Field Trip Day 1 - Anchorage Area
Overview of Trip
Anchorage, Alaska is located in south central Alaska and has a number of forces
shaping the geology of the area. It is positioned where two crustal plates are colliding,
making earthquakes very common. Volcanoes across the inlet expel ash that settles
around Anchorage. At least 5 glaciers have covered and shaped the area over the
last 2-3 million years. The ocean, with 30’ and greater tides, is working the shoreline.
The arctic environment results in seasonal freeze-thaw cycles. Recent heavy urbanization
has effected the ground water resources and modified erosional processes.
This report is being prepared as a requirement for the "Field Trips in Geology"
class, presented by Dr. Kristine Crossen during the Summer 1997 Semester. Kathy
Lemke instructed this first field trip.
Stop 1 - Sitka Street Park
Location: 15th Street and Sitka, 1/4 mile west of Lake Otis
Description of Area: The Sitka Street Park is located south of Merrill Airport,
adjacent to a large marsh area to the south & west and apartments to the east.
The park has been extensively landscaped and there is a 15-20 feet tall grass covered
berm between the park and 15th Street. Several streams pass around the park and
they were sheen free with water fowl in them. Black spruce are abundant in the marsh
area. A snow disposal site and fenced retaining pond is east of Sitka street.
The interesting thing about this stop is the presence of an old dump site north
of 15th street. During the early days of Anchorage urbanization, trash and other
waste materials were dumped off the end of the old airstrip. No efforts were made
to contain any effluents out of or liquid wastes placed into this dump site. Water
fowl in several of the ditches were noted to have an abnormally high birth defect
rate and attention was focused on pollution from the old dump site. During the winter,
snow does not stick to the ground due to the heat generated from the decomposition
process and methane is produced. Several wells to monitor water quality and pump
contaminated water out of the ground are located north of and adjacent to 15th street
and the retaining pond.
Interpretation: Ground water contamination can be a very serious problem. This is
somewhat mitigated for residents of Anchorage because their water source is Eklutna
Lake. However, wildlife in marsh areas can be effected and homes in the Hillside
and South Anchorage area are pulling their drinking water from the ground.
The principal area of recharge for Anchorage area aquifers is along the fill deposits
located at the base of the Chugach Mountains. Runoff from the mountains and rain
over the fill deposits percolate into the ground and then to the sand and gravel
layers underlying Anchorage. Water wells in the Hillside area, generally drilled
below Hillside Drive, encounter these glacial system sands and produce clean water
at typical rates of 5 GPM. Wells located higher up the hillside may encounter bedrock
and expected rates are only 2-3 GPM after fracturing the rock. This is due to lower
porosity of the rock containing less water and slowing the recharge ability of the
rock around the well. There are numerous layers of clay contained in the sand layer
and this helps slow downward migration of contaminates. Septic systems pose a special
problem. If located in bedrock, fractures will be required for adequate effluent
percolation. These cracks in the rock do little to filter the effluent and may contaminate
nearby water wells with sewage. Recommendations include limiting 1 drain field per
acre, minimum spacing between the drain field and water well, and using new designs
that rely less on ground percolation. Subsidence in the Anchorage bowl area has
been minimal. The sands have low clay content and, except for the occasional ice
wedge, there are few problems with sinkholes.
The current landfill located near Eagle River is on compartment 3 of 5 planned pods.
It is considered a state of the art landfill and has liquids pumped out and disposed
of in an environmentally sound manner. Adequate clay exists below each pod to minimize
ground water contamination. The biggest problem is that it is filling up quickly.
Efforts to reduce the amount of waste have been hampered by the poor economics of
recycling. Another landfill site may have be developed much sooner than planned.
Stop 2 - Alaska Native Hospital
Location: The abandoned Alaska Native Hospital is located on the south side of 4th
Avenue.
Description of Area: We parked at the back of the parking lot of the abandoned Alaska
Native Hospital. A fence is positioned along the back of the parking lot. A steep
embankment overgrown with alders, birch trees and smaller shrubs is north of the
fence. Several patched cracks were noticed in the building.
Interpretation: Anchorage is located in a Convergent/Oceanic-Continental plate intersection
area, making it a prime place for earthquakes. The Pacific plate is moving northward
and collides with the Arctic plate along the Alaska Peninsula eastward to south-central
Alaska. The 1964 quake was originally estimated at 8.2, but was upgraded to 9.2
after the quake data was re-evaluated using modern techniques. Damage was severe,
but the low population (at least compared to today’s 250,000 people), warm weather
and it being a holiday kept the death toll to a minimum. The Government Hill Elementary
School was destroyed in a slide. The clay layer that helps protect the aquifer from
surface contamination liquefies when shook and allows soil to move. A much more
stable soil is the outwash gravels that are not prone to movement or liquefaction.
The Native Hospital area is thought to be at very high risk of major damage in the
event of another large earthquake. The "head" is loaded with the heavy
building - when the soil starts moving, it could really go with the added mass to
impart momentum. The slope is unstable - there is no toe, just a steep embankment.
These risks, plus the deteriorating condition of the building, resulted in the construction
of a new native hospital on more stable soils.
There are several gaps in the earthquake record that may indicate a large earthquake
is due. There has been minimal activity in an area known as the Yakataga Gap, which
could result in very large strains building up. If these stresses release suddenly
instead of gradually over time, a large earthquake could result.
Stop 3 - Alaska Railroad Depot
Location: The Alaska Railroad Depot is located in an industrial development along
Ship Creek. Take A Street north across the bridge, bear left at the first exit ramp
and follow the road around to the depot.
Description of Area: The depot is located south of the banks of Ship Creek. There
is constant train traffic and vibration could be felt whenever a train passed by.
Quayana Park is south of the depot and has a smooth, gently graded grassy slope.
Vegetation was pretty uniform across the slope - no indication of preferential growth
along fault lines. There were several flatter areas in the slope, but no lobate
shapes were noted that might indicate sagging soil. Several old pipes are located
at the top of the slope, possible monitoring or water extraction wells. The only
building is the Martin Arms, which survived the 1964 earthquake, a parking lot is
located at the top of the slope. The slope further to the west was not graded and
a retaining wall is present at the side of a road. Fences and trees in the area
are plumb.
Interpretation: This was an area of major damage during the 1964 earthquake. Many
structures east of the Martin Arms were damaged or destroyed. The slope has been
graded at a shallow angle and water was extracted to minimize the risk of the slope
sliding during another quake. Headloading along parts of the slope are minimal,
but new construction is going up and adding to the load. Parking lots are idea for
these slopes - minimal loading. The Bootlegger Cove formation is responsible for
the slope sliding.
The Bootlegger Cove material is composed of glacial silt, clay and sand deposited
during one of the more recent glacial periods into glacial fed ponds and lakes.
Mud in this formation liquefies when shaken. During a strong quake, large blocks
of soil will start to slide on this clay, break away from the surrounding material
and slide downslope. Significant weight at the top of the slope or lack of a confining
toe aggravate this tendency. During the 1964 quake, 10’ vertical and 20’ horizontal
movement were measured in this area.
A retaining wall gives some indication of soil movement. While trees and fence posts
in the graded slope area are generally straight and plumb, the retaining wall is
bulging outward. It appears to be of very heavy duty construction and looks like
it is attached to several large beams sunk into the soil bank. Soil is noted to
be slumping across the area and several trees have a bow shape indicating movement
over time. This slope has not been graded to a shallower angle and is experiencing
significant movement.
Stop 4 - Earthquake Park
LoLocation: Several stops were made to look at the results of earth slides resulting
from the 1964 earthquake - the L Street and Turnagain Heights slides - and look
for Bootlegger Cove clay outcrops. The first stop was at Elderberry Park, located
at 5th & N Street. The second stop was Earthquake Park located at the western
end of Northern Lights Blvd. Finally, we stopped at a small roadside park approximately
1 mile west of Earthquake Park.
Description of Area: Elderberry park is similar to Quayana Park - a cleared out
area that has been graded to a gentle slope. There are quite a few buildings and
houses in the area. The coastal trail now cuts through the area and the ocean is
located west of the trail. The ocean has eroded into lower portion of the slope.
Earthquake Park is located below a parking lot. There was construction going on
to repair portions of the trail and parking lot that had fallen over the bluff into
the park. The park area is heavily vegetated with small birch trees and scrubs (
and lots of mosquitoes!). Earthquake Park has not undergone the landscaping the
other areas have and the remains of the old soil blocks can easily be seen.
The small parking lot of the 3rd stop was located on a bluff eroded away by the
ocean, a 75’ - 100’ high bluff dropping at 60 to 70 degrees to the ocean.
Interpretation: At the L Street slide during the 1964 quake, there was major failure
due to no toe on the slope. The ocean had eroded the toe away. The area has become
very build up with residential houses, which would sustain major damage or complete
destruction during a large quake. Several members of the class that live in the
area noted how hard their homes shake when even a small earthquake strikes versus
the minimal force felt when on glacial fill or bedrock.
At the Earthquake Park area, the gently rolling ground shapes gives a good impression
of how the earth broke up into large blocks, then slid downhill to the ocean. Clay
dug up at the 3rd stop appeared to be mostly clay and silt with a small sand content.
During the 1964 quake, 75 homes were destroyed, some sliding 2000’ and the road
was cracked as far back as Northern Lights.
Stop 5 - Point Woronzof
Location: Point Woronzof is located at the end of Northern Lights Blvd.
Description of Area: The Point Woronzof parking lot is positioned about 200 yards
from the ocean. At the edge of the parking lot, there is a ~100’ bluff dropping
at a 60-70 degree angle to a narrow (20’ - 50’ wide) gravelly beach and the ocean.
This bluff is completely devoid of vegetation and there are many indications of
recent rock and dirt slides. A poorly maintained dirt & gravel road leads down
to the ocean from the parking lot. There is sand and well rounded gravel and cobbles
located in layers in the bluffs above the ocean and along the beach.
Many interesting features can be observed in the bluff face. Lack of vegetation
due to constant material removal and limited weathering make it a great outcrop
to observe. Layers composed of everything from mud to fist size and larger cobbles
are present, in all layers from 1" to 10’+ thick. Numerous bedding features
and small faults are present. Observed rocks stacked on top of each other at an
angle (imbrication), indicating stream deposits. Coal and rocks containing Pyrite
were found.
Other interesting features were discussed. Across the inlet, Mt. Spur, Redoubt,
Hays and Illeanma can be seen on a clear day. These volcanoes are present due to
the subduction of the Pacific plate and its subduction. The temperatures and pressures
along the plate junction is great enough to cause magma to extrude out to the surface.
A major deposit of Hayes ash about 3000 years old is present on top of the Elmendorf
formation.
The Chugach mountains are located east of Anchorage. Most of the peaks have a pointy,
craggy appearance, due to glaciers carving the sides of the valleys away. Peaks
with flatter summits, such as Flattop, were overrun by a glacier and had the top
truncated. Scrape marks are easy to find on Flattops summit.
Interpretation: Erosion of the shoreline at Point Woronzof has been monitored since
1909 has averaged 2’ per year over the past 44 years. This erosion is due to natural
slope failure. Strong tides undercut the bluffs and quickly remove material, preventing
rocks from piling up and providing support to the slope. Vibration from aircraft
passing overhead can be severe and may be contributing to slope failure. The soils
are loose, completely unconsolidated glacial till fluvial deposits and are very
easy to scoop away by hand. Intermittent layers of mud are present and are slightly
more resistant to weathering/erosion.
These materials were deposited during one of the glacial periods. This area was
under ice and remnants of several morains have been identified in the Anchorage
area. Anchorage was a low area during the last glacial period and was filled with
lakes. These quiet areas allowed clays to settle out. Uneven deposits and varying
clay body thicknesses are due to the uneven bottom of the lakes. The presence of
easily pulverized coal in this vegetation poor material indicates that the coal
was transported from its source rock to this area in one piece, then broke up and
redeposited in the stream channels.
Mud flats that are infrequently flooded are located to the SE of Point Woronzof.
Even with the strong tides washing in and out of Cook Inlet, prevailing currents
may be redepositing the eroded material here.
Stop 6 - Kincaid Park Motocross Area
Location - From Jodphur Road, turn into the Kincaid park Jodphur entrance. Drive
past the large parking lot on the right and continue to the end of the road. This
area is popular with motorcycles and there may be a number of them in the area.
Park and walk past the bleachers to the bluff overlooking the ocean. Turn right
and the sand dune is located approximately 200 yards past the fence.
Description of Area: The motocross track is located in an old quarry. A fence has
been erected to keep motor cycles and ATV’s off the sand dune, but it has been torn
down in several places and tracks can be seen going up the dune slope. Vegetation
is sparse and the soil appears to be a mix of sand and gravel. The wind was blowing
5-15 mph from the south which appeared to be the direction of movement of the sand
dune. The terrain not disturbed by the quarry appears to be generally flat leading
up to the coastline, then there is a 50’-70’ ridge of sand deposited on the edge
of the bluff. There is a large broach in the embankment and a steep slope was present
from the edge of the old quarry down to the ocean. This slope was covered with grass
and looked to be relatively stable. Large mud flats are present by the ocean and
were covered with vegetation, indicating the ocean has not been eroding these areas.
The sand dune itself is free of vegetation and is covering trees as it moves to
the north. Sand could be seen moving across the surface of the dune, blown by the
wind. Numerous ripples were observed where a miniature sand dune would be built
up, then sand would tumble down the leeward side. This gave the effect of the ripples
marching across the dune surface, transporting material to the downwind side. The
fence supports keeping motorcycles off the dune have had material around them blown
away and shifted towards the dune. Numerous soda cans and pieces of broken glass
were observed as having been sandblasted - paint was scoured off and sharp edges
rounded over. There was a thin layer of very coarse, well sorted sand on top of
the dune. Smaller, finer material appears to have been blown away by the winds.
An old vegetation layer can be seen between the sand dune and the old slope surface.
Numerous bird holes are located in the upper 5’10’ of the dune, an indicator that
the material has a loess or more silt like texture. A weathering resistant layer
was observed, possibly made of ash and/or clay (lethe?).
Interpretation: The sand dune probably started when the old quarry removed vegetation
from the sandy ridge that sits at the edge of the bluff. Lack of vegetation allowed
the prevailing winds from the south to start moving the sand and it has been redeposited
in the downwind side of the remaining ridge where wind velocity drops and the wind
carrying capacity decreases. The steep NW face of the dune indicates the direction
of movement is to the NE.
These sandy bars on top of the bluff probably have the mud flats as their sand source.
The flats are not being flooded by the ocean. During droughts, these mud flats will
dry out and high winds will pick up sand and redeposit it on top of the bluffs.
Assignment: Assess the safety of your home during an earthquake.
Our home is located in the mid Hillside area, below Hillside drive and between Huffman
and DeArmon. Sediment here is classified as "poorly sorted material of mixed
origin". I can verify this as I’ve dug several holes around the property and
quickly hit a poorly sorted material with well rounded rocks up to 10"-12"
in diameter. This is a fairly stable soil and shouldn’t slide during a quake. Shaking
appears to be minimal and we don’t seem to notice quakes as much as other folks
located in the Anchorage bowl area.
ThThe construction of the house is another matter. The front of the house has a
large loft area and is made of glass. The open span will allow the side walls of
the house to flex outward and possibly drop the roof. The large glass windows will
certainly break and falling glass will be a major hazard. The house is sheathed
with 3/8" plywood which will resist bending somewhat. Unfortunately, windows
in the side and back are rather large and reduce the effectiveness of the plywood.
Overall, not the best house design for an earthquake area. Get lots of sun through
the windows and have a nice view, though.
References:
Chernicoff,S., Fox,C. and Venkatakrishnan,R., 1997, Essentials of Geology, Worth
Publishers
Conner,C. and O’Hare,D., 1988/1993, Roadside Geology of Alaska, Mountain Press Publishing
Company