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Faulted Structures -- Geology 101

This page describes faulted structures. This includes fault blocks, fault lines, splinters, overthrusts, scarps, etc.

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Faulted structures are found where sections of the earth's crust meet one another. Ever see a dried mud puddle...completely dried and cracked into a pattern of individual pieces? Now imagine what would happen to the individual pieces of dried mud if you kicked that cracked crust around. Pieces would collide, some would overlap, some would stand on end. It would no longer resemble a cracked pattern, but rather a chaotic surface. Now, simply substitute the earth's crust for that dried mud puddle, and deep tectonic movement for the kicking motion...welcome to earth, 3rd rock from the sun.

Many of the fault phenomena described on this page can be best understood by referring to the illustration immediately below:

diagram showing landforms inherent to fault blocks that the hiker will encounter

Alluvial Fans

alluvial fan photo courtesy of New Mexico State University

Alluvial fans -- as they relate to fault blocks -- are formed by water run-off where a fault block mountain begins to erode onto the floor of the plain below. This is obviously an aerial photo.

Bajadas

Seen in basin & range areas where numerous alluvial fans have formed where the fault block range is eroding. The range erodes with deep angular patterns, leaving alluvial fans as run-off. When a number of alluvial fans merge together, the resulting sloping plains are called bajadas.

Canyons

Faults play into the formation of many canyons. Where a fault opens up, a canyon begins to form. The Grand Canyon, for example, is the result of a combination of uplifting, fault action, and erosion.

topographical image shows fault geology

Fault

A fault or fault line is anywhere two or more segments (plates) of the earth's crust meet. As the earth is a geologically active planet, the pressures, heating, contracting, or whatever deep in the earth will cause the plates to move along the fault line. While we're all familiar with the various faults in the western USA, the eastern slackpacker crosses faults without even realizing it. The topographical image at right is a prime example; this is an area of gently rolling hills in Quebec that appears nondescript at ground level. From above, as this image clearly shows, a distinctive fault line runs diagonally north/south, and bisects the lakes and scarps.


Inselbergs

Outlying hunks, sort of icebergs if you will, of a range that has eroded away. Usually found "in front" of fault block type mountain ranges that have eroded away. The inselberg resembles a "mini mountain." In the diagram at the top of the page, inselbergs are visible on the pediment in the "old age" segment of a fault block mountain range.

Pediment

In basin and range country, the pediment is the broad sloping area just in front of the range. It gradually slopes into the bajada (see above).

Playa

A shallow lake found in depressions in the basin and range country typical of the Western United States.

Scarp

Upward thrust mountain, hill, or ridge line on the edge of a fault line. The front of tilt block or fault block mountains or ranges. The photo immediately below shows the front of the Wasatch range in Utah. It is a mature scarp. A young scarp will have a steep, solid face. A mature scarp exhibits the deep v-shaped drainage and alluvial fans seen in the photo below. An old scarp is marked by sloping pediment, inselbergs, and well worn mountains that resemble buttes more than mountains.

photo of the Wasatch Range shows a mature scarp marked by v-shaped ravines and alluvial fans.  The front of the Wasatch Range marks the line where the faults meet.

Slot Canyons

Faults and erosion create deep, narrow canyons; found mostly in the Colorado Plateau of Utah and Northern Arizona. Noted for stylish rock formations and flash floods.

Thrust Fault

A geologic oddity in which an older, lower layer of earth is displaced over a younger layer. One of the most notable thrust faults is found on the eastern edge of Glacier National Park, Montana. Known as the Lewis Overthrust, it is where precambrian shield thousands of feet thick has been pushed over the top of Cretaceous material that is much newer. It is evident in Flattop Mountain, where the banded layers of rock lie at a distinct angle. Chief Mountain is slightly east of the edge of the Lewis Overthrust; it is called an outlier because it is actually part of the overthrust material.

Unconformity

Reflects tectonic activity that has turned a layer of rock "on end," and then more recent layers have formed right beside it. Appears as a vertical stratification next to a horizontal stratification.

photo of an unconformity

Roadside Geology -- If you travel at all, you absolutely have to start building your library of Roadside Geology Books. These are the fascinating geological wonders that professional geologists know about, but us amateurs drive right past without a clue. At the very least you ought to get the guide for your home state. If you are a rock collector, or just an armchair geologist, these books are more important than your GPS. The link goes to Amazon, so you can click safely. Your purchase earns a few cents toward operating this website, at no added cost to you.

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