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11 October 2016

A virtual field trip to Portrush, Northern Ireland

One of my favorite places in Northern Ireland is the east side of the peninsula that hosts the tourist town of Portrush. There, two early schools of geological thought engaged in a battle. The opposing sides were: the Neptunists, who thought all stratified rocks, and in particular basalt, must form from precipitation from the sea, and the Plutonists, who thought some rocks, including basalt, formed through intrusion of molten rock …

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12 June 2014

Slickensides in the Vulcan Quarry, Manassas, Virginia

While at the Vulcan Quarry in Manassas, Virginia, a few weeks ago, the principle interesting feature I saw on the individual blocks of rock we sorted through was slickensides. I saw dozens and dozens of examples, in both the hornfels and the diabase, but here are five nice examples to share: No sense of scale on this last one, because I didn’t feel safe edging up to the quarry wall …

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10 June 2014

Diabase intrusion at the Vulcan Manassas quarry

When I was flying back from Phase I of “Border to Beltway” in Texas this past March, I was delighted to photograph a bunch of local geology from the air, including this prominent diabase quarry in Manassas: I had never been to this particular quarry before, but as it turned out, it was one of our final destinations on Border to Beltway’s second phase, last month. Here’s a closer view …

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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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16 August 2013

Friday fold: Sandy Hollow, Montana

Today, a view to the southwest, from close to the hinge of the main anticline in the Sandy Hollow area, a classic geological field mapping locality in southern Montana: Triassic-aged Dinwoody Formation dominates the main part of the scene. Note how the strike and dip of the positively-weathering strata wrap around. Our Rockies students mapped here this past July, trodding on hallowed ground familiar to generations of field camp students.

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12 March 2013

Five new GigaPans from Thoroughfare Gap

Yesterday, I took five new GigaPans at Thoroughfare Gap, a water gap where Broad Run cuts through Bull Run Mountain, the eastern limb of the Blue Ridge Anticlinorium at my latitude. The rocks here are the Cambrian-aged Chilhowee Group, with bedding tilted moderately to the east during Alleghanian mountain-building in the late Paleozoic. To the west is the crystalline core of this massive regional fold, and to the east is …

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27 July 2012

Friday fold: Spray Mountain Group siltstone

Today, we return to Banff National Park, to the outcrops next to the parking area for Bow River Falls… Zoomed-in closer to the thinner layers at left: These strata (shale and siltstone) were laid down in the quiet aftermath of the Permo-Triassic extinction, as terranes colliding with the edge of North America (far to the west) began shedding clastic sediment into the basin to the east. The lack of critters …

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24 July 2012

Tool casts from the Sulphur Mountain Formation of the Spray River Group, Banff

Yesterday I showed you salt casts; today I’ll share a different kind of cast: the infillings of small scorings in the sediment made by tumbling pebbles or sticks or other “tools,”tumbling down a current. These small gouges were later infilled from above by a younger deposit of sediment (frequently coarser in grain size). You’re looking at the bottom of a bed here: These are strata of the Sulphur Mountain Formation, …

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31 May 2012

Plane views: Wyoming

I wanted to pick up where I left off months ago with some airplane photos as I was flying back from my California field class (over spring break). All views are to the north (out the left side of the airplane as we flew from Reno to Minneapolis). We crossed from northern Nevada into Wyoming, and started encountering Laramide uplifts. Here is a look at bedding dipping off to the …

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15 May 2012

Diabase dike in diabase

Seen in rip-rap on the side of Naked Creek, a week ago yesterday: This boulder is exotic to its current location. It is typical of medium- and coarser-grained diabase from the Culpeper Basin, a Triassic rift valley east of the Blue Ridge. The main minerals are plagioclase (light-colored) and pyroxene (dark colored).

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