ROCKY POINT
The
basement rock called Rocky Point was investigated for orientation and geological
significance
The outcrop
consists of metasediments, which are greenstones, marbles, quartzites and
shists intruded by geothermally precipitated quartz veins. They are
essentially vertical, with the fault plane on the south side, with a directional
orientation of 120-140 degrees from north- yielding an average trend of 130
degrees towards the Eerkes Spring on the east side of the island. Eerkes
is underneath the
Just south of the
northernmost outcrop at Rocky Point, there is an excellent expression of the
scraping of the Vashon Glacier on the metasediments surface. There are
about 40 parallel linear scrapings, some a few feet long. This trend
points in a 250 degrees direction from north, yielding the same direction as
Goose Rock glacial movement, Penn Cove orientation, and the mountain glacier
movement which moved down from the Cascades in one of the latest downslope ice
gougings. This collection of scrapings are just below the cliff of till on
the west beach cliff, and show that significant (two or more meters) rubble was
dropped from the ice which scraped the rocks as the glacier melted. This
confirms that not only did the Rocky Point fault exist during the glaciation,
but that it was exposed to scraping, covered by till later and finally
re-exposed by erosion from storms in recent times.
Since there is only
a thin layer of till and then soil on top of that, it is assumed that the till
is Vashon. Further, since no N-S scraping is observed on the southern
metasediments surface, the southern part of the rock must have been below
the surface of the ground at the time of previous glaciers. The
metasediments protrude into the air with jagged layers now, so must have not
been disturbed by other glacier movements. This is not definite, but hints
that the rocks were subsurface during times more than 40,000 years b.p. (of course, the top portion of the present
meta-sediments has been worn down by an unknown amount, after glaciation, but
the north exposure definitely shows the latest glaciation). If this is
correct, the rate of movement of the metasediments upward could be estimated; it
is suspected that the thrusting of the Pacific plate is steadily moving
northward- now expressing itself at
There are some parallel glacial grooves (widths of a
foot or more) in other parts of the Rocky Point. These roughly point in
the 070 degrees from north also. In addition, there are some glacial
grooves on the order of a few inches, which point almost along the bedding
direction (130 degrees from north), but these are suspect as being deviations
from the main direction of glacial movement, caused by the stronger quartzites
influencing locally the movement of the ice. LIDAR mapping was available after this
field work, and remarks about the latest two movements are listed
below.
This Rocky Point outcrop is the best location on the island for
easy viewing of a compressional fault with extreme angle of expression, and for
viewing the influence of glaciers on a hard rock surface. Both scraping, plucking and smooth grooving can be found
with minimum effort.
CONCLUSIONS;
a. The outcrop represents a block of detached rock
from Pre-Tertiary, and is not attached to other bedrock, since there is a base
which is at about the same elevation as the top of the stratigraphic
column;
b. The Rocky Point outcrop is indeed vertically-oriented, the
original bedding planes pointing toward SE, and has been fractured and
secondarily veined with silicate deposits- these have partly crossed each other
with some displacement, hence represent two separate episodes of
fracturing;
c. The silica fractures represent thermal events, since either
hot water or nearby hot veins of silica have created them- they are separate
(later) from the metamorphism, since they have not flowed along the bedding
planes;
d. The whole outcrop appearance represents high grade metamorphism,
with quartzite and greenstones forming at great depths;
e. The outcrop has been exhumed by great stresses,
certainly not by glaciation, but the outcrop has been grooved and scratched by
at least two separate episodes of glaciation. From LIDAR, it is clear that the
main N-S glaciation was earlier and its grooving was partly erased by a later
mountain glaciation moving 250 degrees from north.
e. The open fracture
(160-170 degrees) occurred after the outcrop had surfaced, otherwise it would
have been filled with subsurface debris; it is a brittle fracture, caused by the
stress which shoved the outcrop to its present position- likely a compressional
stress under the middle of the outcrop, causing it to buckle and break at the
top.
We will try to answer the
following questions:
1. Why is the south fault
boundary of the metasediments more obvious and active (younger) than the north?
The south flay appears on the LIDAR, but the north one does
not.
2. Since the greenstones lie
between two NW-SE lateral faults, and the base of the stratigraphic column is
likely to the south (because of reefal indications to the north), does this
indicate that the block of Mesozoic rock was rotated counter-clockwise, looking
toward the east? And if the fault is mainly lateral,
how did the Mesozoic sediments get to sea level?
This Rocky
Point outcrop is the best location on the island for easy viewing of a
compressional fault with extreme angle of expression, and for viewing the
influence of glaciers on a hard rock surface. Both
scraping, plucking and smooth grooving can be found with minimum
effort.
Conclusions after the
Analysis of
Rocky Point outcrop, just south of the NAS Whidbey installation:
1. The meta-sediments found in the south of the outcrop
are generally thin beds and jagged (upward into the air), and have no indication
of having been smoothed by the N-S major glacier, but do have scratches (not
grooves) running 070-250 degrees from north- caused by the last mountain
glacier;
2. The fracture in the middle of the outcrop runs 160-170 degrees
from north, is open by at least a foot, and continues under the till to the
south, with dampness coming from under the till. The walls of the fracture are
somewhat tilted from one side to the other, indicating slight movements allowing
angular blocks to shift inside the fracture. There is no significant
displacement, but the shearing tends to open the fracture almost along a N-S
axis, out into the tidal zone (this could possibly be due to plate stresses- a
flexing or compression from underneath the outcrop, but since opening of
fractures can occur due to extension, this has to be suggested only as one of a
number of scenarios);
3. The greenstones on the
north side of the outcrop are indeed more massive (such as a lava flow, not
random ash or lapilli), but could be softer than the meta-sediments- hence they
might be lower than the elevation of metasediments because of erosion proceeding
faster (they certainly are softer than quartzites, which are found to the
south);
4. The Possession Till occurs just north of
the metamorphic rocks, is regular and level, until a 3 meter drop occurs-
exposing outwash sands, sitting on top of the Possession at sea level. I assume
that this is a small normal fault, down to the north. This would fit in with the
idea that the outcrop and its surroundings have been shoved upwardly in the
Pleistocene.
5. There are silicate veins running across the meta-sediments,
sometimes cutting each other. There is only a few inches displacement,
horizontally, right lateral- which could have been caused by the N-S glacial
thrusting. This is the reverse of the large scale thrusting noticed elsewhere on
the island.
Since the topography
indicates that the fault trace on the ground surface is about 1 km to the south
of the crystalline rocks, inland, I take it that this whole phenomenon
represents a shallow thrust. With this in mind (searching 1 km south of
the 130 degree line on the map- stretching from the Rocky Point to Eerkes
Spring), I looked for road breaking on the east side of the island. I
found this to occur at the scarp just north of the Seaplane Base ( S1, T32, R1E)- the peninsula representing a sort of a
Tombolo. There are many breaks in the asphalt, indicating that the fault
has a wide zone of influence. It would be reasonable to assume that
My analysis is that the saddle at the
Naval commissary represents a fault gouge zone, where movement takes place in
several dimensions- NW, moving the city towards the San Juan Islands; downward
in the saddle, by down dropping and erosion; thrusting under the town; and
rebound from the weight of the last major ice age. We will try to
determine relative movements and rates with future comparisons with the rest of
the island. Just now each of these is demonstrated by the following
features:
1. USGS Publication 1643 shows that
left-lateral movement occurs in the large north portion of Whidbey, in the
Strawberry and Utsalady Points faults- but I believe the main NW movement is
demonstrated by Crescent Harbor itself;
2. Erosion is brought on by the tides
and storms- but the storms are from the SW (the protected direction), so
faulting at the bay bottom must accelerate the process;
3. All of the cliff
studies nearby show that thrusting occurs at Rocky Point;
4. The highest point on the island is near
REVIEW After a 21 May 04
hike to Rocky Point:
1. Rocky Point metasediments
outcrops are separated from the Pleistocene sediments by a fault on the south
side, running NW-SE, which shows readily on the LIDAR map as a linear running
toward
2. The fault on the north
side is not shown on LIDAR, but is seen as a vague expression in the cliffs- it
makes the metasediments appear upthrown, with till and sands lower to the
north;
3. The whole outcrop appears to be shoved upward, somewhat more
in the mid-section, yielding an open fracture in the middle into which angular
blocks have fallen;
4. quartzites and shists
occur more to the south, and calcitic greenstones to
the north- making it likely that the top of the section is to the
north;
5. Numerous SW-NE glacial
scratching occur especially in the north, while the N-S grooves are vague;
and
6. There is increasing erosion northward,
with the highest cliffs on the southern part of the km outcrop.
