LiDAR and Reading Landscapes

 

Before reading it would be worthwhile to click on the LiDAR image below and give it some thinking.

LiDAR image of a section of west Kitsap County (via Puget Sound LiDAR Consortium)

 

The first thing I noticed on this image is the glacial striations across the upland landscape from the north northeast to the south southwest marking the direction of ice flow across the landscape. On the ground, these ice lineations are very subtle and could never be discerned without a very detailed survey (although it should be noted that this is not true elsewhere in Kitasap County). I noted erosion channels where low areas between very modest drumlin ridges intersect the steep shoreline bluff. It appears that water is concentrated at these spots leading to erosional features on the bluff slopes.

 

I found the 'U' turn of the stream in the south central part of the image fascinating. I also noted deep-seated landslide scarps on the slope downstream of the 'U' turn. That may not be a correct interpretation. Alternatively the scarps may represent various harder sediment layers that have created a step like slope.  Another possibility is that the features are remnants of former shorelines associated with either a glacial lake or sea level before the area rebounded from the mass of glacial ice. If I were to assess that slope, I would start with at least those working hypothesis and perhaps a couple more. A bit of arm waving thinking, but all pose possible testable theories.

 

The other feature labeled above are the bowl-like features on the northeast part of the image that I think may be kettles or glacial ice wasting deposits. Based on some observations I made, I am leaning to that interpretation versus deep-seated landslides.

 

The last thing worth mentioning is the valley on the northeast of the image. Note the terraces indicating down cutting of the stream down to a lower level. This is a common feature of central Hood Canal streams and rivers. The stream level adjusted to a lower relative water level (or to land uplift). But now the lower stream valley is drowned; another common feature of small streams in central Hood Canal. Why? A subject for a future post.

San Juan Islands National Monument - a Better Map

Salish Sea Communications had an alert regarding planning/management meetings for Washington State's newest National Monument. I had previously posted a map of the monument that showed the outline of the designated monument area. But this map labels many of the small points and rocks as well as larger tracks that are part of the monument. Click the map to enlarge to see all the various BLM land now designated a National Monument. It would an interesting goal to set foot on all the various small bits of the monument.

Note the map says National Conservation Area. The map was apparently created prior to the Presidential Order back when advocates still had hope of Congressional action. The chair of the applicable House Committee, Doc Hastings never scheduled a hearing despite the state support.

Ripple Cross Sections Near Port Townsend

The shore bluff facing the Strait of Juan de Fuca in Port Townsend provides some great cross sections of a very wide variety of sediment deposits (Click on location at bottom of post). I came across this cross section of a set of ripples.

Ripples in sand deposit with U.S. quarter for scale

Same set of ripples from a bit back (note quarter) with other mostly sandy layers topped with a layer that has been highly deformed by soft sediment deformation

The soft sediment deformation may indicate sediments deformed by very rapid deposition before they became very compacted. The ripple deposits may mark a bit of a transition from preglacial sediments to early glacial advanced outwash. Besides the nice form of the preserved ripples, the ripple unit contained bits of charcoal.

Bits of charcoal within deposit

More charcoal within sand deposits

The presence of charcoal hints at a preglacial deposit. Mapping by Gayer (1976) and generally adopted by Schasse and Slaughter (2005) do not distinguish the bluff units lumping them into a broad category of "undifferentiated glacial and non glacial deposits" due to the scale of the maps. The top of the bluff is mapped by Schasse and Slaughter as glacial marine drift. That is the sediment at the top of the bluff was deposited while the area was submerged during the late stages of the last glacial period.  The mass of glacial ice had pressed the local land surface downward hundreds of feet and hence the area was submerged for a brief time before the land rebounded. My observations of the upper bluff are consistent with Schasse and Slaughters interpretation although the marine drift at this location is a bit thin and there may be patches of till - a tough call.

The ripples at this location almost looked like tire tracks

More tire tracks. Note the ripples grade upward from silt dominated to sand dominated

Cedars are our friends

I was reminded last week of an old geology friend, Chuck Ziegler. I assisted Chuck during his graduate school work in the Northwest Cascades. The reminder came while I was traversing up a fairly classic slope that is common on many of the Salish Sea coastal bluffs - cliffy on the bottom where waves had eroded and under cut the slope, steep mid slope with slopes of 45 to 60 degrees covered with trees and thick brush on up to a vertical cap underlain by glacial till.

With the very top of the bluff capped with concrete like glacial till, my route to the top of the bluff was blocked. I moved laterally along the base of the vertical obstruction looking for a way up and hoping I would find one before some obstruction would force me down the bluff or worse force a complete back track across difficult terrain I had already traversed.

A well located western red cedar, though small, combined with a few other hand and foot holds saved me from a long nasty slog through the steep woods. As I wrapped my hand around the thin branch I remembered Chuck saying, "Cedars are our friends". The strong fibrous wood of the western red cedar makes a good natural trusty rope when available.         

My hand hold before pulling myself up

Glacial till cap on a woodsy shoreline bluff

Lummi Formation Notes

Had a chance to look at some Lummi Formation. The Lummi Formation crops out on the east side of the San Juan Islands and of course Lummi Island where its named is derived. The formation is a chunk of Cretaceous to Jurassic ocean floor. It is one of numerous blocks and slices of terrains that have been accreted onto the North American margin. Where the story of this unit and others gets complicated is just what happened after the initial accretion. 

The particular portion of the formation I was observed was primarily a portion that is a turbidite sequence. Turbidites are sediments that have been deposited on the sea floor via density currents resulting from under water landslides. The sediment creates high density fluid in the water and flows down and out across the slope of the ocean floor. The sequence of events starts with sediment piling up on the shallow sea floor near the coast from rivers and streams. The loose saturated sediment then collapses as an under water landslide. As the density current propagates across the deep ocean floor, the heavy sediment is deposited first and then the sediment fines upward. If the site of deposition is at just the right spot, a sequence of gravel fining up to fine silt will be deposited, but more often only part of the range of sediment is deposited in a given spot or the upper part of the sequence might get eroded by later density currents. Sites far from the landslide will be dominantly fine grained silts and clays. Sites that are closer may be dominated by gravel and sand. Repeated slides will take place as sediment gets piled up again along the near shore by streams and rivers. The slides might be correlated with repeated earthquakes along the coast that will readily trigger under water slides, but the slides can take place without quakes as well.

 

For the most part the section I was looking at consisted of alternating silt and sand layers suggesting a fairly distal location out away from the high energy flows or at least on a quieter part of the turbidite fans that were built out onto the sea floor.

The bedding orientation was very easy to measure at this site with alternating beds of siltstone and sandstone.

The Lummi Formation has been metamorphosed and on some of the bedding plain surfaces faint lineations were present.

 

A common feature at this particular spot was rip up clasts of dark silt embedded within the sand stone. In this case the density current ripped up chunks of previously deposited silt that was then deposited within the sand.

When the only units are alternating siltstone and sandstone determining which was is up in the original formation can be tricky. But a layer of conglomerate grading into sandstone allows for that determination.

Conglomerate grading upward with pen pointing towards the original up direction.

In this case up is slightly downward and to the right.

The dark 'pebbles' are small rip of clasts of siltstone that were eroded by the density current that carried the gravel.

Steptoe Butte

Steptoe Butte

Steptoe Butte rises above the rolling Palouse of eastern Washington. It is a bit tricky to get a sense of scale. When I first spotted the peak I thought I was seeing Mount Hood, but that would have been an absurd view given how far east I was. The butte rises approximately 1,100 feet above the surrounding rolling hills, but its isolated location makes it stand out as an impressive high point.

The peak is underlain by Precambrian rocks that Waggoner (1984) placed in the Ravalli Group. Due to its isolated exposure the interpretation is a bit tentative.

The butte lends its name to a geologic term. A steptoe is an isolated island of older rock surrounded by younger formations. In this case the billion year plus rocks of Steptoe Butte rise above 15 million year old basalt lava that buried everything else in the area. The butte also rises above vast loess deposits of the wind blown sediment that blew out of the Columbia Basin and settled as a thick blanket of silt over the Palouse creating the rich farmland of the Palouse region of Washington State. Why the butte was not buried in a layer of thick loess is a question that adds to the curiosity of this isolated peak.

The grass and wheat covered ground in the image above is for the most part underlain by the Palouse silts and as such are rich active farmland. The dark band of forest in the low area in the mid distance between the viewer and Steptoe Butte is the Rock Creek drainage. Here the palouse silts were scoured out by ice age floods leaving the area not very viable for wheat growing and instead it is a mix of grazing and pine forest.  

Steptoe Butte was named for Colonel Steptoe. However, the butte should not be confused with the hill where Steptoe and his men fought off an Indian attack before escaping near the town of Rosalia.    

Leaving Eden and Some Gaelic with Carolina Chocolate Drops

Paul Krugman put this up for his Friday Night Music.

It is lovely, they also  do Gaelic: