On entering a little grove of box elder and cottonwood trees, they saw around a corner a vast chamber cut into rock, two hundred feet high by five hundred feet long by two hundred feet wide. "And this," Powell marveled, "is all carved out by this little strem, which only runs during the few showers that fall now and again in this arid country"

p208 Edward Dolnick Down the great unknown - John Wesley Powell's journal of Discovery and Tragedy through the Grand Canyon - Harper Collins Publishers New York 2001

Geomorphology n. The study of the evolution and configuration of landforms.¹

In other words, geomorphology is the study of the land around you and how it gets its shape. This seems to be a simple concept. Yet imagine how long it takes to build a single skyscraper or a rail road or freeway. What about tearing down a building and removing all the debris?

Consider the amount of dynamite used to bring down the Kingdome in Seattle? Now imagine mountains 20 times taller than the highest skyscraper and if all you had was water and climate to achieve the same thing.

Fortunately - with geological events you have time. That is time to create canyons that are over a mile deep and time to lift mountains 5 miles up.

There are all sorts of things that are involved in shaping the earth's surface. What goes on beneath the surface as well as what happens once a structure is exposed to erosional forces? Tectonics and uplifting forces may involve crashing plates, faults, magma or simply ice wedges to force hills, elevated plateaus, bumps in the road and mountains to form. Rocks and the earths crust are affected by continental and local "rebound" caused by removing surface weight allows for land to float up.

Rain, streams, rivers, ice, chemical reactions, biological agents (e.g. microprobes, earthworms and overturned trees), wind, solar energy and even gravity help remove and shape what we eventually see. All of these are slowly working to erode rocks and move soil and sediments. Sometimes more catastrophic events such as glacial floods, meteorite impacts, collapsing volcanic chambers and earthquakes have an immediate affect on the landscape.

A complete list and definitions of all of the forces and shapes that result as well as the explanations of how they are formed is significant. More important - how these affect how humans, animals and plants live and have adapted is amazing.

The next time you are walking, or driving, riding your bike, skiing or even looking out you window, look around you and start to ask yourself "Why do things look like they do?" and "How did they get there?" In other words, study of the land around you and how it gets its shape.

The following is a short description of one landform prevalent in the Ruby Valley in Sheridan Montana.

Sheridan sits on top of several alluvial fans

Alluvial Fans

Alluvial fans are unconsolidated sediments and rocks created by stream deposits and debris flows. Deposits may be locally sorted but are generally unsorted. Locations are generally along mountain fronts where a stream emerges.

The Overall morphology of an alluvial fan consists of a fan head at the top, an intersection point and the fan lobe. The aerial profile is usually fan shaped with the narrow end emerging from a mountain front. Looking towards the mountain the shape appears to form a low arc.  The intersection point of the fan is the point at which streams are no longer incised and streams tend to be braided or shows signs of tributaries.

Fan sizes range from several kilometers to in some cases up to 100 kilometers. Steepness or gradient is dependent on source materials (debris size) and the cause of deposition (amount and intensity of water discharge).

Alluvial Fan Formations

There are two main forms of deposition which create the alluvial fans and one minor. The primary forms of deposition include those caused by streams and those caused by debris flows. The third factor involved is caused by sieve deposition.

Alluvial fans formation requires accumulation of sediments/debris in the watershed above the fan, and large influxes of water to transport the material.  The transporting mechanism is either by debris flows or streams.

Debris flows have a high viscosity, carry higher loads and primarily occur only above the intersection point.  Studies indicate that debris flow contribution to alluvial fan build up is higher in areas of lower precipitation - around 11 inches, where contribution may be upwards of 40%. They only make up about 5 - 10 % in areas where rainfall exceeds 19 inches. Other factors influencing debris flow is the topography and the Lithology of the surrounding area. Having clays and silt are important components in creating debris flows.

As stream flows can adjust their sediment load based on stream power, they generally flow as long as slope exists. For fan development, high water influxes, readily available transport loads and over bank flooding are needed.

In the case where the fan soil is highly porous streams may vanish and "sieve deposition" occurs. The water carrying the material flows underground and deposits any load (that is not in solution) on the surface.

To find out more on how the Sheridan Alluvial fan might have been formed - you can read a paper on it by clicking here. Alluvial fan development in Sheridan Montana

The alluvial fan being studied is located near Sheridan, Montana and appears to be the result of three major streams Wisconsin Creek (southernmost), Indian Creek and Mill Creek (northernmost) creating a combined fan. The fan is approximately 8.5 kilometers long and is constrained by what appears to be earlier landforms. The location is in the Ruby valley (45° 27' 41 N 112° 12' 03 W)