We received a tip yesterday that a landslide recently became active, probably in the storms that blew through the area in mid to late November. The landslide occurred in a quarry that has been inactive for some time. The highway south of the quarry is State Route 112.

The quarry is an old clay mine that use to supply clay to the cement mills in Seattle. It is actually still owned by a cement company that has a working plant in Seattle. The report we got was the landslide had rocks that were actively rolling town a scarp and some of the pictures we received are amazing.

Photo by Dave Parks, Department of Natural Resources

Photo by Dave Parks, Department of Natural Resources

The good thing, so far this landslide isn’t threatening any houses, pipelines, powerlines, or such. If it starts progressing uphill, it could eventually impact State Route 112, but at the moment it seems unlikely. The area of the site is part of the Miocene-Oligocene marine sediments of the Pysht formation of the upper Twin Rivers Group, which is locally interbedded carbonaceous and fossiliferous claystone, siltstone and some fine grained sandstone. A clay layer within the Pysht formation at the site was mined for cement manufacturing in Seattle. Also the Burke Museum has a whale skeleton (Miocene) from this quarry, probably by Jim Goedert. The Pysht formation also contains many near shore fossils, like bivalves and gastropods. So, we will keep our eyes out for another whale skeleton (although we might need a bigger truck). Before you think this would be a great place to head out, know that this area is extremely dangerous. Rocks are currently raveling down the scarps and toe of this landslide and sections of the landslide can move and give way at any moment. It isn’t worth your life to try and collect fossils here. The ground is torn apart and you could easily fall into a covered (or uncovered) crevasse and get trapped. Sudden movement of the landslide or the snapping of tree roots could knock over trees without warning. This area is also private property and no tolerance will be given to anyone caught in this area, especially given the extreme danger that this area faces. If you would like to collect fossils from the Pysht Formation (or anywhere like that), contact the DNR Geology and Earth Resources Division, they can help you find safe and legal locations to collect on.

Advertisements

It has been awhile since I completed a Landslide of the Week. I think the Sanford Pasture Landslide is a good candidate since it has gotten so much press lately and what we know about it is fairly limited (at least, in publications).

The formation of the Sanford Pasture Landslide started back in the late Miocene and early Pliocene epochs, where the eruptions covered much of Eastern Washington with basalt, known as the Columbia River Basalts. Between the eruptive cycles, sandstones, generally fluvial in origin, deposited on top of the flows, only to be covered by the next pulse of magma. These are known as interbeds and are suspected to be Ellensburg Formation. At the Sanford Pasture Landslide, the dominant flows of the Columbia River Basalts are the N2 and R2 flows of the Grand Ronde Basalts, some of the last recorded flows of the eruptive cycle. Much of the deposits were lain horizontally, but as we know them today, the geologic units are folded and faulted. This is accomplished by stress from the subductive oceanic plate pushing its way underneath the continental crust that we live on here in Washington State. The force of the collision compresses Washington State, forming wrinkles and faults as the stress is dissipated through the plate. In the Naches area, this folding resulted in the formation of Cleman Mountain as a steeply dipping anticline. The area was not able to just fold to reduce the stress on it, it faulted as well, forming the Nile Thrust Fault. The failure mechanism is something that we probably do understand. The oversteepened anticline combined with the weak interbed layers of sandstone created a perfect weak plane for the above rock to slide on. An earthquake, probably on the Nile Thrust, or perhaps something larger like a Cascadia Subduction Earthquake, probably reduced the restraining forces enough to start the material moving downhill, depositing where we see it today (more on that below). These events occurred after the Columbia River Basalts and interbeds were lain in place, giving us a limiting age on the landslide. Given the flow age, coupled with the folding and faulting of the area, the general estimation of the landslide is 2 million years old.

Determining the age of a landslide is often difficult. Dates can be acquired through a couple of different methods, most often coring into sag ponds, or lake bed deposits (on older landslides that have dammed rivers), or by coring old tree snags that have been drowned. The goal is to find datable material or stratigraphic reasoning to determine a specific of general age. For the Sanford Pasture, there are no found lake bed deposits up valley of the landslide initiation and the landslide is too old to support sag ponds that formed during its initial movement. The general thought is that the landslide occurred prior to glacial times.

The Sanford Pasture landslide moved across what is today the Naches Valley and deposited material almost a mile inward from the valley’s edge. During this time, the Naches Valley was less incised and contained much less water (remember, no lake beds deposits), so whatever damming of the paleo-fluvial system here, it was minor. During the age of glaciation in the Quaternary Period (predominantly alpine glaciation influences at the Sanford Pasture). Advances and retreating of the glaciers, combined with their constant run-off carved much of the valleys and fluvial systems we see today in the area. I should point out, I don’t think any glaciers have reached the Sanford Pasture Landslide area. The melt water flowing through what is now the Naches Valley would have eroded out the landslide and continued to incise into the valley, exposing in-place Columbia River Basalt Flows on the western side and eastern side of the valley. Unfortunately, all of this erosion created yet another unstable element into the system. The eroding river removed much of the lateral strength that the landslide had when its mass continued for another mile. It literally shortened the landslide by half. In response, the Sanford Pasture landslide didn’t fail as one large piece, but as smaller failures within the older landslide material.

This image of the Sanford Pasture Landslide is a quick drawing of the possible major landslide events. There are dozens of smaller events throughout the landslide. The most difficult part to figure out is the northwest section of the landslide, that appears to have gone through a series of deformations, probably more than I have drawn here. That is something we are going to try and unravel down the road. It is difficult to determine if the last major movement was on the eastern or western section of the landslide. The only sag pond that exists on the landslide is on the eastern side, known as Dog or Mud Lake. This makes me suspect that the last major movement has been on the eastern side. Other evidence also suggests that the morphology is younger, less stream development and incision on the eastern side. Regardless, the western side is the side where the Nile Landslide initiated off of and probably has a much more active, smaller landslide activity.

The area where the Nile Landslide has occurred has experienced several large landslide events. Looking at the history, the Nile Landslide is probably the 4th in a series of movements in the area (Sanford Pasture, Largest block in purple, smaller block in green, then Nile Landslide). That is the larger movements. Further evidence looks like smaller landslides have been recent in the same area as the Nile, maybe being able to form and move every couple of hundred years (not sure how far back this might go, but maybe a thousand or two years, depending on when the major movement of the largest block in purple and smaller green block occurred). Granted, that is a bit of speculation. In the 1940’s photo, there is clearly areas without vegetation that look hummocky that might indicate recent movement, like within the last 50 years. Comparing that 1940’s photo to today, areas that were once void of vegetation now are supporting sparse tall trees, indicating a possible regrowth period. Maybe we are looking at something that is geologically common here.

The last work, Sanford Pasture Reactivation. This has been pushed around in the media about State Geologists concerned about future movement of the Sanford Pasture Landslide. They are right, we are concerned, I being on of them. The removal of lateral support by the Nile Landslide could reactivate something larger uphill. Remember, this is really torn up landslide material, it has its strength reduced and it looks like it is sliding on something that is fine grained. Reactivation of the Sanford Pasture Landslide, worst case scenario, would completely block the Nile Valley, forming a massive lake (Lake Naches?) behind the debris. The threat would then continue into the competency of the material to hold the water, a race to safely dewater the lake and the possible major dam-burst flood into the Yakima Valley. The destruction of that last one would be unlikely, but something we have not seen the likes of in modern society.

Seattle-area news Video On Demand | Seattle News, Local News, Breaking News, Weather | KING5.com

Posted using ShareThis

It is a strange thing to see how TV works from the production side. The interview I had was maybe 15 minutes or so, but it all boiled down to about 10 seconds of film on air. I am glad to see that this landslide is getting the attention it deserves. This is a major event in Washington’s history, at least, landslide history.

Features of the Nile Landslide

Features of the Nile Landslide

This is a generalized map of the different features present in the Nile Landslide. The main landslide mass is located in red and deformation at its toe is yellow. The salmon color block is an area where cracks were noticed along the hillside. The orange is a block of material that was pushed and cracked by the Nile Valley Landslide main landslide body. The diverted Naches River is blue in color. These offer a representation and a general boundary of the features. More detailed delineation will be conducted once the landslide is safe to work on.

The Nile Landslide is part of a much bigger landslide system. The Sanford Pasture Landslide is perhaps the second largest landslide in Washington State, it is about 6 miles long an over 2 miles wide. The Malaga (aka Stemilt) landslide is the largest landslide in Washington State.

Sanford Pasture Landslide

Sanford Pasture Landslide

That little red blob at the bottom left of the map, that is the Nile Landslide that just occurred. The Nile Landslide is actually on a secondary failure (probably resulting some point after the initial movement of the bigger landslide), probably on the toe of that landslide. The worry is that the removal of mass on the toe might weaken the upper material by the removal of lateral material. There is a lot of rock up there and if the landslide propagated up or actually triggered a larger block up slope, the result could be the entire valley filling up with sediment. Bare in mind that there is a very low chance of this occurring, but it is something we need to checking out.

The age of the Sanford Pasture Landslide is unknown, but probably Holocene. The landslide, when it moved, crossed the valley and flowed up onto the other valley wall. This might have been done is a rapid, catastrophic failure. The landslide probably formed lakebed deposits upstream and sometime in the near future, we might be able to date this landslide.

I will heading out into the field again soon. I will try and keep you all updated. Most of our photos will be posted on our DNR flickr site located here:

Western edge of the Nile Landslide

I am going to try and post some new photos either tonight or early tomorrow morning.

Landslides can be destructive, destroying houses, infrastructure, and kill or injure people. However, we don’t usually think about landslides being bad for your health.

Washington State has a complex geology. Much of the western Cascades is made up of accreted terrains , composed of both oceanic and continental rocks. Parts of these terrains contain asbestos (which occurs naturally, despite a relatively large number of people believing it artificial). Asbestos is a fairly blanket term for a wide variety of minerals, some harmless, some very dangerous. The most well known example of the dangers of asbestos can be seen in Libby, Montana, where vermiculite mining with occurrences of fibrous tremolite asbestos caused widespread health problems and death for many of the residences and workers.

In Washington State, asbestos outcrops across the state. Most of the outcrops are small, uneconomical to mine or develop and probably pose little danger with limited exposure. However, some larger deposits occur in Snohomish, Skagit, Whatcom, Kittitas, and Klickitat Counties. These deposits can cause weakness within rocks and are sometimes associated with weak, friable material, places where we would expect landslides to occur. The prime landslide that contains asbestos in Washington State is the Swift Creek Landslide in Whatcom County. The landslide material is composed mostly of serpentinite, a friable, weak rock in terms of stability with high amounts of chrysotile. Its origin was probably an uplifted oceanic plate that was probably composed of ultramafic material, such as dunite that was then metamorphosed and transformed into serpentinite. The landslide has produced a significant amount of material which has been transported downhill into the valley below, depositing chrysotile laden sediments. These sediments, especially during flood events, deposit in places where people can come into long-term exposure, which can result in long-term health problems.

Swift Creek might be the most well known landslide to contain asbestos in Washington State, but since asbestos occurs throughout Washington State, many other landslides have the potential to contain asbestos. This map represents deep-seated landslides that have the potential to contain asbestos within them.

Washington State Asbestos-DSLS Map

Washington State Asbestos-DSLS Map

This map is not a perfect representation, as available data is scarce. The map was created by overlaying identified asbestos occurrences found in Bulletin No. 37, Inventory of Washington Minerals (Valentine and Huntting, 1960) with the 100k geologic units (with slight modification on unit selection). The units that were identified with asbestos occurrences were then intersected with deep-seated landslides from DGER Washington’s Statewide Landslide Database (the database is located within the menu). These deep-seated landslides are of all ages, from relict to active. Points were then selected at the centroid of the polygons to create a point file of landslides that potentially contain asbestos materials. It isn’t a perfect method by any means, but it at least gives us an idea that more of these landslides probably exists throughout Washington State. I am in the process of intersecting the landslide layer with ultramafic units known to contain serpentinite, which will help expand and potentially more accurately capture landslides potentially containing asbestos.

Reference
Valentine, Grant M.; Huntting, Marshall T., reviser, 1960, Inventory of Washington minerals; Part I–Nonmetallic minerals; 2nd edition: Washington Division of Mines and Geology Bulletin 37, Part I, 2nd ed., 2 v.

I am often puzzled by what information has never been put together before. I met with the Emergency Management Division today to discuss how we can better communicate landslide information to each other, among other things. Before the meeting started, I wondered what information might be most useful to these individuals I was meeting with and thought, well, an active DSLS map might help open their eyes. Well, it would have, if that information existed. I looked at our landslide database and found that it wasn’t so easy to pluck out the active landslides or even the better known landslides. So, I entered a new attribute that should show up during the next update for Landslide Name (attribute code: NAME), so someone could look up a well known landslide. I was also surprised to find so many of the well known landslides have never been added to the database. I was struggling to find the locations of a few of these landslides, such as the Everson Landslide that occurred on February 8th, 1997. That landslide impacted a natural gas line, resulting in a large explosion. Luckily, no deaths were reported (I could see a heart attack from this event).

Active or Recent Deep-Seated Landslide Map, Washington State

Active or Recent Deep-Seated Landslide Map, Washington State

The map doesn’t include a legend (or scale for that matter). The deep-seated landslides that are active or recent are highlighted in blue. The red polygons are landslides that are in the database, but are either shallow or landslides that are not active or recent.

So, I have decided to put together a active deep-seated landslide file and hope to cover most of the well known landslides we all know and love.