Geology From The Great Courses

Front 1

Who invented the Humid Cycle of Erosion, and describe the cycle

Back 1

William Davis (1850s-1930s)...
an area is uplifted above sea level and is then worn down by running water and gravity until the region is worn away (base leveled)

Front 2

Define a stream's base level.

Back 2

Every stream has a base level, an imaginary plane below the surface down to which the stream is actively carving its channel , with the stream's moving bedload

Front 3

What are two types of stream base levels and define them

Back 3

1) ultimate base level is the lowest point to which a stream will carve.

a) for exterior streams this will be sea level
b) for interior streams it could be anything, even below sea level.
2) temporary base levels, with streams draining to a lake or pond, that body of water is the temporary base level, until the lake/pond fills up and stream continues onward

Front 4

Describe how a temporary base level is removed

Back 4

for temporary base levels, that stream wants to carve itself down to the lake’s elevation. So it is dumping its load into the lake and pretty soon that lake will be filled up. The bottom rises and the water starts to shallow, you start seeing grasses growing in that edge of the lake, and you see that’s a marsh. Sooner or later as this continues, the marsh builds across the lake, the open water gets smaller and smaller. Maybe trees start growing into the marsh and it becomes a swamp (a marsh w/mostly trees). It continues w/more load dumped and it becomes mushy under one's feet (that’s a bog).

2) another example, receding waterfalls

Front 5

Are the Great Lakes lakes

Back 5

no; they are inland seas.
these lakes will not fill in like other lakes; they are ephemeral.

Front 6

What is the key difference between the Great Lakes and the oceans

Back 6

The Great Lakes do not have tides.
Like the oceans they do have
-currents
-waves
-and can create storms

Front 7

Using the example of a waterfall describe how temporary base levels can be removed

Back 7

Waterfalls have a rock ledge sticking out with a stream running down to the edge of the waterfall, (the temporary base level). The stream is trying to carve the elevation down to the lip of the waterfall. This, too, is temporary. With mass wasting of the freeze-thaw-freeze-thaw, water breaks the rocks off the edge of this cliff. As you break off the edge, the waterfall moves upstream and the angle of the waterfall ledge lessens, so in effect the waterfall becomes smaller. The waterfall looks and becomes more like a rapid. So a waterfall w/physical weathering becomes a rapid.

Front 8

What was the rate of erosion for Niagara Falls historically

Back 8

Niagara Falls eroded about 3 feet a year. When they were first created after Glacier Wisconsin they used to be 7 miles downstream. Now much of the water is diverted for hydro-power and rocks have been bolted down to slow erosion.

Front 9

Define slot canyons and how are they formed

Back 9

Slot canyons are significantly deeper than they are wide; form in arid regions generally in sandstone and limestone with intermittent creeks. They form when an area is far above its base level (as a result of mountain building/collission of continents) Stream energy is directed at cutting or moving of the bedload and in a slot canyon you'll get high vertical walls.

Front 10

Why doesn't the east coast of the U.S. have slot canyons.

Back 10

Mass wasting. Instead under the humid climate, east coast valleys form V-shapes. The bottom of a V-shape valley is as narrow as a slot canyon, but mass wasting (freeze-thaw-rock slides) opens the area up to a V-shape valley. There is more rock removed from mass wasting than stream erosion--slot canyons illustrate this

Front 11

What is the first stage of the erosion cycle for Humid Regions~Davis

Back 11

The Youthful Stage.
The stream is way above its base level and is cutting straight down down into the resistant rock later

Front 12

What landform is created in the second stage of the erosion cycle

Back 12

The floodplain.
The stream's energy, while still cutting downward is now cutting side to side cutting back the hills a bit and cutting the valley flat. This newly cut area will be covered in debris during flood. it is the flood plain.

Front 13

What is the third phase of the erosion cycle for humid regions

Back 13

the stream meanders, increasing in sinuousity and becomes muddier, murkier, and waterfalls and rapids start to disappear. hills that were once at steep angles become shallower and rounder. streams are no longer ruining the hills--it is mass wasting that is working on the hills. The topography is being subdued

Front 14

The fourth cycle for Davis~humid regions is?

Back 14

Maturity phase. The stream is going back and forth creating a valley much larger than the stream--creats a broad valley. Abandoned meanders are oxbow lakes.

Front 15

What is the fifth phase of erosion

Back 15

Old age phase. In the really big valleys there will be swamps. An example of old age is in the Southern raches of the Mississippi Valley as it approaches its base level in Lousiana.

Front 16

What can happen to a stream before it passes the old age phase of Davis's humid cycle of erosion

Back 16

Rejuvenation.
Before it gets down to its ultimate base level, the sea level, streams can get lifted up in a mountain building exercise, like the American Southwest

Hint 16

In the southwest, up until about 10,million years ago, the entire Southwest was down to sea level; streams in maturity meandering to stream. Then 10 million years ago, southwest lifted 10,000 feet. It wasn’t b/c of plate tectonics. There is not consensus. But Renton thinks heat is accumulating, the normal heat flow to surface was impeded by granite, rocks heated up, expanded, rocks density went down and the whole area was uplifted 10,000 feet.

Front 17

Facts about the Grand Canyon

Back 17

Made over 10 million years ago and uplifted 10,000 feet, as a result of rejuvenation.
The main stream is just in the maturity erosion phase as there are not wide valleys.

Front 18

Facts about Bryce and Zion

Back 18

Bryce canyon and Zion canyon, you can see the result of the rejuvenation of the Colorado river over the last 5 million years. We’re 5,000 feet down into it. We’re halfway down to the baselevel. It has a long way to go.