Stories of Other Worlds
How to Build a Landscape: Glacial, Shoreline, and Karst Processes--Text
Earth Science Essentials
by Russ Colson
Lecture Recap
Compare the two pictures from the lecture below, the first of a pond in northwest South Dakota and the second of two ponds in northcentral Kansas
Consider the picture below from the Salt Lake City airport.
Glacial Features and Processes:
Consider the two pictures of mountains below, one from the Colorado Rockies and one from the Appalachians in Tennessee and North Carolina. Can you recognize any differences in the type of landscape? Try to describe one or two of the differences before continuing.
Smoky Mountains TN and NC on left and St. Mary Lake in Glacier National Park, MT on right
There are at least two differences: the landscape from the Rockies is not drained, that is, there is a lake where water can't go continuously downhill to the sea, whereas the gullies and valleys in the Smokies are dendritic (branching) and go continuously downward.
Also, notice the pointy-topped mountains in the Rockies compared to the more rounded mountains in the Appalachians.
These are two common differences between a landscape carved by running water (the Smoky Mountains) and one carved by glaciers (Glacier NP). Glaciers don't erode or deposit sediment relative to a base level and so the landscape is not well drained, whereas rivers must go down continuously in order to flow, so landscapes carved by running water will be well drained.
Glaciers tend to make sharp peaks because of the way that glacial ice scrapes the rock as it flows down the slopes of the mountains.
As ice flows down the side of a mountain, it forms a cirque, a steep-walled cliff with a basin at the bottom, as shown by the drawings below. If the basin fills with water after the glacier melts, it's called a tarn lake.
Below is a picture of a cirque as viewed from above with a tarn lake in the basin at the bottom.
Near Evans Peak, Colorado Rockies.
A feature with a cirque on three or four sides is called a horn. The mountain seen across St. Mary Lake in the picture above is a horn.
A narrow ridge with cirques on both sides is called an arête.
To be called a glacier, ice must be moving. As it moves, it carries sediment which is deposited at the toe of the glacier where the ice is melting.
If the toe of the glacier remains in one place for a time (meaning that the rate of ice moving is balanced by the rate of melting), the sediment will accumulate in a moraine. Take note that the ice does not push the sediment in front of it like a bulldozer, but rather the sediment is carried within the ice and deposited where the ice melts.
A moraine at the end of the glacier is called a terminal moraine. If it's at the side of the glacier it's called a lateral moraine.
If the sediment occurs as a band within the glacier, it's called a medial moraine.
Below is a picture of a moraine in Glacier National Park MT (the glacier is no longer at this location).
Hill of sediment deposited at the edge of a former glacier, Glacier NP.
As a cirque glacier is in retreat (meaning that it is melting faster than new snow accumulates so that the end of the glacier retreats upslope), it can leave a series of moraines that create dams that make a series of lakes arranged in a line. These are called paternoster lakes.
Paternoster lakes near Bear Tooth Pass between Wyoming and Montana.
We learned that running water forms 'V'-shaped valleys in the mountains. Glacial valleys tend to be 'U' shaped. Below is a U-shaped glacial valley on the coast of Alaska. There is a second clue that this valley is glacial rather than river, related to base level. Before continuing, can you explain this?
Glacial 'U'-shaped valley, near Skagway, Alaska.
The valley is not eroded down to base level (the sea) as erosion by a river would tend to do. This kind of valley is called a hanging valley because it is hanging above its base level. Hanging valleys can produce spectacular waterfalls in the mountains, like the waterfalls of Yosemite Valley or the Swiss Alps. The picture below is taken from the train going up to the Eiger in Switzerland. The waterfall on the right can be seen emerging from a hanging valley in the picture on the left. Notice also the U-shape to the larger valley and the rugged, pointy-topped mountains, typical of glaciated mountains.
The features above relate to alpine glaciers—those in the mountains. However, during various periods in Earth's history, glaciers have extended over non-alpine regions of continents (and still do today in Antarctica and Greenland). As we've talked about in previous units, the landscape in the regions that were glaciated is often 'lumpy' because sediment within the ice is unevenly distributed, leaving behind a lumpy, undrained terrain when the ice melts. This is why Minnesota is the land of 10000 lakes.
Lumpiness can also form when blocks of ice get covered up by sediment and then, as the buried ice slowly melts, the sediment sags into a depression called a kettle. If water fills the depression it is called a kettle lake.
Below is a satellite view of kettle lakes in Massachusetts near Cape Cod.
Photograph of glaciated terrain taken by astronauts of the space shuttle mission of Feb 7 2007.
Consider the picture below taken northeast of Bear Tooth Pass and southwest of Red Lodge, Montana.
Consider the picture below showing Mount Hadley in the background. Would you say the mountain was shaped by glaciers or by running water?
Mount Hadley rising above the Apennine plain at the Apollo 15 landing site, 1971. Photo of the Lunar Rover and James Irwin taken by astronaut David Scott, NASA.
Shoreline Features and Processes:
The landscape of shorelines results from the complex interplay of sediment supply (erosion and deposition), uplift or subsidence of the land, rising or falling of sea level through time, the direction, strength, and persistence of wind, and tidal effects. All this is much too much for us to tackle in this short summary.
We're going to take a look at a few shore processes and features related mainly to the effects of shoreline erosion and deposition.
Let's start with a comparison of two different shorelines (not to the same scale). The top image is of the Maine coastline (rotated so that up is to the northwest) taken by the Terra satellite Feb 10, 2013 (white color of the land is from a heavy snowfall).
The lower picture is of the Gulf Coast of Louisiana and Texas taken by the MODIS camera of the Terra satellite Jan 15 2002.
Before continuing, identify a couple of features of the shorelines that differ.
The Maine coastline is irregular with many convolutions. The Gulf coastline is smooth. In addition, where rivers enter the sea in Maine, there is an embayment of the sea into the land (for example, the largest embayment at left center is where the Penobscot River flows into the sea). In contrast, along the Gulf coast, there is often a delta where a river flows into the sea (for example, the Mississippi River delta is in the center right).
Here's a question: Since we know that sediment is deposited where rivers flow into the sea (because the energy of the water falls and sediment settles out below base level), why aren't the embayments along the coast of Maine filled in with sediment? Why isn't there a delta formed there?
One reason is that the embayments are geologically recent. They formed when sea level rose after the last ice age and the sea flooded up the river valleys, making the embayments. Sediment is in the process of filling these embayments.
Here's another question: Notice that there are several embayments along the Gulf coast also, for example, Galveston Bay is at center left. However, at Galveston Bay the beach line continues smoothly across the mouth of the bay. What's with that?
The Gulf coast is a depositional coastline—huge amounts of sediment are being carried into the Gulf by the Mississippi and other rivers and this sediment is carried westward by longshore currents. The current carries sediment across the mouth of the bays, smoothing out the shoreline. This feature is called a spit.
Cape Cod in Massachusetts is one of the United States' most famous spits. This picture is from a photograph taken by astronauts of the space shuttle mission of Feb 7 2007
The straightening of a coastline results from erosion as well as deposition. Waves refract (bend) in such a way as to erode points of land poking out into the sea and deposit sediment in the embayments.
We can apply what we've learned about refraction in previous units to this problem. As a wave approaches land, the depth gets shallower, which causes the waves to slow down.
Based on the idea that waves refract toward the normal when they go from higher velocity to lower velocity (in this case, perpendicular to the shore) that we learned previously, predict the direction of wave refraction in the following case.
The refraction of waves near land has the effect of increasing the erosion of points of land that protrude into the sea and increasing deposition in the embayments of the sea into the land as shown below. Sometimes the erosion cuts off part of the protrusion of land, making a small island called a sea stack.
The straightening of a shoreline by combined deposition and erosion can be seen in the Waipeo Valley of Hawaii. In the image below, the sediment fills the valley, flat at near base level, and the cliffs show where waves have eroded into the areas formerly protruding into the sea.
Sea stacks are common along the coasts of Washington and Oregon. The picture below comes from the coast of Washington at Olympic National Park.
Sometimes erosion by waves can undercut the part of the rock poking out into the sea, making a sea bridge, like the elephants trunk at Etretat, France. A sea stack is also seen on the left. Notice the steep, eroded cliffs.
Notice the straight coastline on the south shore of Martha's Vineyard in the image below from the space shuttle mission of Feb 7 2007. This coastline is subject to erosion and those parts of land that stuck out into the sea eroded preferentially while deposition occurred in the bays. Deposition of sediment carried by longshore currents formed spits across the mouth of the bays. In contrast, the northern irregular coastline still shows evidence of the flooding of river channels by the rising sea after the last ice age. This area is sheltered from wave erosion and deposition by longshore currents. Even in this area, the bays are gradually silting up due to transport of sediment by the rivers.
Which one of the beaches below is most likely a depositional beach?
Karst Features and Processes:
Karst refers to a landscape that is shaped by dissolution of the underlying rock. On Earth, this underlying rock is most typically limestone which dissolves in slightly acidic water. This dissolution causes cracks in the rock to widen allowing increasing amounts of water to flow underground. The cracks can widen into caves, and the caves may collapse, producing sink holes like those seen in the picture below from Orlando Florida. Sometimes houses and highways fall into the earth when sinkholes form.
Karst topography, NASA Landsat image—2014, Orlando FL
In Florida, the sink holes are often filled with water, making lakes. In other places the sinkholes may be above the water table and so remain dry at the surface. Although the landscape looks somewhat like the kettle lakes seen in the picture from Massachusetts above, the two landscapes are quite different. Glacial topography is lumpy and undrained. Karst topography is lumpy, but drains through underground conduits and caverns that are dissolved through the rock.
In addition to sink holes, features typical of karst topography include streams that suddenly disappear underground and natural bridges where most of the roof of an underground river has collapsed leaving a small portion of the roof as a bridge.
One of the most famous karst terrains in the world is found in China. Notice in the image below that the river channels are not continuous at the surface, much like the undrained glacial landscapes. However, in the karst terrain, water can drain out of what appear to be closed basins (oval pock marks in the image below) into the underground drainage system
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Guangxi China—NASA Landsat 8 satellite image 2013
From the ground, the landscape of the Guangxi region has an unearthly feel that has inspired many artists. Check it out at Google search Guangxi
Although it's not exactly a landscape feature, I want to talk a little bit about how caves and karst topography typically form (on Earth anyway). Most caves form in limestone when the limestone dissolves in acidic water.
Usually the acidity comes from carbonic acid. Carbonic acid forms by combining water with carbon dioxide:
When there is lots of CO2.in the water (often from plant life in the ground above), then this reaction goes to the right and calcite dissolves. When CO2 is low, such as when it bubbles out of the water down in the cave chambers, the reaction can go to the left and the calcite will undissolve. Most speleothems (cave formations) form by this reverse reaction.
Entrance to Mammoth Cave, Kentucky, formed when acidic water dissolved corridors in the underlying limestone.
Flowstone and cave popcorn formed of calcite that precipitates from the water when the acidity falls. Mystery Cave, MN
The Guangxi region of China is like the glaciated landscape of northern Minnesota in that
One last challenge on stream, glacial, and shoreline landscapes:
Match each of the pictures below with one of the following landscapes
Last updated Oct 13, 2015. All text and pictures are the property of Russ Colson, except as noted.
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