The purpose of your visit is to recognize how the geology here was
altered by the Great Alaska Earthquake of March 27, 1964. An
informational kiosk at these coordinates on the paved bike path
provides the necessary information to complete your learning about
Girdwood’s “Ghost Forest” of silver-gray trees.
The Seward Highway, from Mile 88 to Mile 91, forms the southern
boundary of Girdwood Valley and its community. The northernmost
corner of Alaska’s coastal rain forest is found here, crowded
in a narrow strip of land between the waters of Turnagain Arm and
the rocky glacial terrain of the Chugach Mountains. Sitka spruce
and western hemlock tower over the forest undercanopy, appearing
gargantuan relative to the black spruce found elsewhere in
southcentral Alaska.
The coordinates bring you near a bridge over what Alaska Dept of
Transportation calls “Tidewater Slough #6.” A small
watershed between the valley’s west-side lateral moraine and
the west-side mountain ridge drains freshwater into this slough. A
large beaver pond straddles the stream about .3 miles north of
here. You’re standing in the midst of a low marsh zone called
the Girdwood Marsh, created as a consequence of the Great Alaska
Earthquake of 1964. This area looked very different prior to the
earthquake, when Girdwood’s original townsite buildings
spread eastwards along both side of the highway from here. A gravel
airstrip paralleled the highway on its south side, across from the
intersection of the Seward and Alyeska Highways. A second-growth
forest of spruce and hemlock trees populated the landscape above
the limits of the high marsh zone. The earthquake of March 27, 1964
changed the landscape dramatically, resulting in what you see
around you today.
The epicenter of the Great Alaska Earthquake of 1964 was located in
Western Prince William Sound about 75 miles east-northeast of
Girdwood. The event caused uplift and subsidence that affected an
area of more than 54,000 sq mile along 500 miles of the
Alaska-Aleutian subduction zone. Girdwood lies in the area of
significant subsidence, so its elevation above mean sea level fell
dramatically. Within two weeks of the earthquake, tidewater had
reached the forested lands around this location. Most of the homes
and businesses in the old townsite found themselves with tidewater
surging across their ground floors at high tide. The influx of
tidewater forced most owners to move their homes and business to
the new townsite, about 2 miles north up the valley from this
location.
Geologic vocabulary terms related to this valley and
event:
coastal rain forest – In North America, a temperate
rain forest is defined as receiving annual precipitation in the 75
to 150 inch range, with a mean annual temperature between 39 and 54
degrees Fahrenheit. A common feature of Pacific temperate rain
forests of North America is the Nurse log, a fallen tree which as
it decays, provides ecological facilitation to seedlings –
seen here in Girdwood Valley
watershed - the area of land where all of the water that is
under it or drains off of it goes into the same place
lateral moraine - parallel ridges of debris deposited
along the sides of a glacier, parallel to its flow
low marsh zone – that area of a coastal marsh
flooded daily by high tides, with relatively high salinity
high marsh zone – that area of a coastal marsh only
flooded perhaps monthly by extreme neap tides, with relatively low
salinity
tectonic plate – one of the independently-moving
sections of the Earth’s crust; geologic events such as
earthquakes, volcanic activity, mountain and trench-building occur
where these plates meet
uplift – an increase in elevation relative to sea
level as caused by plate tectonics
(movement of the earth’s crust)
subsidence – a decrease in elevation relative to
sea level as caused by plate tectonics
(movement of the earth’s crust)
subduction zone - an area of the Earth’s crust
where one tectonic plate moves under another tectonic plate,
sinking into the Earth's mantle, as the plates converge
deposition – the ‘falling out’ of soil
particles from water (in this case, tidal waters of Turnagain Arm)
which occurs as the velocity of the water slows enough that gravity
overcomes the ‘transport ability’ of the flowing water
and soil particles are left behind by receding tidal waters
second-growth forest - A forest or woodland area which
has re-grown after a major disturbance such as fire, insect
infestation, timber harvest or windthrow
Learning Questions to be answered with
specific information from the site kiosk &
visit:
1. What is the magnitude of the ‘Great Alaska Earthquake of
1964’?
2. How far did the land sink here?
3. What specifically caused the death of these ‘ghost
trees’?
4. What has caused these dead trees to be so well-preserved?
5. How long ago did a prior ‘great quake’ occur
here?
6. Name the 3 distinct habitats here?
a.
b.
c.
Please email your answers to the cache owner. Do not include
your answers in your online log.
Not required but feel free to a photograph a view of the
Ghost Forest that way we can all see the different seasons and
maybe some of the inhabitants.
Placed under Chugach State Park Permit #09-076