You are browsing the archive for unconformities Archives - Mountain Beltway.
3 September 2019
An unconformity at Bacon Cove, Newfoundland
At Bacon Cove in eastern Newfoundland, there is a nice example of an angular unconformity between Ediacaran and Cambrian sedimentary rocks.
31 May 2019
Friday fold: Point Arena, California
It’s Friday, and I’m grateful that Bret Leslie of the NRC has stepped up with a Friday fold contribution from coastal California: Bret says this outcrop is: from my trip last month to the Sonoma and Mendocino coast. The first is the cliff below the Pt. Arena lighthouse. The marine deposits (now a marine terrace) are unconformably on top of the folded Miocene sedimentary rocks in the second photo. Here …
25 June 2018
Mafic volcanics atop felsic instrusives: Sonora Pass, California
Sonora Pass, California, is a lovely place to examine a volcanic-on-plutonic nonconformity that spans about 80 million years of missing time. Let’s check it out on a photo-rich virtual field trip!
6 October 2016
Oddball Icelandic rocks, part I: A green ignimbrite
Iceland does basalt really, really well. But there are a few non-basaltic igneous rocks to be found there, too. One of them is a green ignimbrite (pyroclastic conglomerate) that crops out in coastal Berufjörður, eastern Iceland. Check it out!
30 August 2016
Northwest Highlands unconformities (3 of 3): Sub-Ardvreck
It’s time to cover the third and final unconformity I observed this summer in the North-West Highlands of Scotland: the unconformity between the Neoproterozoic Torridonian Group below and the Cambrian Ardvreck Group above. Where I saw it, south of Loch Assynt on the mountain called Beinn Garbh (north of Canisp), it actually is displayed alongside the sub-Torridonian unconformity. The mountain hosts a “double unconformity”! Here is a view, looking south: …
22 August 2016
Northwest Highlands unconformities (2 of 3): Sub-Torridonian Group
Let’s now profile the next unconformity in the sequence of pulses of erosion and deposition recorded in the North-West Highlands of Scotland. For a reminder, here’s a cartoon cross-section through the four relevant units: Today, we’ll look at the contact between the Lewisian Gneiss and the Torridonian Group (Diabeg Formation), marked with a “2” on the cartoon above, where it is exposed along the shore of Loch Assynt. There are …
16 August 2016
Northwest Highlands unconformities (1 of 3): Sub-Stoer Group
First in a series profiling the three unconformities to be found in the North-West Highlands of Scotland. Today: the sub-Stoer unconformity as exposed at Clachtoll. Explore a Proterozoic buried topography topped with coarse, angular breccia.
26 February 2016
Friday fold: Siccar Point video from BGS
The British Geological Survey just came out with a new video on Siccar Point, featuring some excellent drone video of the site (in very good weather!). In addition to the unconformity, one of the things you will appreciate about the video is an excellent end-on view of a plunging synform exposed just above waterline: You’ll get a much better sense of its shape by enjoying the motion of the drone’s …
7 October 2015
The New Market / Lincolnshire contact
Yesterday, I put a photo up here on the blog, and asked you to figure out where the formational contact was in that image. Here’s the image I showed you: It turns out that my plan to have readers upload their copies of the image didn’t work as well as I had planned – apparently you all don’t have as complete a suite of control options as I do. Shocker! …
28 May 2014
B2B Coastal Plain trip
Last week, the “Border to Beltway” field exchange team went to Dora Kelly Park in Alexandria, Virginia. There, a ravine reveals the boundary between the Coastal Plain and the underlying metamorphic rocks of the Piedmont… My NOVA colleague Ken Rasmussen joined us for the day, leading us deep below the soil profile to examine the basal nonconformity beneath our youngest geologic province: The ravine was dark, so the next few …
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.