Earth’s Surface Processes

and their effects on landscapes

Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

Why do these places look the way they look?

Because of a combination of the kinds of materials in the Earth at each place and the kinds of surface processes that have affected those materials. Surface processes include volcanism, weathering, erosion and transport by mass wasting, glacial ice, wind, and moving water, and deposition by those same agents. We will take a brief look at some of those processes that create and modify landscapes and some of the landscapes they have created.

Weathering

We talked about weathering as part of sedimentary rock formation, sedimentary processes like weathering and erosion also leave behind interesting and important

features of the landscape.

Most importantly, of course is soil. This is where we grow our food and we cannot do without it.

But there are other landscape features that are simply interesting or beautiful and

these we will focus on in our examination of surface processes – weathering, erosion, and deposition.

We’ll begin with weathering

When limestone (or marble) is chemically weathered by slightly acidic groundwater it dissolves – just like any calcite dissolves when you put it in acid.

When the resulting cavities in the rock are small we call them “vugs”, but when they are big …

Scale: Keys

We call them caves. This is inside Seneca Caverns in Pendleton Co., WV. Scale: Ordinary floodlight

Mass Wasting

The Force of Gravity

As this stream in western Virginia undercut its far bank the support for the cliff that was there was lost. Eventually the weight was too much for the strength of the rocks and they fell. This was a rockfall. Other types of rapid mass movement include “slides”, “avalanches”, and “flows”. These things differe in how the material moves – coherently or in a jumble; touching the ground or not; saturated with water or dry …

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The water is currently cascading onto and across fallen rocks, but the undercut of the cliff that is likely to lead to the next rockfall event is evident just to the right of the falls. In this way the waterfall has retreated up its valley, leaving a gorge downstream. Rockfalls along the side will also continue to widen the gorge downstream.

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Appalachian Trail in New Hampshire. (Franconia Range)

On the southwest side of Whitewall Mountain, NH the AT crosses a large pile of rubble formed by the falling and avalanching of rocks from the steep cliff above. How the steep cliff got there in the first place we’ll come back to; for now we are interested in the scree or talus at the foot of the slope. It got there because of mass wasting off the steep rock cliff above. Over time the cliff (and the one on the far side of the mountain) will “retreat”, lowering the landscape.

The Zeacliffs on Whitewall Mt., White Mts National Forest, Lincoln Co., NH

The scars on the face of this mountain in the Great Smoky Mountains near Newfound Gap were created by debris slides and/or avalanches. Whenever you are in the mountains and see a “beware of rocks” sign there’s some reason to expect that the hillsides ahead might do something like this, or you might find rocks in the road because they already have.

US 441, Great Smoky Mountains National Park, Swain Co., NC

Here’s another example of debris avalanche scars in southern Colorado near Mesa Verde.

Caviness Mt., San Juan National Forest, La Plata Co., CO

The “harder” layers of rock in Canyonlands National Park in eastern Utah resist weathering and erosion better than the “softer” ones do, but even they eventually are eroded by mass wasting. The evidence for this is clear in the photo. The steep cliffs are hard sandstone. The gentler slopes are softer mudstone and shale. The talus on those softer layers is debris from the cliffs above, which builds out until it reaches the next cliff and begins to fall off that. The talus actually protects the softer rocks to a certain exent and doesn’t allow them to erode as fast as they would were the talus not there.

Canyonlands National Park, Wayne Co., UT

You see the same thing in the Grand Canyon in Arizona (or any other broad canyon cut into flat rocks). Most folks believe the story that the Colorado River eroded the Grand Canyon, but it is really only partly true. The river provided the depth, and the steep sides, but mass wasting is responsible for most of the width of the canyon. The canyon is far wider than it is deep, so truly, gravity has done most of the eroding and the river has mostly carried away the debris.

Grand Canyon National Park, South Rim (looking ~north), Coconino Co., AZ

Glacial Ice

Moving because of gravity

Glacial ice weathers and erodes rock both by prying open cracks in which it freezes and by plucking and dragging rocks from and along its base, grinding them to powder in the process.

Glacially eroded valleys (troughs) have characteristic rounded shapes, both at their bottoms and at their heads. Where two or more valleys or heads meet this creates sharp ridges called aretes or sharp topped mountains called horns.

Rocky Mountains , San Isabel National Forest near Leadville, CO

The broad, rounded bottom of a glacial trough is evident in this picture, as are a number of aretes and horns.

Looking East from Independence Pass, Lake Co, CO

The head of a glacier once sat in this bowl-shaped cirque near Leadville Co. The cirque is on the side of a horn.

This is a big glacial trough in the Cascade Range of Washington.

Aretes, cirques, and horns are obvious in this photograph from the Rocky Mountains in Montana. One cirque holds a lake called a tarn.

You don’t have to go to the west to find glacial landscapes. New England has them as well.

Mt. Katahdin, Baxter State Park, Piscataquis Co., ME

Cirque with a small Tarn beside Mt. Katahdin, ME

Arete, up close and personal, leading northeast off of Mt. Katahdin, ME. An approach trail for the AT follows this jumble of rock.

Mt. Katahdin, Baxter State Park, Piscataquis Co., ME

The notches and valleys in the White Mountains of New Hampshire are all glacially eroded. Remember the Franconia Range? The scree is falling off the side of a glacial trough. See the rounded, flat bottom?

The Zeacliffs on Whitewall Mt., White Mts National Forest, Lincoln Co., NH

Most of Maine is actually not mountains. The glacial ice here was more continuous than in the valleys of the Rockies and scraped the surface almost flat over much of New England. Peaks (horns, etc) of very resistant rock survive, but much of the landscape is actually lowlands with many, many, many lakes. (Minnesota is called the “Land of 1000 Lakes” for the same reason, but the ice was more extensive and thicker and there are no mountains at all in Minnesota).

Base map from Google Maps

The present Greenland Ice Sheet averages about 4-5 miles in thickness, but it is only a tiny remnant of the Ice that once existed in this region.

All the sediment eroded by continental glaciers can only be carried to the edge of the ice, no farther. The resulting terminal moraines show us where the ice ended.

Base map from Google Maps

The blue shows the maximum extent of the huge Laurentide Ice Sheet during the last glacial advance. Cape Cod and the nearby islands and Long Island are largely or entirely moraine deposits. Inland, the ice and moraines dammed originally northward-flowing rivers and diverted them westward, forming what is now the Ohio River. People ski on glacial deposits in Wisconsin and Minnesota.

Deserts

Wind and water moving because of gravity

Geologists like working in deserts because there is no irritating soil and pesky vegetation to cover and hide the rocks. The originally continuous sedimentary rock beds that have been carved by erosion (wind and water) are evident in this picture. The visual beauty of this place is in the rock that is no longer here. The geological beauty is in the rock that is still here. Most landscapes in most deserts are bare rock! Weathered material is easily washed or blown away.

Dry though they are, a great proportion of sediment movement in deserts results from mass wasting and moving water. Dunes and dunefields are actually not very common in many deserts. This stream in Egypt might see water only once every few years, but because there is little vegetation to hold the regolith in place, when it rains the stream carries huge amounts of sediment. The deepest part of the bed is obvious because the groundwater below it supports a few hardy plants.

There is one very famous desert River that supports a huge population in a narrow belt across the biggest desert on Earth.

The peasant owner of this little Egyptian felucca named it the Jimi Hendrix. How many Egyptian musicians can you name?

This is one of the most extensive dunefields in the deserts of the western US, and it almost fits on one picture. The Sahara and the Arabian Deserts have much more extensive areas of dunes.

Streams

Water moving because of gravity

Moving water has energy, which, in physics is the ability to do work. Work, in physics, is simply the movement of matter from one place to another. Much of the energy of a stream is spent moving the water itself, but in many cases the stream may have more energy than enough to do that work and so must do additional work as well. Much of that additional work is the erosion and transport of sediment – additional matter to move from one place to another, over and above the water. The form of a stream results from the need to balance the stream’s energy with its load – the sediment it is moving. If the stream comes to a place where its energy is more than necessary for the load already being carried, then it must erode more sediment. If it comes to a place where its load is too great then it must deposit some of its load. Erosion can occur at the bottom of the stream (downcutting) or at the sides (lateral erosion). We’ll look at five types of rivers or river-related features and see what energy/load (abbreviated E/L below) balance causes them.

Canyons form where elevations are high and slopes steep. It should be easy to figure out what the E/L relationship here is. This is the Black Canyon of the Gunnison in southwestern Colorado. The water moves very fast across the steep slopes and has a great deal of energy. It erodes very effectively. It should also be obvious that the bulk of the erosion is downcutting and not lateral. Most of the width of the canyon results from mass wasting of the steep sides The resulting talus is obvious along the lower parts of the slopes (and as rapids in the river).

Compare the sharp-bottomed, V-shaped valley of this stream with the U-shaped, flat-bottomed glacial valleys you saw earlier. Even if the river were not in the bottom of this canyon we’d know it was a stream valley and not a glacial trough.

Egan Range, Nye Co., NV, near Lund, NV – Image from Google Earth

Where a stream flows out of a canyon in the mountains (light blue arrow) and reaches a relatively flat landscape beyond the canyon mouth, it must slow down on the gentler slope. The sediment load it brings from the mountains is thus too much for it to carry and most must be deposited. As the stream deposits in one place it clogs istelf up and must switch to a different direction. This happens over and over, resulting in a multitude of channels crossing a building pile of sediment, thickest near the mouth of the canyon. The resulting feature is called an alluvial fan. This one is outlined in red. Make sure you see the branching channels across the fan, spreading its discharge across the fan.

Do you see why it’s called a “fan”?

Beyond a fan, or, in fact, anywhere that a stream’s load exceeds the energy available to move it farther, a similar feature (but not fan-shaped) will occur. Channels repeatedly clog and shift their positions, splitting and rejoining, to create a braided stream. This is Medano Creek where it flows along the side of the Great Sand Dunes. There is so much sand it clogs the stream and forces it to braid.

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Here’s what Medano Creek looks like on Google Earth. Can you see why this is called a “braided” stream?

The little stream in the bottom of Cloudland Canyon, GA is braided. Why should it be?

Most lowland rivers, far from any source of abundant sediment, are like the Mississippi – the “Old Man”. This image of the Mississsippi/Arkansas border region from Google Earth shows their typical form – one curve after another, back and forth. They are called meandering streams. The curves cut themselves off and leave curved oxbow lakes near the river, these eventually fill in and make swamps (the green curves that are obvious near the river). The river sits in a wide floodplain, heavily farmed in the Mississippi Valley, heavily forested in most cases. Over time the river swings from one side of the valley to the other, or sometimes switches rapidly back to an old course on the opposite side. Ancient meanders are particularly evident on the east side of the valley here.

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Google Earth image from the eastern boundary of Sumter Co, GA, just south of Andersonville

The Flint River is also a meandering stream. Its floodplain is heavily forested and the adjacent uplands farmed. (Including tree farms, evident on the west side). Notice that there are often obvious sandbars on the “inside” of meander bends. There is a separate slide show on these sand bars. In meandering streams the energy is more than enough to carry the load (if the stream were straight) and so they erode. Since they are already on a low flat surface this cannot be downcutting erosion, so instead they cut laterally, creating the meanders and thereby balancing their load and energy.

Google Earth images

When a stream reaches the sea (or any body of standing water) its current all but disappears and its energy drops to nearly nothing. Any sediment load it brings to this point is almost immediately dropped, forming a delta. The Nile Delta (left) is the source of the name, dating back to the Greek pharaohs of the latter Egyptian Empire. Can you see why they called it that? The Mississippi (right) brings more sediment to a quieter seashore and so the sediment builds outward into a more elongate delta there.

Seashores

Water moving because of wind (waves) and gravity (tides)

Ultimately rivers bring sediment to the coast. There are other ways for sediment to get to a coast, if the conditions are right, but rivers deliver most sediment that is eroded from land to the sea. As we’ve just seen, that sediment can form a delta, but in many cases the delta of a river is very small or non-existent. Even at large deltas like the Mississippi the sediment does not stay forevermore in the delta. Instead, it is moved away from the river by marine processes. Sand moves along the coastline by the activity of waves. Mud is moved offshore in a variety of ways (and back inshore in one way that we’ll get to). We don’t have time to go into this in any detail, but the net result of wave action is to move the sediment parallel to the shore and away from a river mouth; the net result of tidal currents is to move it toward and away from the shore.

On some coasts, and for a variety of reasons, the rate of sediment delivery is too small to balance the energy of the waves and tides. The excess energy, just as in a stream, is expended eroding the shore. Most of this results from waves breaking directly on the exposed face of the shore. The picture shows the resulting type of landscape, at low tide an erosional coast.

The waves (at high tide) undercut the cliffs, which then mass waste down to where the waves can reach the resulting talus. The sea cliffs are thereby continuously renewed and continuously retreating. In some instances the undercutting can be extremely localized and a sea cave results. As the cliffs retreat they leave a wave-cut platform at the level below which the waves do not effectively erode. These can be exposed at low tide (as it is here) if the tide range is great. The flat surface at the top of this shore is simply an old, uplifted wave-cut platform called a marine terrace.

Where the marine processes balance the load supplied to them a depositional coast forms, as on the Georgia Coast. The wave-created longshore transport brings the sand along the shore to build barrier islands, paralleling the shore. The tidal currents chop these into short segments, keeping the sand from making a continuous ridge and forcing islands instead. The tidal inlets in this picture are indicated by yellow arrows. They move sediment into the marshes between the islands and the mainland, and offshore onto the continental shelf. Along coasts where the waves are relatively stronger than in Georgia and/or the tides relatively smaller, the islands are much longer, with fewer inlets. The white arrow indicates the principle direction of longshore transport on this coast. Some of the sand on Georgia’s beaches may have come from much farther north, perhaps as far as New York.

Google Earth image

One Major Sediment Source (Altamaha River)