Tom Bain's Earth Insight Cache -- check out the post on carbonate "cannonball" concretions in the Ohio Shale! Very impressive... looking forward to future posts.

EDIT: The URL is http://earthinsightcache.blogspot.com/

Labels: blogs, concretions, geology

A couple days ago, I showed a photo of plumose structure here, a feature that sometimes forms when rocks fracture (i.e. a joint is formed). I invoked the image below to show the relationship between the plumose structures and the concentric "ribs" that sometimes show up on a joint (here labeled as "arrest lines"). The point was to show how they were mututally perpendicular.

But the diagram shows something else, too: that the delicate topography of the plumes becomes more exaggerated away from the main surface of the joint, and they grow into twisted "hackles" along the edge of the joint. Joints have ruffled edges! These hackle fringes can also be spotted on many rock surfaces, if you're looking for them.

Here's a photo I took a couple of weeks ago, in the Silurian Needmore Formation (exposed in the Massanutten Synclinorium between Waterlick, VA and Seven Fountains, VA). It shows a series of hackle fringes parallel to one another, showing the growth of the fracture surface over time.

Here it is again, with the Photoshop "contrast" dial turned up to 11:

The high-contrast view helps bring the hackles into high-relief, and also illuminates the subtle plumose structure. Looks like this surface formed from the top, down. As I read it, this joint started on the right side of the image and propagated leftwards as time went by.

(The hematite nodule at left is a bonus feature.)

Labels: concretions, joints, structure

Outside of Shamokin, Pennsylvania, is a coal strip mine that has had the coal stripped away. Under the coal was a Pennsylvanian (in the time sense of the word) carbonaceous shale (the Llewellyn Formation), which is now preserved in lovely undulating Appalachian folds. Thanks to the removal of the coal, these fold surfaces appear in three dimensions -- a rarity for structural geologists like myself. The area is known as "The Whaleback" because of one anticline (center) with a shape that evokes a surfacing cetacean:

I went to the Whaleback last fall on a fossil-hunting trip with the The Calvert Marine Museum Fossil Club. In today's post, I'll take a look at the structure, and in a later post, I'll show you some photos of the fossils themselves. Here's some of the guys on the trip:

At the north end of the excavation, a cross-sectional view of the absent upper levels is preserved, showing this syncline. It once continued towards the camera's perspective in the air, a downflung fold between the Whaleback anticline and the neighboring anticline which made up the background "wall" in the first photo.
Okay, that's it for today. Tune in soon for the fossiliferous sequel.

Labels: appalachians, concretions, pennsylvania, structure

Geological Travels in Northern Ireland, Part VII:

Ground moraine being used (quite appropriately) as a golf course, east of Port Rush.

An old quarry south of the road between Bushmills and Port Rush. This is easily accessible from the parking area for White Rocks, a popular surfing beach. (Yes, they surf in December in Northern Ireland!)

Well-exposed here is the unconformity between the Cretaceous-aged "Chalk" (the Ulster White Limestone) and the overlying "Lower" Basalts (Paleogene in age).

The ancient topography is revealed in the undulations of the unconformity surface: prominently featured here is an ancient valley that was topped off with basaltic lava during the eruption. Valley depth in this photo is about 80 feet.

The limestone ("Chalk") here was quarried for lime. Lime is the binding agent in cement and mortar, and it is produced from the burning of limestone. Disused kilns from the burning process were still situated in the quarry. The area was lousy with flint nodules, like the one here. I collected a beautiful one that looked like a cross between a sausage and a powdered donut, but security confiscated it from my carry-on luggage on my flight back home.

Labels: antrim coast, basalt, chalk, concretions, flint, geology, glacial landforms, northern ireland, unconformities