DNR Geology Update

May 29, 2009

We are moving forward after our budget cuts and are in the process of reorganization. We lost a few good people and some great people (most will probably be moved around to another position in DNR), but it seems we are still keeping our core functions alive and well. This is nothing new for the Geology Division, as we have had a lot of up and down swings, but we have always been able to pull through.
Luckily, I am still working on landslides and I hope to continue to do so for many more years down the road. I completed a major update for the landslide database and it should be up on our ArcIMS site soon. If you would like a copy, please contact me at:
isabelle.sarikhan@dnr.wa.gov

I guess I am still catching up on landslides in the news. The Greenleaf Basin Landslides occurred sometime near or before January of 2008.

Greenleaf Basin Landslide in the Bonneville Landslide

Greenleaf Basin Landslide in the Bonneville Landslide

The landslide is within the scarp of the Bonneville Landslide (the famous Bonneville landslide that formed the Bridge of the Gods, coincidentally, and not to be confused from, the Bridge of the Gods, which connects Highway 14 to Highway 84 – Washington to Oregon). The Bonneville Landslide failed probably in conjunction with the 1700 Cascadia Earthquake.

Greenleaf Basin Landslide - Prefailure View

Greenleaf Basin Landslide - Prefailure View

The above orthophoto is a 2006 photo of the area where the Greenleaf Basin Landslide occurred. The landslide cut into the scarp and incorporated numerous mature trees (probably over 50 years old).

(The photos are from Don Nelson)

Landslide in distance, note the scarp of the Bonneville Landslide Complex

Landslide in distance, note the scarp of the Bonneville Landslide Complex

View of the scarp and material from the landslide

View of the scarp and material from the landslide

A clue to why this area is unstable, look at those springs!

A clue to why this area is unstable, look at those springs!

A good view of the woody debris that came down with the material, people for scale (in red)

A good view of the woody debris that came down with the material, people for scale (in red)

A closer view of the springs and change in lithology

A closer view of the springs and change in lithology

A close up view of the scarp

A close up view of the scarp

As you can tell in the pictures, the lithology of the landslide probably helped to create an unstable environment. The top layer is probably a Quaternary basaltic andesite flow (referred to as the basaltic andesite of the Cascade Landslide) which is on the Eagles Formation (a Miocene continental conglomerate) The springs are probably representing a perched layer of water and in December of 2007, we had quite a bit of rain. The average rainfall for the Bonneville area is 12.72 inches (for the month of December). In 2007, the rainfall was 19.45 inches. The beds are gently dipping to the south (most of the photos are looking north and the dip isn’t very apparent in them). The increase in pore pressure along the contact probably help to pry away the upper rock and combined with the dip, the mass of rock lost what resistive forces it had to stay in place.

An interesting side note. The layering of an incompetent over competent lithology, combined with a dipping bed, is probably a major component why the Bonneville Landslide Complex exists where it is. The Stevenson Landslide (also known as the Rock Creek or Piper Road Landslide) is also on a similar situation.

Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Hornby Landslide, Glenoma Area, Lewis County

This is one of the more fascinating landslides that occurred during the January 2009 storm. Numerous debris flows and avalanches dotted the slopes above Mark and Jon Hornby’s farm.

Map of landslide in Glenoma Area, Lewis County

Map of landslide in Glenoma Area, Lewis County

Hornby Landslides

Hornby Landslides

Series of Events

At about 9:00 am on January 8, a large debris flow moved into Mark Hornby’s farm pasture, plugging a culvert and covering it with mud and debris. About a half an hour later, another landslide came off the slope, nearly striking Mark and his brother, Jon. I think this landslide also struck a bull and carried it a ways across the pasture. There full story can be found here.

View of debris flow deposit near house; DGER/DNR Photo

View of debris flow deposit near house; DGER/DNR Photo

The debris avalanche/flow in the middle of the clear cut slope is very shallow, less than 2 feet of incision in many places. The landslide incised down to bedrock in most places, which was probably why the landslide was so shallow. When it reached the pasture, the landslide turned into a short debris flow and then transformed into a hyperconcentrated flow and made its way across the Hornby’s Farm pasture. The landslide ponded against Highway 12, flowing to the Hornby’s driveway and then onto Highway 12. One of the oddities discovered in Glenoma was that many of the hyperconcentrated flows that reached the valley floor were entrapped into roads by snow berms from plowed snow.

Hornby Landslides; DGER/DNR Photo

Hornby Landslides; DGER/DNR Photo

January 7-8 Storm Summary

In December of 2008 and into January of 2009, cold air from British Columbia created an ideal condition for snowfall across Washington State. Snow accumulations preceding the storm were low in the Puget Lowland, with at only inches on the ground in most places. On January 7, a stream of moisture originating from around Kauai (Hawaiian Islands) flowed into Washington State, bringing warm temperatures and high amounts of rain, rapidly melting what snow remained in the lowlands and eating away at the snowpacks in the mountains. By January 8, the largest evacuation in the state’s history was under way, forcing more than 30,000 people living in the Puyallup River area to flee. The town of Orting, with a population of more than 26,000, was almost completely flooded. For the second year in a row, flood waters closed Interstate 5 in Centralia/Chehalis. In the rest of the state, rivers were also flooding—the Stillaguamish, Snohomish, Chehalis, Naselle, Hoko, Cedar, and Cowlitz were the most significant, peaking above the 100-year flood level.

King and Snohomish County were least affected, as a rain shadow from the Olympic Mountains shielded their low-lying areas. In the Puget Lowland, rainfall totals ranged from 1.5 inches in Seattle to 5 to 7 inches in southwest Washington and 3 to 6 inches in the northwestern counties. As the storm progressed into the Cascades, the higher elevation forced the clouds to release water as they moved over the mountains, leaving more than 20 inches of rain in two days. The rainfall saturated slopes, many already wet from melting snow, triggering debris flows and debris avalanches throughout most of western Washington. Areas sensitive to high-intensity storms, such as Glenoma, Concrete, and Van Zandt, were the site of numerous large debris flows, blocking roads, limiting emergency response, and destroying homes. In the end, more than 1,500 landslides were reported or recorded from Washington Division of Geology and Earth Resources (DGER) field and aerial surveys.

Logging and Landslides

DGER and AEG hosted a field trip in the Glenoma Area (field trip guild). The purpose of this stop was to discuss logging and landslides. Unfortunately, the conversation never got going very well. It certainly caught my attention when I was looking over the photos coming in. One of the first things that caught my attention was the prominent deep-seated landslide on the west side of the clearcut. It is difficult to see if it is active from a photo, but when I first saw it from the aerial photo I thought it probably had some recent movement (within the last 100 years, maybe?). In the subsurface, there are places of thick, mostly unconsolidated pumice. So, is this logging related? It is possible. Was it illegal? Probably not. I didn’t do any detailed ground survey of this area, but just at a general glance, I cannot think of any forest practice rules that they might have broke. Maybe we need to look at if the FP rules are protecting our slopes, especially in Lewis County.

One of the more interesting landslides I have ran across was a debris flow triggered during the December 3rd, 2007 storm west of Pe Ell, Lewis County. Kelsay and I were conducting a reconnaissance of SW Washington to try and find out just how bad the landslides were. We pulled up through Pe Ell to find two debris flows that had come across State Route 7 and surrounded a house.

Pe Ell Debris Flow - Impact to House

Pe Ell Debris Flow - Impact to House

We parked and I decided to hike up the western debris flow as Kelsay went to look at the eastern debris flow. Unfortunately, I didn’t see the ditch that was covered in mud and quickly went up to my hips in mud (and since I do all my field work in a skirt, that was about as awful as it got!).

Western Pe Ell Debris Flow

Western Pe Ell Debris Flow

Past falling in the mud, the hike up was fairly easy and the scarp was amazing. A thin layer of soil and dirt has slid off of a hollow area (not to be confused with the DNR Forest Practices definition of a bedrock hollow), that is a volcanic tuff.

Western Pe Ell Debris Flow Scarp

Western Pe Ell Debris Flow Scarp

The soil is between 6 inches and two feet, depending on which side of the scarp you are at. The convergent topography with intense precipitation probably greatly contributed to the landslide moving. The rainfall in this area was probably between 16-20 inches during the storm (the majority falling on December 3rd). Intense rain + shallow soils + impermeable substrate = landslide. Actually, that is the formula we saw again and again for almost all landslides during the December 3rd storm.

The other interesting thing to point out, the area was recently harvested. The lack of canopy coverage can increase the rate that rainfall will reach the ground (from a timed delay to no delay). On weak storms and wet winters, this could increase landslide activity, but we haven’t seen it very many compelling cases around Washington State (but there is a nice study from Canada). However, during the December 3rd storm, the intense precipitation and lack of canopy might (and I will go out on a limb and say almost certainly) have increased landslide activity. To what extent and what increase, that remains to be seen.

Laszlo Toth

May 21, 2009

Today in geology weirdness news – in 1972 (May 21st), Michelangelo’s statue Pieta in St. Peter’s Basilica in Rome was vandalized by a Hungarian geologist named Laszlo Toth.

From Wikipedia:
“Wielding a Geologist’s hammer and shouting, “I am Jesus Christ — risen from the dead”, he attacked the statue, and removed the Virgin’s arm at the elbow, knocked off a chunk of her nose, and chipped one of her eyelids. He was never charged with the crime, in view of his apparent insanity. On January 29, 1973, he was committed to an Italian psychiatric hospital. He was released on February 9, 1975, and was immediately deported to Australia where he had studied prior to the attack; Australian authorities did not detain him. He is believed to reside in Melbourne, Australia.”

Michelangelos Pieta

Michelangelo's Pieta

This immortal act has spawned into American culture. He inspired at least two books by Don Novello, and was used in the development of the character Father Guido Sarducci, who appeared frequently on Saturday Night Live.

The attack did shed some positive light. The Vatican announced that in repairing the statue, a previously unknown monogram or secret signature of Michelangelo – an “M” fashioned from the skin lines – was discovered on the palm of Madonna’s left hand.

We tried getting into the Pilchuck headwaters yesterday to no avail. Roads were washed out and gates were locked. We did see a number of landslides on the Sultan Basin Road heading up to Olney Pass. These landslides bare the mark of a large rainstorm event and almost certainly moved during the January 7-8th, 2009 storm event

Sultan Basin Road Landslides

Sultan Basin Road Landslides

The picture has a backdrop of 2003(? I think it was later than that) Snohomish County LiDAR.

The most interesting of these landslides is a debris avalanche at the bridge crossing at Olney Creek. It was probably dealt a one-two punch, the swollen Olney Creek was probably eating away at the bank (and probably has been for years) and the saturated ground allowed enough driving forces to overcome the resistive forces. It also moved a good amount of timber into the creek, which might cause a problem down the road by creating a debris dam behind the bridge.

Sultan Basin Rd Debris Avalanche.  Photo by Carol Serdar

Sultan Basin Rd Debris Avalanche. Photo by Carol Serdar

Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Mt. St. Helens Debris Avalanche, Skamania County

Today is May 18th and it seems fitting that the Mt St Helens Debris Avalanche would be the landslide of the week. The Mt St. Helens Debris Avalanche is perhaps one of the most well known landslides in the world.

Area: 23 sq.mi.
Volume: .67 cu. mi.
Age: 29 years

Mt St Helens Debris Avalanche

Mt St Helens Debris Avalanche

An Excerpt from: REPORT: Eruptions of Mount St. Helens: Past, Present, and Future

“May 18, a Sunday, dawned bright and clear. At 7 a.m. Pacific Daylight Time (PDT), USGS volcanologist David A. Johnston, who had Saturday-night duty at an observation post about 6 miles north of the volcano, radioed in the results of some laser-beam measurements he had made moments earlier that morning. Even considering these measurements, the status of Mount St. Helens’ activity that day showed no change from the pattern of the preceding month. Volcano-monitoring data-seismic, rate of bulge movement, sulfur-dioxide gas emission, and ground temperature-revealed no unusual changes that could be taken as warning signals for the catastrophe that would strike about an hour and a half later. About 20 seconds after 8:32 a.m. PDT, apparently in response to a magnitude-5.1 earthquake about 1 mile beneath the volcano, the bulged, unstable north flank of Mount St. Helens suddenly began to collapse, triggering a rapid and tragic train of events that resulted in widespread devastation and the loss of 57 people, including volcanologist Johnston.

Debris Avalanche
Although the triggering earthquake was of slightly greater magnitude than any of the shocks recorded earlier at the volcano, it was not unusual in any other way. What happened within the next few seconds was described by geologists Keith and Dorothy Stoffel, who at the time were in a small plane over the volcano’s summit. Among the events they witnessed, they

“noticed landsliding of rock and ice debris
in-ward into the crater… the south-facing wall
of the north side of the main crater was especially
active. Within a matter of seconds, perhaps 15 seconds,
the whole north side of the summit crater began to
move instantaneously. … The nature of movement
was eerie…. The entire mass began to ripple and
churn up, without moving laterally. Then the entire
north side of the summit began sliding to the north
along a deep-seated slide plane. I (Keith Stoffel)
was amazed and excited with the realization that
we were watching this landslide of unbelievable
proportions. … We took pictures of this slide
sequence occurring, but before we could snap off
more than a few pictures, a huge explosion blasted
out of the detachment plane. We neither felt nor
heard a thing, even though we were just east of the
summit at this time.”

Realizing their dangerous situation, the pilot put the plane into a steep dive to gain speed, and thus was able to outrun the rapidly mushrooming eruption cloud that threatened to engulf them. The Stoffels were fortunate to escape, and other scientists were fortunate to have their eyewitness account to help unscramble the sequence and timing of the quick succession of events that initiated the May 18 eruption.

The collapse of the north flank produced the largest landslide-debris avalanche recorded in historic time. Detailed analysis of photographs and other data shows that an estimated 7-20 seconds (about 10 seconds seems most reasonable) elapsed between the triggering earthquake and the onset of the flank collapse. During the next 15 seconds, first one large block slid away, then another large block began to move, only to be followed by still another block. The series of slide blocks merged downslope into a gigantic debris avalanche, which moved northward at speeds of 110 to 155 miles an hour. Part of the avalanche surged into and across Spirit Lake, but most of it flowed westward into the upper reaches of the North Fork of the Toutle River. At one location, about 4 miles north of the summit, the advancing front of the avalanche still had sufficient momentum to flow over a ridge more than 1,150 feet high. The resulting hummocky avalanche deposit consisted of intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake. Covering an area of about 24 square miles, the debris avalanche advanced more than 13 miles down the North Fork of the Toutle River and filled the valley to an average depth of about 150 feet; the total volume of the deposit was about 0.7 cubic mile. The dumping of avalanche debris into Spirit Lake raised its bottom by about 295 feet and its water level by about 200 feet.

1983 Photo North Fork Toutle River

1983 Photo North Fork Toutle River


Mudflows and Floods

Volcanic debris flows-mobile mixtures of volcanic debris and water popularly called mudflows -often accompany pyroclastic eruptions, if water is available to erode and transport the loose pyroclastic deposits on the steep slopes of stratovolcanoes. Destructive mudflows and debris flows began within minutes of the onset of the May 18 eruption, as the hot pyroclastic materials in the debris avalanche, lateral blast, and ash falls melted snow and glacial ice on the upper slopes of Mount St. Helens. Such flows are also called lahars, a term borrowed from Indonesia, where volcanic eruptions have produced many such deposits.

Mudflows were observed as early as 8:50 a.m. PDT in the upper reaches of the South Fork of the Toutle River. The largest and most destructive mudflows, however, were those that developed several hours later in the North Fork of the Toutle River, when the water-saturated parts of the massive debris avalanche deposits began to slump and flow. The mudflow in the Toutle River drainage area ultimately dumped more than 65 million cubic yards of sediment along the lower Cowlitz and Columbia Rivers. The water-carrying capacity of the Cowlitz River was reduced by 85 percent, and the depth of the Columbia River navigational channel was decreased from 39 feet to less than 13 feet, disrupting river traffic and choking off ocean shipping.

Mudflows also swept down the southeast flank of the volcano-along the Swift Creek, Pine Creek, and Muddy River drainages and emptied nearly 18 million cubic yards of water, mud, and debris into the Swift Reservoir. The water level of the reservoir had been purposely kept low as a precaution to minimize the possibility that the reservoir could be overtopped by the additional water-mud-debris load to cause flooding of the valley downstream. Fortunately, the volume of the additional load was insufficient to cause overtopping even if the reservoir had been full.

On the upper steep slopes of the volcano, the mudflows traveled as fast as 90 miles an hour; the velocity then progressively slowed to about 3 miles an hour as the flows encountered the flatter and wider parts of the Toutle River drainage. Even after traveling many tens of miles from the volcano and mixing with cold waters, the mudflows maintained temperatures in the range of about 84 to 91 degrees (F); they undoubtedly had higher temperatures closer to the eruption source. Shortly before 3 p.m., the mud- and debris-choked Toutle River crested about 21 feet above normal at a point just south of the confluence of the North and South Forks. Another stream gage at Castle Rock, about 3 miles downstream from where the Toutle joins the Cowlitz, indicated a high-water (and mud) mark also about 20 feet above normal at midnight of May 18. Locally the mudflows surged up the valley walls as much as 360 feet and over hills as high as 250 feet. From the evidence left by the “bathtub-ring” mudlines, the larger mudflows at their peak averaged from 33 to 66 feet deep. The actual deposits left behind after the passage of the mudflow crests, however, were considerably thinner, commonly less than 10 percent of their depth during peak flow. For example, the mudflow deposits along much of the Toutle River averaged less than 3 feet thick.”

Interstate 90 through the Snoqualmie Pass/Hyak area is a very unstable area and has resulted in numerous rockfalls each year. WSDOT has conducted a Report to the Governor on Snoqualmie Pass (and other highways) and can be accessed here. According to the report, at least 5 fatalities have been caused by rockfalls on Interstate 90:

7/25/1970: 1
2/5/1998: 1
9/11/2005: 3
Source: WSDOT, Transportation Data Office, Collision Data and Analysis Branch

So, there shouldn’t have been much of a surprise when Interstate 90 was closed due to a rockfall. Here is an excerpt from WSDOT:

“Falling boulder closes eastbound I-90 at North Bend for five hours

Date: Tuesday, May 12, 2009

HYAK – Eastbound Interstate 90 closed to all traffic at North Bend earlier today. The road is now open. WSDOT crews closed the road at 2 a.m. after a large boulder fell off the hillside, bounced over the concrete jersey barrier and came to rest in the eastbound lanes. The boulder did not hit any vehicles, but a commercial truck struck the boulder.

WSDOT’s geotechnical experts climbed to the top of the hill to assess the situation and determined the road was safe to reopen at 7:15 a.m., Tuesday, May 12.

“We only opened the road when our experts told us it was safe for drivers,” said Paula Hammond, Washington State Secretary of Transportation. “Safety is our number one concern.”

An initial review of the site shows it is not on WSDOT’s list of unstable slopes. A more likely cause is the freeze thaw cycle in the area. …

“This can happen,” said Don Whitehouse, WSDOT Regional Administrator. “When you build a road through a mountain pass, rocks will fall. Our job is to make the area as safe as possible for drivers.”

Between 6 a.m. and 7 a.m. geotechnical experts hiked up the rocky slope, found two loose boulders, and pushed those onto the roadway below. Crews cleared the debris and reopened the road to all traffic.”

The boulder involved in the rockfall is quite small:

WSDOT Rockfall Highway 90

WSDOT Rockfall Highway 90

WSDOT Scarp of Rockfall Area

WSDOT Scarp of Rockfall Area

WSDOT Damage from Boulder

WSDOT Damage from Boulder

Luckily, no one was killed in this event, unlike the rockfall in 2005.

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.

Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Greider Lake Landslide, Sultan Basin, Snohomish County

Greider Lake is located in the DNR Morningstar Natural Resources Conservation Area (NRCA).

Greider Lake Landslide Location Map

Greider Lake Landslide Location Map

Area: ~366,000 sq. ft.
Width: ~1,400 ft
Length: ~3,000 ft
Depth: ~ 250 ft
Volume: ~1,050,000,000 cu. ft.
Age: Unknown
Trigger: Earthquake

The Greider Lake Landslide splits Greider Lake into two separate lakes, Little Greider Lake and Big Greider Lake. The lake is probably a tarn, which formed during alpine glaciation in this cirque.

Greider Lake Landslide Toe

Greider Lake Landslide Toe

Greider Lake Landslide Scarp

Greider Lake Landslide Scarp

Although it is difficult to determine, the landslide may have moved after the lake was formed.

Greider Lake Landslide_lidar

Greider Lake Landslide 2006 NAIP Orthophoto

Greider Lake Landslide 2006 NAIP Orthophoto

The landslide would have created a large seiche in the lake and probably formed a hyperconcentrated flow or a debris slurry as it moved down hill. The reflection ponds at the base of the hill might be a plunge pool formed by the seiche outwash of Greider Lake.

Greider Lake Landslide Plunge Pool

Greider Lake Landslide Plunge Pool

The red lines in the map represent landslides mapped in the Sultan Basin.

Glacial lake bed deposits form much of the valley floor where Spade Lake is located. There is a faint terrace that might have been deposited after a debris dam formed from the Greider Lake seiche outwash.

Geology of the Sultan Basin

Regional Geology

Regional bedrock that includes the Sultan Basin watershed belongs to the Western Mélange Belt, part of the Western and Eastern Mélange Belts (WEMB) terrain. The WEMB includes Mesozoic (late Jurassic to early Cretaceous) marine sedimentary rocks, along with lenses of Paleozoic limestones, Mesozoic intrusives, and other rock types in fault-bounded bodies that were tectonically juxtaposed (Tabor et al, 1993). The WEMB rocks underwent high pressure, low temperature metamorphism in the late Cretaceous orogeny at about the time they were juxtaposed against the Northwest Cascade System terrain to the North.

Local Geology

Bedrock in the Sultan Basin watershed is mainly composed of the Western and Eastern Mélange Belt (Phipps et al., 2003; Dragovich et al., 2002; Tabor et al., 1993). The oldest units in this watershed are derived from the Stillaguamish Ophiolite (a slice of oceanic crust that has been thrust onto continental crust) suite. Sedimentary rocks were deposited during the late Jurassic to early Cretaceous (170 to 100 million years ago) periods (Carithers and Guard, 1945). The older sedimentary rock formed from thick silt and mud deposited in a marine setting. This unit appears to have had subsequent submarine landslides, resulting in chaotic bedding called mélange (Tabor et al., 1993; Cowan, 1985). Most of the units in the Sultan Basin have been metamorphosed so such features are locally difficult to discern. Younger, continentally derived sediments composed of mostly sand and gravel, of the late Cretaceous and early Paleocene lay unconformably (above a time break in a depositional sequence) on the older rocks (Hedderly-Smith, 1975). Peridotite (dark green to black granite-like rock) intruded around this time into the older marine sedimentary rocks. Hydrothermal systems and submarine eruptions (similar to black smokers) formed from intruding magma, creating large pyritic deposits (such as the Lockwood Pyrite deposit) and overlaid the marine sediment with volcanic (for example, ash and basalt) material (Olson, 1995; Snohomish County, 1979). These rocks were then exposed to regional metamorphism (exposed to heat and pressure). The metamorphism changed the marine sediments into primarily argillite (metamorphosed siltstone), phyllite (metamorphosed mudstone) and chert (white to gray rock) (Yeats, 1964). Sedimentary continental rocks changed primarily into argillite, quartzitic sandstone and meta-conglomerates. Peridotite has metamorphosed into serpentinite (light green to dark green and black dense rock with waxy luster) and talc.

This unit was imbricated (thrust as slivers) into the North American plate by an accretionary wedge (Wells and Heller, 1988; Jett, 1986). The timing for this event is not well known, but is constrained to somewhere between early Cretaceous to the early Eocene (Tabor et al., 1993; Frizzell et al., 1987). This was primarily done by faults, many of the faults responsible for this imbrication can still be seen trending northwesterly within the watershed, where they form saddles and linear drainages. Most or all of these faults are no longer active. Severe folding also occurred during imbrication.

The Bald Mountain pluton is composed of granodiorite (light gray granitic rock) intruded into the area in the early to mid-Eocene (55 to 49 Ma). Contact metamorphism can be seen near the edges of the pluton and marine metamorphic rock, resulting in gneissic margins (light gray large grained metamorphic rock) (Dungan, 1974; Carithers and Guard, 1945). This unit locally occurs around Bald Mountain and no further outcrops occur to the east.

Miocene batholiths (Vesper Peak stock and the Index batholith) intruded into the Stillaguamish Ophiolitic suite. These intrusions are primarily composed of tonalite (light gray granitic rock). The Index batholith caused widespread hydrothermal alteration and metamorphism throughout the ophiolitic units in the Sultan Basin (Baum, 1968). It appears, either a product of weathering or regional intrusive patters, that the southern part of the Sultan Basin experienced higher temperature metamorphism than the northern part, as seen by the minerals at the Sultan King mine and the .45 mine. High temperature minerals, such as magnetite and molybdenite are found at the Sultan King mine. Low temperature minerals are found in the .45 mine, such as galena and ruby silver (Carithers and Guard, 1945).

As the batholiths cooled, metal-bearing solutions and subsurface waters flowed into the metamorphic sedimentary rocks, following cracks from the intruding batholiths, sheer zones and faults. As these solutions lost pressure and temperature, they precipitated ore minerals in veins (Carithers and Guard, 1945). Due to the long history within the watershed of faulting, shearing and intrusion, no common structure exists for these veins to follow.

Glacial Material and Recent History

The Sultan Basin consists of continental glacial drift, alpine glacial drift, alluvium and talus. About 14,000 years ago, the Puget Lobe of the Cordilleran ice sheet, which represents the most recent advance of continental ice sheet, flowed into surrounding valleys. The deposits of this glaciation are called the ‘Vashon Drift’ locally. Tongues of the Vashon glacier dammed valleys that were tributaries to the Puget Lowlands, creating large ice dammed lakes. Glaciers advanced up the Pilchuck River system and the Sultan valley, covering the northwestern portion of the watershed (Tabor et al., 1993; Booth, 1990). This blocked the paleo-Pilchuck River, creating a large ice-dammed lake, known as glacial Lake Sultan and depositing outwash (deltas) and lacustrine (lake deposits) on the north flanks of Blue Mountain to Bald Mountain and upstream to midway on the south fork of the Sultan River, Elk Creek and Williamson Creek drainage. This rising lake eventually overflowed, washed over Olney Pass, and deposited fluvial outwash across the plains in the west and south parts of the Sultan River watershed.

As the glaciers retreated, the terminal moraine (sediment collected at the end of the glaciers) locally called the ‘Pilchuck Plug’ blocked the upper drainage of the Pilchuck River, creating the new Sultan River watershed (Coombs, 1969; Bliton, 1989). The Sultan River established a channel, rapidly incised into the glacial material, cut into the bedrock, and became entrenched. This incision is probably due to easily eroding glacial material and isostatic rebound of the bedrock in the area. Relict meander bends and channels can be seen near the main channel of the present Sultan River.

Alpine glaciation, predominately after the last of the major continental glaciation, formed much of the topography in the upper Sultan Basin. Alpine glaciation deposits make up most of the river valleys (beneath river alluvium). Alpine drift can also be traced up much of the valleys, from Bear Creek to the upper mountain peaks of the Sultan Basin. A steady decline in alpine glaciation has occurred within the watershed and almost all alpine glaciers have disappeared. Currently, only one major glacier northeast of Copper Lake exists.

Slope Stability and Geology

Geologic units within this area have affected general slope stability. The marine metasedimentary units (including the Stillaguamish Ophiolite suite), present predominately in the southwest section of the watershed, have beds striking between N 5 to W 20 with a variety of dipping beds (due to tightly folded and faulting). This unit has been observed in field and aerial photo interpretation to have a higher landslide frequency, specifically when bedding is near vertical. Historically, hundreds of debris flows have occurred in areas where these geology factors are present. Vertical beds are known to occur near Blue Mountain Ridge, but dips seem to gradually decrease towards the South Fork of the South Fork of the Sultan River. Bedding that is dipping into the mountain or at angles that are not at or near vertical have not been shown to produce landslides, but field verification should be considered in areas where this geology is present.

Regional metamorphic units observed within the watershed have not been shown to increase or decrease slope stability within the watershed. These rocks trend along the peaks and ridges along the eastern edge of the watershed, from Hard Pass to Crested Buttes and north to Gothic Peak and Headlee Pass. The beds strike N 15 to W 20 and dip from vertical at Headlee pass, to 35 E near Crested Buttes. These rocks are very resistant to erosion, have a lower landslide activity and can create steep cliffs above the valley (Carithers and Guard, 1945).

Altered peridotite and serpentinite (dark green to black rock that feels slippery), although it has not locally been shown to cause slope instability, has created major slope stability issues in other areas (for example, Blewett Pass). These ultramafic rock types occur primarily in a strip north of Red Mountain, 500 to 1,000 feet wide and striking northwest. However, pockets of ultramafic rock can occur throughout the watershed (Tabor et al, 1993).

Tonalite (light gray granitic rock) from the Index Batholith and Vesper Stock have caused major rock topples. A prominent feature present within the rock is three strong joint planes. These planes can aid in rocks breaking into rectangular blocks or wedges, as large as 15 feet on each side (Carithers and Guard, 1945). Most rock topples recorded were independent of harvest or road construction and are generated by erosion of the basin. However, two major rock avalanches were located in this area, one dividing upper and lower Greider Lakes, the other west of Vesper Peak. A brief field investigation of these rock avalanche sites has led to the theory that these landslides have been seismically triggered. Both landslides are located adjacent to fault zones. Large regional earthquakes from outside of the watershed could generate major rock topples within this watershed in the future.

Reference
Baum, L. F., 1968, Geology and mineral deposits, Vesper Peak stock area, Snohomish County, Washington: University of Washington Master of Science thesis, 75 p., 1 plate

Bliton, William S., 1989, Sultan River project. IN Galster, R. W., chairman, Engineering geology in Washington: Washington Division of Geology and Earth Resources Bulletin 78, v. I, p. 209-216.

Booth, Derek B., 1990, Surficial geologic map of the Skykomish and Snoqualmie Rivers area, Snohomish and King Counties, Washington: U.S. Geological Survey Miscellaneous Investigations Series Map I-1745, 2 sheets, scale 1:50,000, with 22 p. text.

Carithers, Ward; Guard, A. K., 1945, Geology and ore deposits of the Sultan Basin, Snohomish County, Washington: Washington Division of Mines and Geology Bulletin 36, 90 p., 1 plate.

Coombs, H. A., 1969, Leakage through buried channels: Association of Engineering Geologists Bulletin, v. 6, no. 1, p. 45-52.

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Dungan, M. A., 1974, the origin, emplacement, and metamorphism of the Sultan mafic-ultramafic complex, North Cascades, Snohomish County, Washington: University of Washington Doctor of Philosophy thesis, 227 p., 3 plates.

Frizzell, Virgil A., Jr.; Tabor, Rowland W.; Zartman, Robert E.; Blome, Charles D., 1987, Late Mesozoic or early Tertiary mélanges in the western Cascades of Washington. IN Schuster, J. E., editor, Selected papers on the geology of Washington: Washington Division of Geology and Earth Resources Bulletin 77, p. 129-148.

Hedderly-Smith, D. A., 1975, Geology of the Sunrise breccia pipe, Sultan Basin, Snohomish County, Washington: University of Washington Master of Science thesis, 60 p., 2 plates.

Jett, Guy A., 1986, Sedimentary petrology of the western mélange belt, north Cascade Range, Washington: University of Wyoming Master of Science thesis, 85 p.

Olson, Duane F., 1995, Geology and Geochemistry of the Lockwood Volcanogenic Massive Sulfide Deposit, Snohomish County, Washington: Western Washington University Master of Science thesis, 118 p., 8 plates.

Phipps, Richard W.; McKay, Donald T., Jr.; Norman, David K.; Wolff, Fritz E., 2003, Inactive and abandoned mine lands–Spada Lake and Cecile Creek watershed analysis units, Snohomish and Okanogan Counties, Washington: Washington Division of Geology and Earth Resources Open File Report 2003-3, 36 p.

Snohomish County Public Utility District No. 1; Washington Department of Ecology, 1979, Sultan River project, Stage II; Application for amended license, FERC project no. 2157–State of Washington final SEPA EIS and FERC environmental report (exhibit W): Snohomish County Public Utility District No. 1, 2 v.

Tabor, R. W.; Frizzell, V. A., Jr.; Booth, D. B.; Waitt, R. B.; Whetten, J. T.; Zartman, R. E., 1993, Geologic map of the Skykomish River 30- by 60-minute quadrangle, Washington: U.S. Geological Survey Miscellaneous Investigations Series Map I-1963, 1 sheet, scale 1:100,000, with 42 p. text.

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