Abstract
of results of 12 June hike near Ledgewood and
1. A large
NW-SE fault cuts the
2. The fault
gouge and flays from the main fault proceed at least 200 meters north of the
collapsed house. At 200-300 meters there is a normal fault of at least 20
meters throw, leaving younger gravels against older fine sands and silts, at a
45 degree angle (down to the south). This is the best presented normal
fault observed on the entire island.
3. There are
small compressional features in the section to the north (at least one large
sand lens wedged between two sections of the younger gravels), but the main
feature is that of extension. South of the park, the Microsoft house
has had at least 5 meters of slumping and storm erosion in three years, and one
km south of the park there exists a cauliflower structure in fine sand and silt
(it is old, in the middle glaciation, and is covered by regular stratification,
indicating that its age could be determined).
4. The age of
beach concrete armament, which has been undermined and
prostrated, can be used to calculate the retreat of the coast due to storms and
erosion. It is greater than 1/3 meter yearly.
5. This is the
best and most convenient place to view:
a. recent slumping, due to NW-SE faulting;
b. retreat of the coast line and erosion of
cliffs;
c. a large normal fault; and
d. road breakage from earth
movements.
Comments as of
16 Jul 03:
The slump area
was reviewed for its relation to the large NW-SE fault, which stretches from The
Admiralty Head at Fort Casey to Baby Island, where it is seen as a hogback
rising up to the NE with a 20 degree dip, and expressed in a compacted sandstone
(similar to Pliocene in strength), which is probably Esperance. My
interpretation of the dynamics is that there is a thrust from the south, causing
the deeper sediments to attempt to ride over the Baby Island Head; this moves NW
in surface expression, because of the western movement of the NA plate. Now
let’s look at Ledgewood, with this hypothesis as a guide:
One mile south of the Edgewood ramp, in
S31 T31N R2E, there are the angular blocks (chair-sized), seen on the North
Hancock hike, which represent earth which has fallen in an earthquake opening in
middle glacier time (40 kybp?). This is on the extreme
south side of the disturbed Ledgewood slump and fault zone, and there are three
sets of fractures (geologists would call these joints), which are regional in
nature by my classification. I only use the word joints to portray local cracks
in granite, etc. which are caused by exfoliation, daily temperature changes, and
events on the meter scale- which are not regional.
The fractures (faults with no
displacement) run NW-SE, so that one can line up with the fracture face, turn
180 degrees and align oneself with the saddle across the bay, at
The slumping then can be expected to go
inland a considerable distance, so long as there is a
significant elevation change (gradient or slope). This is the case with the new
Baumer house and lands, where the earth has been
scraped to expose the slumping indications. He can expect that he will have
continual creep, as well as the beach-front movement (which has already moved
some 10 feet or so since I have witnessed the alteration of the
landscape).
I have found that slumping is accompanied
by high alkalinity of the well water, in the nearby zones surrounding the open
fractures. I anticipate that this is mainly HCO3 (bicarbonate or baking soda
when precipitated with Na- sodium), which gives your water a nice bubbly taste.
This is caused by precipitation entering the open fractures, which are
exaggerated around a fault, and the acidic nature of rain dissolves the calcite
and subsurface cement to form bicarbonate. The excess HCO3 may be a harbinger of
movement in slumps, and I am testing this hypothesis. Parallel to the large
slumping at Possession is a NE anomaly of alkalinity in the well water (up to
twice the average value).
We found the blue clays in the tidal
zone, but I found no significant peat, which is our marker for stratigraphic
location; this zone is in the middle glacial cycle, by my reckoning. Also, by my
reckoning, the fault at Ledgewood is active, since the asphalt hiway has to be
continually repaired, and Baumer’s beach and grounds continue to move. I don’t
think it is up to the 1 cm/year rate; possibly we could adjust our thinking to
the mm/year scale. I will later analyze the well water around the SW line toward
Remarks taken
from an investigation into the local faulting, requested by one of the local
property owners near the beach at Ledgewood:
FAULTING (see attachment 1) October
2003:
The Baby
Island- Ledgewood- Admiralty Head fault is our most certain fault. It runs
NW-SE, and can be seen and sampled just off the
I
believe that the fault is still active, based on the road breakage and
slumping at Ledgewood; there is indication that it is left lateral from a
rotation of a driveway in the village of Ledgewood, where I measured the bending
of nails attaching the driveway to the main slab. The nails had bent more
than 1/4 inch to the right (yielding a left lateral movement of the slab,
causing the nails to bend in the opposite direction) and the main
observations support the conclusion that it is compressive and causing uplift at
selected locations like Admiralty Head.
Other faulting on the main
body of
May 7, 2004, the hiking group reviewed
the stratigraphic section north and south of the Ledgewood Beach Park, in S30 & 31, in T31N,
R2E, on Whidbey Island, after the winter storms had caused new slumping and
spalling of the cliffs, on this west facing beach.
The first general impression was that there
is an undulation of the gravel deposits both north and south of the collapsed
house. The phenomenon of compressional squeezing adjacent to the fault surface
ahs caused the an almost sinusoidal appearance of the beds of at least three
meters up and down movement of a particular horizon. In addition, there is
occasional normal faulting both down to the south and down to the north. Just at
the collapsed house, one can see that both faulting and distortion of the sand
and gravel beds occurs. The homeowner attempted to offset the slumping by
installing pipes in the spoil to dewater and speed draining of the soil. Hence
his danger must have had warning signs over a considerable time. But the house
splintered in falling and his installations all slumped considerably over the
thirty or so years since installation. Concrete bulwarks in the neighborhood
also cracked and broke, but this must have been augmented by large
storms.
The Esperance sand is the most consistent
horizon which is used to analyze these movements. It usually has iron staining
and exists at the base of the last glaciation period, before the advancing
Vashon ice. It has been observed elsewhere on the island, and usually has aerial
oxidation of the iron compounds. This red to yellow color is not common on the
island, since most deposits are dull grey. However, there are seeps which bring
iron carbonates into the air, causing new iron oxides to appear, and one must
allow for this, particularly at seeps draining sandstones.
The chair-sized angular blocks were
viewed again, since they had spalled off and were fresh in appearance. These
rearranged siltstones (relative to similar deposits nearby) were at odd angles
to their neighbors, had no rounding to the edges, and must have fallen into
large crevices as earthquakes opened the earth with large fractures allowing the
silts and fine sand deposits to fall and become partly surrounded by pea gravel
before being re-buried. The stratigraphy above this exposure is again almost
level and regular, indicating that the fractures closed before the last
glaciation. This again is in the equivalent of the Esperance
sand.
THIS WHOLD KM. LENGTH NORTH TO SOUTH
SECTION EXHIBITS PRESENT LEFT LATERAL NW-SE FAULTING WHICH HAS MOVEMENT GREATER
THAN 1 CENTIMETER/YEAR.
.
Harold L.
Overton