Yesterday, I asked you to figure out what I was getting at here:


The answer is that I was trying to depict the fundamental difference between the two different classes of glacial landforms by showing the two different actions glaciers can take on rock: either they can carve it up, or they can carry it off a ways and dump it.


Glacial landforms may be broadly grouped into erosional landforms (like cirques, aretes, horns, and hanging valleys) and depositional landforms (such as moraines, eskers, drumlins, and kettles). Erosional landforms dominate in areas of alpine glaciation (like, say, the Patagonian Andes). Depositional landforms dominate in areas of continental glaciation (like, say, Wisconsin).

If any educators want a full-size (i.e. PowerPoint-ready) version of this image, shoot me an e-mail. I'll send you one, and I won't do so at a glacial pace.

Nobody really guessed it. But there were some great guesses regardless, and perhaps the big lesson is that the Analogy Is In The Eye Of The Beholder. Thanks to all who contributed ideas to the discussion!

Labels: analogies, art, contest, geomorphology, glacial landforms, glaciation, graphics, patagonia, wisconsin

My post on Silver Spring geologizing caught the eye of NOVA Geoblog reader and geocacher James R., who was inspired to initiate a new geocache site on Northwest Branch. You can check out James' work here. ...And you can check out the rocks by visiting the site in person!

Labels: geomorphology, maryland, piedmont, rivers, websites

Our third Rockies training hike took place Saturday. Six of us hiked White Oak Canyon, in Shenandoah National Park. It was about an eight mile loop, with six big waterfalls on it. There were a lot of plungepools where other hikers were swimming.

There wasn't an astounding amount of geology on the trail: it was mostly Catoctin Formation, with a few outcrops of underlying Grenvillian granitoids. A few nice amygdules; no columns.

The waterfalls sure were purty, though. Here's Jason at the uppermost falls (86 feet tall):


Me departing from one of the lower falls:

Photo by Chris McMahon

I got home tired and sore from this hike -- it was a good time, but I slept well last night as a result!

Labels: blue ridge, falls, geomorphology, national parks, proterozoic, shenandoah, virginia

Wow... I can't believe it's taken me so long to process through my Billy Goat Trail photos from a month ago! Here's the final batch: a collection of images of potholes. Here's a typical pothole in metagraywacke of the Mather Gorge Formation:



Differential etching of the mica-rich and quartz-rich layers suggests that sand or silt is responsible for much of the carving of potholes: picture a liquid tornado with suspended grit, focusing abrasion on a specific area of the sub-river bedrock. Later, the brittle fins of quartz may be snapped off by shearing stresses when larger clasts smash into them, such as the pebbles and cobbles seen above.

Here's a trio of itty-bitty potholes in migmatite:


And, lastly, a nice waterfall-carved (now dry) series of chutes and plunge-pools, again carved into migmatitic metagraywacke:


...And a more zoomed-in shot to give a better sense of the complicated topography here:


I'm heading out on the Billy Goat Trail again today, and also Thursday and also Friday... a busy week of field-tripping. Hope you can make it outside too!

Labels: geomorphology, hydrodynamics, rivers, sediment