can be found in anorthosite as well.[1] Its appearance is generally light grey or whitish, and has relatively coarse grains (mm to cm).

Anorthosite is an intrusive igneous rock formed through the crystallization and accumulation of anorthite within a magma body. It is abundant on the Moon, and lunar anorthosites are believed to have accumulated on top of a magma ocean early in lunar history. A relatively dense magma will act as a heavy liquid, and cause the less dense anorthite to float, separating the original magma from the crystallized anorthite. These types of crystallization processes, where the magma becomes separated from crystals it produces, are called fractional crystallization, and can cause the resulting magma to be enriched in some elements or components (such as SiO2). Even with massive basalt flows, fractional crystallization can cause an occasional rhyolite flow as well, but I’ll leave discussion of the rhyolites of the North Shore for another day.

Pictured above is the view from Corundum Point, a large block of anorthosite at Split Rock State Park. Below is a close-up view of some of the anorthosite, as well as a benchmark which has been placed in the anorthosite block [Thanks to Jessica Ball (@tuff_cookie) for giving me the idea of photographing the benchmark]. Despite being far from the ocean, Minnesota is home to National Ocean Survey benchmarks.

Anorthosite with survey point, Split Rock State Park, MN.  Image credit: Bill Mitchell (CC-BY).
Anorthosite with survey point, Split Rock State Park, MN. Image credit: Bill Mitchell (CC-BY).

The name Corundum Point suggests the presence of corundum—a mineral used in abrasives—and it comes from a mining operation on the site in the early 1900s. However, the point is actually anorthosite, which was much less useful for abrasives. Between the incorrect mineral identification and a fire which burned down the crushing house, the operation was eventually shuttered.

***

[1] Mark D. Schmitz, Samuel A Bowring, Trevor R Ireland, “Evaluation of Duluth Complex anorthositic series (AS3) zircon as a U-Pb geochronological standard: new high-precision isotope dilution thermal ionization mass spectrometry results” Geochimica et Cosmochimica Acta (2003), 67, p. 3665–3672. DOI: 10.1016/S0016-7037(03)00200-X

Update Upon further study, it appears that the naming convention of Split Rock State Park is to call this point Corundum Point. However, Google Maps displays this point as Split Rock Point, with Corundum Point a few hundred meters to the northeast. Regardless of the arbitrary common name, the benchmarks are on the point to the southwest.

Various Interesting Articles

Thin section photomicrograph of a gabbro, (crossed polarizing filters).  Image credit: Siim Sepp (CC-BY-SA).
Thin section photomicrograph of a gabbro, (crossed polarizing filters). Image credit: Siim Sepp (CC-BY-SA).

There have been a couple of interesting articles I’ve come across recently, which are worth mentioning.

First, Emily Lakdawalla has an excellent summary of the Pluto discoveries from both the American Geophysical Union’s Fall Meeting and the [NASA] Division of Planetary Science meeting. There’s a lot of new stuff there, and it’s pretty exciting.

Second, the Joides Resolution blog (the Joides Resolution is an ocean sediment coring vessel) has a series of posts (1, 2, 3) on geologic thin sections. Not surprisingly, the thin sections pictured are from rocks such as gabbros or sheeted dikes, which are expected in oceanic crust and in ophiolites (oceanic crust exposed on land). There’s a great exposure of the Coast Range Ophiolite just west of Patterson, CA, in Del Puerto Canyon, which is described in a recent blog post by Garry Hayes.

Third, Dave Petley has a great post on The Landslide Blog about the recent landslide in Shenzhen, China. I find landslides fascinating, and always learn something when I read The Landslide Blog.