SOIL PEDESTALS

Early Indicators of Erosion

by Greg Geehan, Spring 2019

Photos & video by John F. Williams except where noted

Soil Pedestals

Early Indicators of Erosion

By Greg Geehan, Summer 2019

Photos &  video by John F. Williams except where noted

what does the rain hit?

​Rain nurtures Salish forests, and the forest structure ensures that the effects of even heavy rainstorms are not damaging.  Tree leaves, needles, branches, and bushes interrupt fast-moving rain drops, cushioning their impact.  The soil underneath receives a soft soaking that filters down into the ground. From there it is either taken up by roots, or it sinks further down to feed aquifers and the headwaters of rivers and streams.

But when hard rain comes down on bare ground, soil erosion is the destructive result. A forest floor can be exposed in different ways.

Foliage like this Oregon grape absorb much of the raindrops’ energy, so the drops reach the ground much more gently.
For more about the water cycle, see After the Rainbow in this issue

Fires are a typical natural cause.  But the worst effects happen where humans completely rework the ground to eliminate all roots and plant debris. When hard rain comes down on bare ground, soil erosion is the destructive result.  ​Examples include building sites and dirt roads — but also cultivated fields.   Soil erosion is the bane of agriculture and farmers keep a sharp eye out for it, especially after a hard rain.

What would a farmer look for as evidence of soil erosion? Rills and gullies are obvious indications.  But the ground can begin to erode even before channel rivulets form.  Erosive water can flow in sheets. 

A rill network courtesy of Dr. Donal Mullan, Creative Commons

splash erosion

Illustration by Lesley Dampier, courtesy of UBC Centre for Teaching, Learning and Technology

Even the raindrops themselves can start the process by what is called splash erosion.  Imagine droplets hitting the ground like mini meteorites and blasting soil or sediment particles up into the air.  On a slope, more of those particles fall downhill from the impact point.  That form of erosion can be difficult to detect.  If the ground is composed of particles that are all about the same size, the sloping ground may look the same, even though sediment has moved downhill.

For more about raindrops, see Shape Up, Raindrop! in this issue

pedestal erosion

But what happens where the ground surface has different sized particles, including some that are too big to be dislodged by even the largest raindrops?

In that condition, the larger particles stay put while the smaller bits move downhill. The result is small rocks sitting on pedestals of preserved sediment. Those small pinnacles of sediment, protected by small rocks, are a clear indicator of raindrop splash erosion. They are often seen along the edges of cut banks on primitive roads. Their formation is called pedestal erosion.

Here’s an example of pedestal formation on the scraped side of a service road.

The yellow square shows the area of the close-up in the next two photos.

In this closeup it’s easy to see the small pebbles as caps on pedestals.

This side view helps show the shape of the pedestals and the slope

An even closer look — the fir needles provide a sense of scale.

Here is a couple of larger stones beginning to make pedestals.

ladies with hairdos

Hoodoo near Wahweap Creek, Page, Arizona by Wolfgang Staudt commons.wikimedia.org

Bryce Canyon Hoodoos by Jon Zander commons.wikimedia.org

Raindrop pedestal erosion features are similar to much larger sculpted landscapes common in arid areas. In the desert southwest, they are called hoodoos. The French call them demoiselles coiffees — ladies with hairdos.

The formation of these landscapes is broadly similar to raindrop pedestals. Easily eroded sediment is washed away except where protected by a hard ‘cap.’

However, hoodoos can be formed by a wide variety of erosive processes. They often begin as fractures in an overlying bed of hard rock. The cracks are widened by freeze-thaw cycles and then running water. Ultimately, little of the overlying bed remains; only the hoodoo caps.

Greg Geehan grew up in Tacoma, graduated from the University of Washington, and earned a PhD in Marine Geology at Scripps Institution of Oceanography. He then worked in the oil industry for over 20 years in a career that took him to Houston, San Francisco, Alaska, London, Kuwait, and Colombia. Upon retirement, Greg and his wife Kathy returned to the Puget Sound area and chose to live on Bainbridge Island partly because of its reputation for supporting education and environmental conservation. Greg is a docent at IslandWood, a member of the Bainbridge Island Kiwanis club, and a board member of the Bainbridge Island Land Trust. He is co-author and narrator of the video Geological Formation of Bainbridge Island, produced and directed by Cameron Snow.

Table of Contents, Issue #3, Spring 2019