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Fault Bend Fold Not every fault on the planet reaches the surface. In fact, most of them don’t. Many faults form at depths of several kilometers below the Earth’s surface and as they move, the rocks around them can be bent and folded without breaking.
Ductile Folding This beautiful outcrop was photographed on the island of Crete by Dr. Marli Miller. The rocks are part of the Plattenkalk series of sediments that form the backbone of that island; they include fine-grained limestones deposited in the ancient Tethys seaway alternating with more siliceous layers that include terrestrial sediments and sometimes buried sponges. These rocks have been uplifted and clearly folded as part of the collision between Africa and Eurasia that is uplifting islands in the Mediterranean.
only week 3 and my maps are already getting so much better 😍 practice really does make perfect
Anticline + Syncline
These are the two major types of folds, an anticline and a syncline. Geologists call folds that are open downward (concave downward) anticlines and folds that open upwards (concave upward) synclines.
If you find a stack of paper or a paperback book you can make these yourself, just gently fold the book into the same patterns. Unlike a book though, the layers of the Earth have an additional space problem. You can fold the book easily up and down because there is nothing containing it either on the top or the bottom. The planet Earth is different; layers in the Earth can’t stretch out or easily move into empty space like the pages of a folding book.
Instead, synclines and anticlines are often found together like you see here. When a syncline is found next to an anticline, it gets rid of this space problem – no single layer is being extremely stretched or compressed compared to any other.
You can simulate this again with a stack of paper or a book; push at the end of the paper and create folds that alternate, then look at whether or not the layers slid relative to each other.
-JBB
Image credit: USGS https://www.flickr.com/photos/usgeologicalsurvey/15199645513/
On the way back from a trip to PA we drove by this amazing road cut. I got so excited my SO stopped so we could get out and look. This is an amazing example of a synclinal fold in the basin and range Province of the Appalachians. I learned that this location is Sideling Hill and it was used as an educational site for a while until it lost funding. One of my cohort told me they went their during their sedimentology and stratigraphy field work course! I’m super jealous. There is a coal seam in their somewhere too, but it was too cloudy for me to even see the top of the structure.
I may as well link to my post on the syncline:
Monkstone point new hedges South Wales uk
Folded
This beautiful exposure of rocks is part of Bays Mountain, a mountain in the Valley and Ridge province of Tennessee. The vertical lines you see across the road cut are the remnant of dynamiting to allow the road to pass; workers drilled holes in the rock from above and placed dynamite into the holes. When they detonated the dynamite, it blasted away all the rock that used to be where the photographer was standing; creating a solid cliff that won’t collapse when a road is built next to it. The vertical lines are the remnants of the holes drilled for dynamiting.
These rocks started their life as sediments during the Ordovician. At that time, much of what is today the east coast of the United States sat underwater accumulating sediments. These layers are a mixture of carbonates and fine-grained shales, accumulated in quiet water far from the ancient coastline. These layers are part of a geologic unit known as the Sevier shale; several hundred meters of sediment much like this accumulated during this time period. A few hundred million years later, the continent of Africa came rafting in and collided with North America, uplifting the Appalachian Mountains and folding and faulting the sediments deposited off the coastlines of the two continents.
These rocks today are part of the Valley and Ridge province; if you travel through this part of the country, you’ll find long mountain ranges running north-south that are the remnant of these folded and faulted sediments.
These rocks have been warped into a syncline. They’ve been bent up on the sides, a type of fold where the youngest rocks are in the center of the fold. The opposite type of fold is an anticline; in that case, the rocks are bowed upwards in the center, so that the oldest rocks are at the center of the fold. Ironically, this syncline is forming the core of a long ridge even though it seems like the rocks have been folded downwards.
In this case, the folding happened so long ago that even though these rocks were a syncline, the folding is not controlling the topography, erosion is. The rocks at the center of the syncline are harder to erode than the rocks on the limbs, so the center of the syncline is standing up as a mountain today. Something very similar happens at the incredible Sideling Hill outcrop farther north (https://tmblr.co/Zyv2Js1yyAl37).
If you look at the image closely, you can see all sorts of fractures that have been filled with newly grown minerals. These are especially common right at the axis of the fold, where the rocks have been bent the most.
-JBB
Image credit: James St. John https://flic.kr/p/Y5jzAf
References: https://bit.ly/2N7JOdY https://on.doi.gov/2tKfmxl
Hilltops — Layers. Experiments by Monograph&Co. // Sophia Ahamed.
The official caption for this video describes this french forest as being trapped in a syncline - the type of fold formed when the outer limbs of an area are lifted up. That’s going to get shared by me just because of the term usage!
The forest of Saoû is known for being the most beautiful perched syncline in Europe. Surrounded by a thick wall of hard clay, the forest climbs to its highest point by way of a ridge pathway culminating in the famous mountain Trois Becs (1589 m); The exceptional biodiversity is due to the almost perfect closure of the syncline behind its high walls as well as the different exposures of its mountain slopes, both north and south. This phenomenon attracts geologists, geographers and other eminent experts from all over. The history of the Forest of Saoû is intimately linked to that of its neighbouring inhabitants.
The formation is the result of a compression of the horizontal layers during the tectonic pushes which raised the Pyrenees about 100 m at the time of the Cretaceous and clearly reveals the different limestone geological layers.
Folds
This mountain shows a beautiful pair of folds in Jurassic aged limestones from Switzerland. This is the south face of the Doldenhorn mountain near Gasteretal, Switzerland, so this photo is looking roughly to the north. The face has been exposed by downwards erosion of the Kander River, which runs near the base of the mountain.
These rocks were once flat-lying limestones. They have been lightly metamorphosed and heavily folded during the growth of the Alps. At this site, the sedimentary layers were literally kinked – the rocks in the middle of the folds have been tilted so much that they’re actually upside down. Geologists would call this an “overturned” limb of the fold. If you start at the uppermost layer on the left, at the kink it bends back to the left, then above the kink you actually have older rocks on top of younger rocks. Normal stratigraphy, where the older rocks are at the bottom, does not resume until the second fold axis is crossed. This photo is about half a kilometer across. It seems enormous from this frame, but this pair of folds is actually only a small part of a much larger structure in the area. These rocks sit on the edge of a dome of metamorphosed gneisses about 50 kilometers wide that was forced into this area during the growth of the Alps. The sedimentary layers on top of this large gneiss dome were bent into one huge fold, and this mountain side is a small fold pair formed during growth of these much larger structures.
A tunnel known as the Lötschberg Tunnel runs almost directly beneath this spot. The first tunnel beneath these hills was constructed in the early part of the 20th century, and suffered several major engineering disasters including both a flood and a collapse when the excavation hit an unknown channel containing loose sediments. Nearly 40 miners died in these 2 disasters. The modern tunnel, dug far below this site, opened in 2007.
-JBB
Image credit: https://commons.wikimedia.org/w…/File:Folding_Gasteretal.jpg
2017: Folding in sediments near Emerald, central Queensland
Recumbent fold
When rocks start to be compressed, sedimentary layers begin to bend, forming common fold patterns called anticlines and synclines. An anticline is a fold where the layers have been bent upwards, a syncline is a fold where the layers have been bent downwards.
If rocks continue to be squeezed, these folds can get steeper and steeper until the folds themselves start being pushed to the side. These Precambrian-Cambrian aged rocks exposed on an island in East Greenland are a recumbent fold. The folding started off more simple, but during the Caledonian Orogeny these rocks were squeezed so much that this fold rolled over fully onto its side.
At this point, you can’t simply tell by looking at this fold whether it used to be an anticline or a syncline. To figure that out, you have to go closer to the rocks and see which unit is younger – if this used to be an anticline, the youngest rocks would be at the center, while if this used to be a syncline the youngest rocks would be at the edge. Alternatively, you could find some sedimentary structure that tells you which way is up – for example, mudcracks that formed on the top of a surface and pinch out with depth could tell you which direction used to be up.
-JBB
Image credit: http://bit.ly/2sHW3Hx
Read more: http://bit.ly/2sHTghl
Structural Geology of a cookie!
Gorgeous structural geology sketches
Structural mapping of the geology at Point Dickinson (at Burmagui)
