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22 September 2011

More moki marbles

More moki marbles: little concretions in sandstone, kind of like the ones I showed you Tuesday from Illinois. But these ones are from the Navajo Sandstone, a late Triassic or early Jurassic erg deposit from the Colorado Plateau. These photos were taken in Zion National Park, near Springdale, Utah (real close to the cross-beds I featured a month ago). They are tougher than the sandstone in which they formed, and …

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21 August 2011

The best cross-bedding you’ll ever see

Setting aside the lack of scale, it really doesn’t get any better than that. Click through to make it huge. This is the Navajo Sandstone, early Jurassic (or late Triassic?) in age. It’s in Zion National Park, Utah. Wind direction was from the right towards the left, as these preserved slip-faces of ancient dunes indicate. The beds are right-side-up, and they have been differentially eroded, causing the cross-beds to jump …

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15 August 2011

Split concretion

Concentrically-zoned ironstone concretion in sandstone of the Morrison Formation, eastern flank of the Bighorn Mountains / western edge of the Powder River Basin, Wyoming, at the Sheridan College dinosaur fossil quarry, last week. The white stuff is caliche. A quick post to celebrate the fact that as of three hours ago, Lily & I are back home in our condo in D.C. It’s been a great summer, and I have …

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25 June 2011

A major discovery

Visiting Butch Dooley and crew on a dinosaur dig in the Jurassic Morrison Formation on Wednesday morning, I did a lot of wandering around the area, dubbed the Two Sisters site. I noticed something in the sandstone at the top of one hillock, and thought it looked like a sauropod footprint: (The depression is filled in with modern mud.) I took a photo, and thought, I need to ask Butch …

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18 February 2011

Friday fold: Jefferson River Canyon

Here’s your Friday fold, straight from the canyon of the Jefferson River, near Cardwell, Montana. Perspective is to the south. East is on the left; west is on the right. Bigger version This is right next to the outcrops of LaHood Conglomerate that I mentioned earlier this week. My Rockies course co-instructor Pete Berquist and I bring the students here for several reasons, including the conglomerate, nearby Lewis & Clark …

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6 December 2010

Gerede segment of the North Anatolian Fault

The author recounts a field trip in October along the section of Turkey’s North Anatolian Fault that last ruptured in 1944. The rock types on either side of the fault are compared, offset markers are illustrated, and several types of landforms particular to strike-slip faults are shown. The post concludes with an examination of the town of Gerede itself, which is built directly atop the fault.

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3 November 2010

Tavşanlı Zone field trip, part 5

After our mind-boggling encounter with the limestone strata turned lurid pink by their high-pressure encounter with subduction, our band of merry geologists set off for a stroll: We began the walk in greenschist + blueschist mélange, as seen here with a Turkish 1-lira coin (26 mm diameter; about the same size as a U.S. quarter) for scale: Then we crossed a structural contact into an ophiolitic mélange thrust sheet, a …

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21 October 2010

Tavşanlı Zone field trip, part 2

Yesterday, I shared a few thoughts about the first couple of stops on the field trip I took earlier this month from Istanbul to Ankara, prior to the Tectonic Crossroads conference. Today, we’ll pick up with some images and descriptions from the next few stops. After lunch, our next stop brought us to a relatively low-metamorphic-grade outcrop of sheared graywacke (dirty sandstone) and shale. As you can imagine, it wasn’t …

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10 March 2010

Pyrolusite on a pterosaur

All the photos I posted over the weekend here were via iPhone, and hence not particularly high-quality, despite their excellent geological content. Now I’ve downloaded the photos from my real camera, and have a few good ones to show. Here’s a succession of photos of the same specimen of Pterodactylus longirostrus, each progressively more zoomed in than the last. It’s a late Jurassic pterosaur (140 Ma) from the Solnhofen limestone …

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4 March 2010

Triassic rifting in the Capitol

My girlfriend’s mom was in town in January, and we took her down to visit the Capitol Building. The tour had a good bit of history, but definitely missed the opportunity to talk geology. I was particularly struck by the columns in the Hall of Statuary: Close up of one column, with my hand for scale: That’s the Leesburg Conglomerate, a Triassic-aged deposit found in the western part of the …

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