click for details and join us on a hike! click here if you are unable to use the state-by-state map know how to walk the walk, but have trouble talking the talk?  try this page.

current camping & hiking close-outs and specials

...deep discounts on better quality gear

click here for the slackpacker web site

Geology 101

The purpose of this page is not so much to explain EVERYTHING about geology, but to provide an introduction to the geological features the average hiker might encounter.

If you happened to stumble upon this page from a search engine, you may be disappointed to discover that this "Geology 101" is not a course. We just call it that so our users know immediately what the content is. Sorry for any inconvenience.

In recent weeks this page has begun to get larger and larger, taking forever to load on the average dialup modem. For this reason, this page is rapidly becoming an entryway to the geological content and definitions, and you will have to click on the links immediately below to access the nitty-gritty geological stuff. The content is divided into types of formations: Igneous, Sedimentary, Erosion, Glaciation forms, Horizontal Strata, Water forms, Wind forms, etc.

Domes and Basins

Includes domes, cryptovolcanic domes, basins, salt domes; domes & basins large-scale and localized.

Faulted Structures

Includes Faults, Fault Blocks, Overthrusts, Bajadas, Inselbergs, Playa, Pediment, Outliers, Uplifts, Scarps, Canyons.

Glacial Formations

Includes Drumlins, kettles, erratics, loess, moraine, etc.

Igneous Land Forms

Includes Dendritic drainage, Dikes, Necks, Calderas, Intrusives, Extrusives, Cinder Cones, Lava Domes; Composites, Lacolithic domes, Lava flows, Stocks, Monadnocks, Pre-cambrian shield, craters, volcanoes, batholiths.

Lake and Pond Forms

Describes geologic, organic, and anthropogenic impoundments; volcanic, glacial, etc. creation of lakes.

On this page, below, are general and specific geological terms not elsewhere classified. Good stuff, lots of info & links...

Alluvial Fans

alluvial fan photo courtesy of New Mexico State University

Alluvial fans are formed by severe water run-off or landslides, and usually appear where a mountain or hillside meets the valley floor or floodplain. This is obviously an aerial photo.

Anticlines & Synclines

Anticlines and synclines result from geologic pressures acting on horizontal beds. They reflect the tectonic history of a region.



Arches, Bridges

To qualify as a natural arch or bridge, the opening should be at least 15" at the widest point, and at least 15" at the highest point. (Smaller openings are referred to as windows, lighthouses, holes, etc.) A natural arch is created by natural forces causing rock to fall away. The most common is moisture freezing/expanding and causing cracks. Other forces that cause natural arches are wind, windborne debris, rain...and in the case of sea arches, waves. A natural bridge is caused by water eroding, running underneath, and creating the formation. In most cases, the name is correct; Delicate Arch is an arch...Rainbow Bridge is a bridge. But the rules can be confusing: A sea arch is usually not a bridge, even though it was formed by water. A waterfall step arch is really a bridge. The natural bridge at Bryce Canyon is really an arch. Angels Window in Kentucky is really an arch. Natural Bridge in Virginia is a bridge remaining from a collapsed cave, Natural Bridge in Kentucky is an arch. Go figure.

Confused? The Arch Millenium Website is an excellent resource for worldwide natural arch information from Guilain Debossens.

Want to see photo evidence of how an arch forms? Here's a link to an excellent article and photo essay on the sea arch at Grand Portal Point in Michigan's Pictured Rocks National Lakeshore, written by Jonathan Gennick.

Another question concerning natural arches and bridges is: What is the difference between a natural bridge and a cave? The answer is, quite simply, if the "tunnel" is longer than the width of the "opening," it's a cave. If the opening is wider than the length of the tunnel, it's an arch or bridge.

Natural arches and bridges are one of the most fascinating geological formations you'll ever see while hiking. The formation "frames" something behind it or in the distance, and it defies gravity. Arches are different from every other rock we are familiar with. Utah's Delicate Arch, for instance, is a weathered piece of lowly sandstone, but it is more beautiful and more priceless than any diamond. If you are interested in learning more about these features, be sure to visit The Natural Arch and Bridge Society website.


Primary examples in the SW United States. These are features up to hundreds of feet high with steep sides narrow, pointed tops. Actually, buttes are what is left of what was once a mesa; the rest of it has been eroded away. What's the difference between a mesa and a butte? see Mesas, below.

Chock or Chockstone

A rock wedged into a crevice. This can be caused by glaciation, fault movement, erosion...virtually anything.


Caves, Caverns

Easiest way to remember the difference between a stalactite and a stalagmite: A stalactite hangs tight to the ceiling. A stalagmite is on the ground and is something you might bump into.

Here's a fantastic website for the average slackpacker to delve into caving: The Virtual Cave. Lots of cave info, and links to most show caves in the USA.

More caving talk: A TAG or TAG-cave is any one of a gazillion caves in the region where Tennessee-Alabama-Georgia meet.



Unfortunately this word is frequently used to describe two similar looking phenomena that couldn't possibly be more different in origin. One is the result of something blasting out of the earth -- while the other is the result of something blasting the earth. Both are called craters. Turn to northern Arizona, where Sunset Crater and Meteor Crater are within 50 miles of each other. Sunset "Crater" is actually an inactive volcano, while Meteor "Crater" is the site of a meteor impact.


Incised Meander

This is the result of a few different geological forces working together. A meandering water body is first uplifted, then cuts laterally through rock to create a "meandering canyon" if you will, until it reaches the water table. The Goosenecks of the San Juan River in SW Utah are an excellent example of an incised meander. For a good photo essay on this phenomenon, please click here.


A group of mountains connected to each other; a group of peaks divided almost near the top by cols or shoulders. The classic is the Jungfrau Massif in Switzerland; the peaks are the Eiger, Mönch, and Jungfrau. Perhaps the best known is the Everest Massif, completed by Lohtse and Nuptse.


Primary examples in the SW United States. These are features up to hundreds of feet high with steep sides and large, flat tops. Actually, mesas are what is left of what was once a flat plain; the valleys and canyons below have been eroded away. What's the difference between a mesa and a butte? Mesas have much larger "tops," and some will say that a mesa is a butte on which standing water can sometimes be found. This can be confusing, because after a rainstorm you can find water atop many buttes. Some say that technically speaking, a butte is taller than it is wide, and a mesa is wider than it is tall. Well, we've seen buttes that don't fit this, so we disagree. You can also look to the Grand Canyon's Shiva Temple, a feature visible from virtually anywhere on the south rim. Shiva Temple has more height than width, but it's clearly more mesa-ish than butte-ish. It's likely that the more romantic "standing water on top" definition came from the southwest USA. It was originally said that you could find game on a mesa, but not a butte, which eventually translated to the idea that you could graze cattle on a mesa (and find water) but not on top of a butte. So, a good rule of thumb if you will, is that if you can fit a water tank and a small herd of cattle on it, it's a mesa.


Rock or formations that take on the appearance of something else, such as, a human face, an animal, a hamburger, etc. Although no longer with us, the Old Man of the Mountains in New Hampshire was arguably the most famous mimetolith. Queen Victoria in Bryce Canyon National Park is another example. Sections of hundreds of mountain ranges around the world are locally nicknamed "the elephant" or "the sleeping giant" because of how they appear from a certain area, are also mimetoliths. On a smaller scale, an individual rock that naturally resembles something is also a mimetoliths. The small button-shaped stones found in Vermont's Button Bay are one example of these. The most descriptive and informative source of information on this subject can be found on this site by R. V. Dietrich, Professor Emeritus, Central Michigan University College of Science and Technology.


See also "unconformity" below. This appears as the "intersection" of a vertical stratus butting up against a horizontal stratus. In an unconformity, it is two of the same type of rock (e.g. sedimentary & sedimentary). A nonconformity is two layers of different types (e.g. igneous & sedimentary). The photo below shows an igneous & sedimentary nonconformity.

photo of a nonconformity

Patterned Ground

Mystics & new age types think this is the result of some energy force from the great goddess of the earth. Not quite.

Patterned ground usually is created in freeze-thaw conditions. The surging of the frozen ground moves stones upwards and outwards.

Aerial view of same region.


Foothills or small, "almost-mountains." These are usually regions of rolling hills located between a coastal plain and a mountain range. Much of South Carolina is a piedmont; but also the region of Vermont between the Green Mountains and the Connecticut River Valley -- definitely piedmont type terrain.


Three basic types of rock are igneous, sedimentary, and metamorphic. Igneous is rock of volcanic or molten origin; examples include basalt, pumice, and obsidian. Granite is another type of igneous rock. Sedimentary is rock formed by deposited material and mild pressure, chemical change, or organic deposits; examples include shale, sandstone, and limestone. Metamorphic is rock that was another type but has since been changed by extreme heat or pressure; examples include slate, marble, schist, and gneiss. Slate began life as shale, marble was once limestone, etc. A real good webpage for understanding the different types of rocks is available here on the University of Oregon's Ask Geoman site.

Slackwater Deposits

Slackwater deposits are the residual left by a flood in calm, backwater regions. They indicate maximum flood levels.

This is approximately a 3' high cross-section view of slackwater a deposit. "Diagnostic features include horizontal bedding, grading of sediment sizes from gravel on bottom to fine grain sizes on top. There is also ash interbedded with the deposit here." courtesy New Mexico State University

outflow channel slackwater deposition.

This deposit was created by catastrophic floods. Vertical scale is approximately 35 feet.


Literally, petrified sand dunes or sea beds. This is the stylish, layered sandstone found in the Canyon country of Utah, Arizona.

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. One of the best websites for learning about slot canyons, where they are located, how to find them, etc. is's Slot Canyon page.


When a substantial part of a mountain, cliff, hill or whatever "breaks away" and slides -- more or less intact -- to the bottom of the slope. It then appears as if a section of the top of the cliff has been cleaved away and moved to the bottom, which is exactly what it is. One of the finest examples of a slump can be seen at Colorado National Monument near Grand Junction, Colorado. The Shawangunk cliffs in New York State are also dotted with countless slumps.


Cave formations that resemble something else. Do a cave tour, and the guide will point out stuff like "popcorn" and "bacon" and "curtains" and "fairyland" and "soda straws" blah blah blah. Oh it's all cool stuff, don't get me wrong, but it can get tiresome if you frequent as many caves as I do.


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.

Want to add your information or website to this page? Please click the "submit a site" button, above left, or email admin(at)slackpacker(dot)com.


Please click here for the Slackpacker web site

Copyright © 2002-2018 Slackpacker