Geology on the Trails
The components of geology, the science of the planet and its history, are always underfoot. But the geology is not
always visible, and even when it is visible, it doesn't always reveal itself in a way
that can be interpreted. In fact, one of the greatest mysteries facing us is how the
planet has changed over geologic time (USGS). Here and there we find pieces of the puzzle that
help to tell the story, places where the evidence can be seen a little more clearly than usual.
So what kind of evidence can you see by getting out and going for a walk? The outline below
provides an answer to that question.
Glaciation - the earth cycles through cold and hot periods over time. During the cold periods, ice sheets
form and move over continents. Glaciers are powerful erosional forces. They leave a great deal of evidence behind after they are gone.
- Striations - scratches in the surfaces of rocks that are made by the debris caught up in glacial ice.
- Glacial polish - flat, shiny rock surfaces that have been "polished" smooth by the scraping of glacial ice and the debris it carries.
- Carved valleys - U-shaped valleys which have been scooped out by a passing glacier. Valleys carved by rivers are generally V-shaped.
Yosemite Valley in California is a famous glacier-carved valley.
- Erratics (pictured) - boulders that have been carried from their original location by moving ice sheets and then deposited somewhere else when the ice melted.
- Moraines - debris deposited at the leading edge of a glacier. A moraine has a specific look about it that makes it clear it did not develop in place through natural processes.
- Potholes - depressions in bedrock that were created by rocks swirling in water pockets under the glacier.
Earthquakes - when stresses in the earth's crust build to a point where the earth moves in response,
you get an earthquake. This occurs frequently along the boundaries of tectonic plates, such as on the west coast of California where the North American Plate and the Pacific Plate meet.
- Offset - an often visible result of an earthquake where there is measurable movement along a fault. A famous example of this is at Point Reyes National Seashore where an old ranch fence was displaced by the 1906 San Francisco quake.
- Sag Ponds - a depression caused by an earthquake fault. They often accumulate water.
- Beheaded Streams - when an earthquake moves the ground enough, it can change the course of a stream, leaving the old streambed dry, but still visible.
Mining - Mining involves not only extracting precious resources from the earth but also figuring out where to find them and how they came to be there.
The process of mining leaves plenty of evidence behind.
- Tailings - debris piles where the bedrock has been discarded. These often contain small amounts of whatever was being mined and are therefore searched by rockhounds looking for treasure.
- Pits - a fairly obvious remnant. Some of these are very large, especially those cause by hydraulic mining, a good example being Malakoff Diggins.
- Shafts - open holes in the ground where the miners dug in. These can be dangerous to explore, but many are open to visitors, such as the shaft at Gold Bug Mine in Placerville, CA.
- Equipment - machinery used in mining that has been left behind. This can include huge stamp mills used to crush gold ore, kilns, or entire towns that supported the mine, like Bodie, CA, which is now a tourist attraction.
Erosion - the process of wearing down surface features through wind, ice, and water action, and sometimes through root invasion by plants. Although this is happening on a continuing basis, over long periods of time erosion can create fascinating displays and is very often the leading cause of some of our most treasured natural landscapes.
- Hoodoos, spires, etc. - interesting rock shapes can result over time, including some very impressive displays, such as the colorful spires of Brice Canyon, Utah.
- Arches - arches can be formed on land or in water and are the result of localized erosion which punches a hole over time. The best place to see arches is in Arches National Park in Utah.
- Sea stacks - rocks in the ocean which are more resistant than the rock that used to be adjacent to them. They are left standing like sentinels in the waves. Sometimes sea stacks are the old supports for an arch which has collapsed. In fact, the only thing left of the arches at Natural Bridges State Park, CA are sea stacks.
- Canyons - these come in all sizes and are cut by running water. In places where the bedrock is soft, the result can be quite dramatic as at the Grand Canyon.
- Waterfalls - large waterfalls often occur where the old riverbed was cut away by glacial ice. This is how Yosemite got its tall waterfalls.
- Marine Terraces - cut by wave action against the seashore, these flat old beaches emerge above the sea when the sea level changes. You can hike from the ocean inland through 5 terraces at Jughandle State Reserve near Mendocino, CA.
Volcanoes - these can be classic cones that erupt or fissures in the surface of the earth that allow magma to escape.
- Lava - dark black basalt or pumice on the ground is a dead giveaway that there was a volcanic event in the area. A good place to see blocky basalt and volcanic glass (obsidian) is the Medicine Lake area of Northern California.
- Craters - these result when an explosive type of eruption occurs. In some cases, the top of a mountain is blown off, creating a large caldera, as at Crater Lake, Oregon. Some very young and obvious volcanic craters can be viewed at Inyo Craters in Inyo National Forest, CA.
- Lahars - a deposit composed of various types of debris, including volcanic ash and mud, which results when a volcanic eruption encounters water, either by melting snow or through rainfall. The water mixes with the volcanic ejecta and creates a mudslide or mudflow, which is often more destructive than lava alone would be. A well-exposed lahar deposit can be seen on the Trail of the Gargoyles in Stanislaus National Forest in California.
- Lava tubes - mostly round channels underground which were created by flowing lava. As the part of the lava exposed to air cooled, it created a chamber through which the internal lava still flowed and emptied out of, leaving a tube. One of the best places anywhere to explore lava tubes is Lava Beds National Park in northeast California.
- Cinder cones - a very distinctive hill or mountain that is often sparsely vegetated. In a young volcanic cone, the material that makes it up (cinders) is loose, making it hard for plants to become established. Cinder cones are very common and often are found in clusters, as in the Coso volcanic field in eastern California. One of the easiest and most entertaining cinder cones for the hiker is Amboy Crater in the Mojave Desert.
Ubehebe Crater in Death Valley
Geothermal Activity - water heated by magma below ground escapes to the surface in a variety of forms, giving evidence that magma is relatively close to the surface in those hot spots.
- Hot springs - warm or hot water emerging from the ground. Naturally-occuring hot pools have traditionally been used as public baths or spas.
- Geysers - the hot water is under such pressure underground that it erupts into the air at intervals. Yellowstone National Park, of course, is your best bet for seeing a geyser erupting, but a very active and predictable geyser can be seen in Calistoga, CA.
- Fumeroles - a hole or vent from which steam or vapors escape. A fumerole is like a geyser except that it emits steam instead of water.
- Mud Pot - a hot pool of mud which often bubbles and spurts as steam bubbles escape. These can be very colorful if the right minerals are present. A great place to see mud pots and fumeroles is Bumpass Hell Trail in Lassen National Park.
Caves - a cave is simply a hole in the ground which may or may not have an opening to the surface. Naturally-occurring caves are formed by lava (see lava tubes) or water.
- Solution - these are the beautiful show caves that contain crystal formations like stalagmites and stalactites. The limestone or marble is hollowed out over time by underground water and the formations are created by dripping water depositing minerals on the ceiling and walls of the cavern. California has several beautiful show caves, including Black Chasm Cavern.
- Sea caves - these are created by waves punching into the shoreline. An excellent example is Sea Lion Caves near Florence, Oregon.
- Wind caves - they are called wind caves even though they are created mostly by water erosion. These are usually shallow and are not true caves. A great place to play in them is at Rock City on Mt. Diablo in central California.
Fossils - the evidence left behind by plants and animals that lived long ago.
The hard parts, and sometimes softer parts, were covered by sediment and turned
to stone over a long period of time. The locations and types of fossils found
in a particular place can give science invaluable information about the history of the place.
- Plants - petrified trees or leaves encased in sedimentary deposits can tell us what kind of environment existed at the time the plant lived. You can hike through a petrified redwood forest in Calistoga, CA.
- Animals - skeletal remains of animals that no longer exist provide important clues in piecing together the evolution of a species. They can also tell you what the animal ate, how it moved, etc.
- Footprints - tracks and foot impressions made in ancient mud preserve a record of a creature who passed by.
Geologic Terms Glossary
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