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7 December 2018

Welcome to D.C. for #AGU2018

A summary of resources to learn about the geology of Washington, D.C. and the surrounding region, in anticipation of AGU’s Fall Meeting being held in the nation’s capital city.

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13 October 2017

Friday fold: ptygmatic vein from Finland (NMNH)

Happy Friday! Here’s a sample from the Smithsonian Institution’s National Museum of Natural History in DC: As the label says, we have a nice example of ptygmatic (“intestine like”) buckle folding here. It comes from Finland. The coarse equigranular crystals in the vein appear to be mainly potassium feldspar and quartz. The surrounding matrix has a pronounced foliation. Note the cuspate “flames” of matrix between the broad “lobes” of folded …

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24 April 2017

The bizarre world wherein we march for science

I marched on Saturday. In spite of the congested conditions in both the local atmosphere and my sinuses, I felt compelled to add my voice and presence to the March for Science, an event that was probably the first of its kind since the Enlightenment, aiming to push back against anti-science attitudes from the current occupant of the White House and his contemporaries on Capitol Hill. I tried to keep …

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2 November 2015

Norman Bowen’s papers

I got a special treat the week before last – one of my students this semester works at the Geophysical Laboratory of the Carnegie Institution in Washington, DC. During our unit on igneous rocks, he was prompted by the “Bowen’s reaction series” discussion to let me know that Norman Bowen’s notebooks were still extant at the Carnegie. He invited me down to check them out, and I readily accepted. Here’s …

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3 August 2015

The Sykesville Formation, in 6 new GigaPans

As part of my work on the GEODE project, I’m always looking for good imagery to teach key concepts in geoscience. One important concept that I’ve been thinking about lately is the principle of relative dating on the basis of inclusions. Just as you can’t bake a loaf of raisin bread without already having raisins in your kitchen, rock units that are included in another rock unit must be older …

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4 December 2013

The Smithsonian Castle and the Seneca Quarry, by Garrett Peck

The Smithsonian Castle is one of the most striking buildings on the National Mall in Washington, DC. One reason for this is its distinctive architecture, but a second reason is its color: a bright, deep red. This color comes from the rock from which the Castle constructed: the Triassic-aged “Seneca Sandstone,” a part of the formation technically known as the Manassas Sandstone. It is overlain by the Balls Bluff Siltstone, …

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5 June 2013

Stone tools of the Piney Branch quarry, DC

Archaeology meets geology in this visit to the Piney Branch valley of Rock Creek Park in Washington, D.C. Cretaceous deposits of cobbles of Cambrian quartzite were quarried by Native Americans and modified into tools thanks to the fact that they break with a conchoidal fracture.

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8 May 2013

A comparative look at the Klingle Valley outcrop

A week ago, I featured six new GigaPans here, of the extraordinary rocks in Klingle Valley, DC. I hadn’t been able to get the next phase of imaging that site ready in time for the post, but here it is: an annotated view of the outcrop. Annotation color code: PINK = Granite contact BLUE = Sericite after staurolite pseudomorphs YELLOW = Outlines of stretched clasts within the metaconglomerate GREEN = …

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30 April 2013

Strained metaconglomerate in Klingle Valley, DC

Following on yesterday’s post about the kink bands within the strained metagraywacke of the Laurel Formation in DC, let’s take the opportunity today to go to Klingle Valley, site of a different facies within the Laurel Formation: a strained metaconglomerate. Though the exposure isn’t as great as the Purgatory Conglomerate, I think you’ll find plenty to hold your attention in these rocks. Close looks will reveal sericite-after-staurolite pseudomorphs (evidence of …

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29 April 2013

Kink bands in highly strained Laurel Formation, Rock Creek Shear Zone, DC

Last week before GSW, I spent several pollen-choked hours in Rock Creek Park, GigaPanning some of the rocks of the Rock Creek Shear Zone. Here are some exposures in the bed of Broad Branch that show lovely kink banding. In at least one spot, you can see a conjugate pair, so these rocks were (1) sheared out in a ductile shear zone, producing the foliation, and then (2) compressed under …

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