P. Kyle House, PhD., Surficial Geologic Mapping Specialist / Geomorphologist
The Afar Triangle is, geomorphologically speaking, truly and intriguingly pathological in its naked expression of crustal extension. Practically the birthplace of humans and soon to be the birthplace of a new arm of the sea.
More info on this amazing place: http://en.wikipedia.org/wiki/Afar_Depression
Here's a little example from my master's thesis area which shows a structural control on drainage along with the strong imprint of glacial flow. A mesozoic shear zone, about 1 kilometer in width, bounds the eastern edge of the Sierra Nevada Batholith. These sheared rocks have attained a transposition foliation which strikes roughly NNW. Due to differential weathering, some lithologies are more readily etched into lowlands, while others stand up as ridges. During the Pleistocene, alpine glaciers flowed out of the Sierras into the Owens Valley to the east, at approximately a 90° angle to the structural grain (ENE). Damming the drainage produced the body of water known as Gem Lake (37.749269 Lat, -119.150943 Long), which highlights the contours of the surrounding drainage by filling it all up with a horizontal layer of water.
Flash Earth location here:
I would love to visit this area. Not too much to say about it. I've posted on this pathological structure before, and it's quite well known. There's a Wiki page that covers the basics, but little detail. This structure is occasionally mentioned as a potential cause for the Triassic/Jurassic boundary extinction, but an abstract from Geology shows that to be unlikely.
Link: http://www.flashearth.com/?lat=51.384302&lon=-68.685925&z=10.1&r=0&src=yh
It seems most of the examples thus far of structurally controlled drainage have been in arid to semi arid areas, so I wanted to post an example from a markedly wet area. The GE image shows the glacially excavated area between Sudbury, Ontario and Killarney on the northern shore of the Georgian Bay. The geological map, from here, provides some context; at the link is a nice time line of events in the region. Of particular interest is the shift from rounded "organic" forms in the Southern Province's folded metasediments to more angular fault- and joint-controlled lakes and streams in the crystalline granites and gneisses of the Grenville Province to the east. The Grenville front can be seen from the bottom middle to the right top of the image, and the town of Sudbury and associated mining operations at the top middle.
This is a fantastic area for canoeing, backpacking, blueberries, and of course most importantly, geology.
Here is another structural control on drainage from Chile. An alluvial fan crosses the neotectonic Atacama Fault northeast of Antofagasta, Chile, completely changing the stream profiles.
The flashearth image is here but the image is higher resolution in Google Earth.
The Volcanic Tableland south of Long Valley Caldera (and north of Bishop, California) is composed of Bishop Tuff erupted about 76,000 years ago during the formation of the caldera. One pathological feature of this area is Pleistocene-aged drainage channels (it was wetter then) that flowed across the top of the tuff’s welded upper layer, but then were disrupted by faulting of the Volcanic Tableland since the Pleistocene.
There’s a lot of other stuff going on in this area. Zoom out a bit to see beautiful suites of en echelon normal faults disrupting these relatively young volcanic rocks.
This area in northern California has a locally profound expression of structural / topographic control on drainages...check the drainage near right-center for example. This pathological landscape is the result of late Cenozoic extension / transtension of the volcanic Modoc Plateau.
Tectonic control on drainage, you say? I give you the Okavango Delta in
Botswana.
More pathological structural control on drainage, this time streams following parallel fractures in the ignimbrites, north of the Salar de Atacama in northern Chile near Lagunita.
Our Dear Leader has set a February challenge of pathological examples of structurally controlled fluvial systems. This image is from Angola, an area known as the Moxico and Cuando Cubango. North of the Cuando River the drainage is in straight, parallel channels flowing south-southeast, where as south of the main channel the drainage is parallel but almost due east.
Many drainages are clearly controlled by underlying bedrock structure. This example from China in the middle of this image is a nearly pathologically perfect example of a fault-controlled river course. It stands in considerable contrast to the smaller drainages to the south (in particular).
Especially for Callan, here is a rock glacier. This is the Marinet Rock Glacier in the Alpes Maritimes of France. The formation of these features is controversial. Some argue that they are permafrost features, others that they are a debris rich ice glaciers whose ice core has substantially melted away leaving mostly rock that moves slowly downhill, probably on some basal ice. This is probably an example of one of the latter.
The flashearth image is here and more information on rock glaciers can be found here.
I see Kyle's Russian thermokarst example and raise him thermokarst plus volcanoes. According to the Smithsonian's Global Volcanism Program: "The Ingakslugwat Hills consist of at least 32 small cinder cones and 8 larger craters covering an area of more than 500 sq km in the Yukon-Kuskokwin delta area of SW Alaska. Numerous small spatter cones and cinder cones of the Ingakslugwat Hills range from 8 to 190 m in height; many are concentrated in the NW side of the volcanic field." 61.43° -164.47° Flash Earth image: http://www.flashearth.com/?lat=61.526696&lon=-164.750237&z=10.5&r=0&src=msa
Not all glaciers are ice. The anticlines of the Zagros fold and thrust belt in Iran are cored with salt diapirs that, when they breach the surface, the salt flows into the valleys between the folds under gravity forming salt glaciers. Salt glaciers move more in winter when it is wetter and have a dark colour from clay inclusions.
Few landscapes on Earth are more pathological than those dominated by thermokarst...some may even call low-hanging fruit on this one, but it is impossible to resist the juxtaposition of a wildly meandering alluvial river and an endless array of thermokarst lakes.
The circular features are Precambrian peralkaline granitic intrusions of the Aïr Mountains of Niger. The intrusion centre-left is bisected by a spectacular circular fracture. The intrusions appear to have a concentric compositional zoning possibly caused by a ballooning emplacement mechanism.
The flashearth image is here.
Sideling Hill, Maryland (39.719205 Lat, -78.283193 Long) is a ridge in the Appalachian Mountains' Valley & Ridge Province. Held up by erosion-resistant early-Mississippian-aged siliclastic strata, it was a barrier to human travel for many years. The hairpin turn in the road traversing Sideling Hill was the site of many accidents. In 1983, a new route was carved through the mountain, resulting in a striking 'notch' that hosts Interstate 68.
This road cut reveals the internal structure of Sideling Hill to be a large syncline. More information about the geology of Sideling Hill can be viewed here: