Field Trip Day 3 - Eagle River Area
Overview of Trip
The objective of this trip is to look at glacial features in the Hiland Valley and
Eagle River valley areas. How development in the area has accommodated these features
is also examined. Both these valleys have experienced recent heavy glaciation. Glaciers
were present 14,000 - 16,000 years ago. Outwash from these glaciers contributed
to the Bootlegger Cove clay and Elmendorf moraine formation.
The dominate geologic feature of the area is the Chugach mountains rising up from
the coastal plain. The Border Ranges is located along this juncture and resulted
in the sharp contrast. The Eagle River fault also passes through Eagle River Valley
and separates the McHugh melange formation from the Valdez flysch formation.
Stop 1 - Nunaka Valley Park
Location: Corner of DeBarr and Beaver Road, near Baxter.
Description: A park located in a residential area. The park area is level, but a
hill is seen to the south and another to the west. The hills are covered with vegetation.
While you can’t tell from this angle, the hills are 3-5 times longer than the width
and are about 30-50 feet high. The hills are about 1-1/2 city blocks long and 1/4
block wide with a N/S orientation.
Interpretation: The hills are thought to be drumlins. Drumlins are sub-glacier features
and are formed when a glacier passes over the terminal moraine of a previous glacier.
The terminal moraine is eroded away by the new glacier and redeposited as long,
asymmetric glacial till piles. Drumlins are composed of glacial till, the same material
as the terminal moraine. They have their long axis oriented in the direction of
the eroding glacier movement.
These drumlins indicate that the area was covered with a glacier. The glacier progressed
over the terminal moraine of an earlier glacier and reformed the moraine as drumlins.
The recent glacier was traveling to the south due to the N/S orientation of the
drumlins. This area is a large drumlin field. These are not classic drumlins - they
have asymmetric features and are instead referred to as “drumlinoids”.
Other terrain features from glaciers include eskers, moraine and roche mountonnee.
Eskers are formed when water channels running under glaciers cut channels in the
bedrock. These channels can become filled with sand and gravel and when the glacier
recedes, the long, sinuous ribbons of clean gravel can remain. Moraine is characterized
by U or lobate shaped features and is composed of till. Roche mountonnee is an erosional
feature formed when a glacier deeply quarries the down glacier slope of a hard rock
formation. They are normally composed of bedrock.
There are two types of material commonly associated with glaciers. Glacial till
is the broken up pieces of rock torn up from the surrounding bedrock and transported
by the glacier. It is very poorly sorted, has no stratified sediments and is composed
of all sizes of rocks, from silt sized particles to boulders. Till is deposited
directly from glacial ice. Outwash is the material carried out of the glacier by
melt water runoff. It has a lower silt content - as runoff carries the sediment,
the heavier, larger rocks drop out of the current while the smaller particles are
carried downstream. The rocks are smooth and well rounded and depositional features
such as graded bedding can be seen in outcrops.
Stop 2 - Elmendorf Moraine
Location: Take the Hiland Road exit from the Glenn highway. Turn left, cross over
the overpass, park to the left in front of the dump entrance. Hike along the service
road to the top of a road cut, located on the north side of the road.
Description: The slope is composed of light colored glacial till. Rocks are poorly
sorted and there is a wide assortment of sizes. Soil to bowling ball sized rocks
are present. No sedimentary structures are apparent.
A pit was dug. There was a thin layer of organic material, with a gray/white layer
under that. A layer with a deep red color that lightened up with depth underlaid
this layer.
Interpretation: The rocks here are part of the Elmendorf formation. Rocks associated
with the Elmendorf formation are generally part of an outwash plane. 12,000 - 13,000
years ago, a glacier terminated in Eagle River valley area and a huge outwash plane
formed. Streams pouring out of the glacier washed the silt away. Along Eagle River,
there are beds of channel and stream deposited outwash. Mt. Susitna has roche mountonnee
features with glacial erratics on top - at least one glacier went out further than
the one that deposited the Elmendorf formation and was at least as thick as Mt.
Susitna is high, 3000 feet.
The Elmendorf formation is underlaid by the Bootlegger Cove (BLC) mud. Fossils indicate
that the environment was fresh, brackish and salty, indicative of a marine estuary
environment. BLC mud gets thinner towards the mountains and thicker by the coast.
It is found over a large area - from Willow to Hope. Fossil dating indicates that
the age is approximately 13,000 BP. A large estuary environment was probably present
13,000 years ago and the quiet, calm waters allowed the mud to settle out of the
glacial streams. Once the glaciers melted, the loss of weight allowed the land mass
to rebound and raise BLC above sea level.
These formations are dated using peat and fossils in old lake and pond deposits.
Large pieces of ice trapped in rock melts and the top collapses, forming a lake.
This results in the hummocky topography of the Elmendorf area and is the source
of most of the lakes in the Anchorage area. Lakes are short term geologic features
and relatively quickly fill in with sediment and become a bog. Using carbon14, peat
on the lake bottom can be dated.
The soil in this area is classified as “Spodosol”. This soil gets it’s name from
the high level of eluviation of the A horizon and the stained B horizon. The high
sand content and a high acidity level from the spruce/conifer needles results in
lots of leaching in the soil. The organic acids percolate into the soil and leach
away the acid soluble minerals. This leaves lots of quartz/sandstone in the top
layer. The minerals are redeposited in lower layers. The C layer is is unaltered
and is underlain by till. The soil is thin because it is only 12,000 years old and
the cold climate results in slower organic material growth.
Stop 3 - Kenai Group Outcrop
Location: Take the Eagle River Road exit off of the Glenn Highway. Turn left, cross
the overpass, turn left on Frontage Road. Park in the turnout at the base of the
hill. Walk down the trail to Eagle River.
Description: Well rounded rocks are along the river bank. Sizes range from pebbles
to cobble and occasional larger rocks. Lots of mud clasts are on the bank. Most
rocks appear to be sandstones from the Valdez formation. Larger rocks showed coarsening
upwards sequences in some of the layering. Across the river, a steep cut bank is
being eroded away and layers of varying thickness are seen. A darker layer is present
in the middle and several layers of what appear to be sandstone are present above
and below the darker layer. The beds are horizontal and exhibit no folding.
Interpretation: The cut bank across the river is an exposure of the Kenai group.
This group is part of the Peninsular terrane and is Tertiary age. It is overlain
by the Elmendorf moraine in the Anchorage area. Coal is present and can get up to
40 feet thick in the Mat-Su valley at at Beluga. Presence of coal indicate a swamp
and with sand, mud and coal occurring together, it could have been a delta environment.
Oil and gas also occur and significant fields have been discovered at Swan River
and in Cook Inlet.
The Peninsular terrane was formed around 20 degrees north and moved its present
position. Kenai group sediments were deposited on top of the Peninsular terrane
sediments after its arrival.
Stop 4 - South Fork Overlook
Location: Drive out Hiland Road, turn on Johnny Road, drive to end.
Description: Nice view up Hiland Valley. Many glacial features noted. Houses are
positioned at the bottom of clear, treeless gullies.
Interpretation: Foundation construction for homes built on these steep slopes often
consist of excavating material from the upslope side of the hill and redepositing
it on the downslope side. This can cause problems if the upslope side starts to
collapse and fall in around the house. Also, the material deposited downslope can
head downhill or compact, allowing the foundation to tilt or crack. Factors that
effect the slope are:
- What’s underneath? bedrock? till? loosely consolidated outwash?
- How much water? snow melt? freeze/thaw cycles? In the spring, snow melt saturates
the soil with water and makes it easier for soils to move.
- Will the vegetation hold the slope together?
Several of the houses are built in avalanche zones. These can be identified by the
lack of large trees and a slope of 25 to 40 degrees. One house has been knocked
off its foundation in the past.
Stop 5 - Solutions to Unusual Problems
Location: Drive out Hiland Road, stop 1/4 mile past Chugach State Park turnoff sign
at old quarry.
Description: Rocks exposed in the old quarry are angular and very poorly sorted.
Rock size varies from dust to car sized boulders. The quarry is located high up
the side slope of the mountain. Several houses are located downslope from the road
in a treeless area.
Interpretation: What is the source of this material? A lateral moraine would contain
poorly sorted rocks, but they should be better rounded. This is an alluvial fan,
formed from rockfall off the adjacent mountain. Material from the quarry was probably
used for road or home foundation construction.
Houses in this area are in an avalanche hazard zone. To reduce the chance of damage,
home owners have piled material on the upslope side of their homes. The idea is
that as snow slides down the mountain, it encounters this barrier and is deflected
away from the house.
Another problem in the area is installation of a drain field for a sewage disposal
system. Gravel that is permeable is necessary to allow water to drain off. The drain
field also needs to be 3-5 feet above the ground water level to avoid contamination
and provide adequate leak off. Several houses have had to install an elevated drain
field. If the field can’t be located below the house, it will require installation
of a pump, increasing maintenance costs.
Stop 6 - Chugach State Park South Fork Eagle River Parking Area
Location: Drive out Hiland Road, follow signs back across the South Fork Eagle River
to the Chugach State Park parking lot. Hike up the trail, heading up valley.
Description: 4 stops were made along the trail.
Stop 1: Approximately 1/4 mile up the trail. A benchmark is located beside the trail.
Several trees have slash marks on their trunks.
Stop 2: Approximately 1/2 mile up trail. Nice view point of Hiland valley. Good
views up and down the valley, lots of glacier features noted. Scrub birch and other
low bushes cover mountainside.
Stop 3: Approximately 3/4 mile up trail. This stop is in a hemlock grove. Trees
have soft needles and purple colored pine cones.
Stop 4: Approximately 1-1/4 miles up trail. Observed numerous glacier features on
the landscape.
Interpretation:
Stop 1: Bench marks were put in by the USGS and are used by land surveyors. The
markers don’t have location or elevation but instead have an identifying number
that can be referenced at their offices. To locate the markers easier, bearing trees
had large, vertical slash’s cut into their trunks. Surveying land in Alaska can
be very expensive. At one time, there was a program called the “Open to Entry Land”
and someone staking a claim could attempt to gain ownership. Often, the survey would
cost more than the land was worth.
Stop 2: We stopped here to examine the valley floor and look for any changes. A
change in topography could be seen. Further down valley, the valley floor was fairly
flat. The floor became much more hummocky as you move up valley, indicative of piles
of moraine.
Vegetation in the area is shrub birch and soft, spongy hummocks. These are normally
found in permafrost areas. This stop is close to treeline. The dividing fault between
McHugh and Valdez formation is located along the ridge to the south.
Stop 3: The hemlock indicates the presence of a coastal forest. A coastal forest
is present along the coast of Alaska starting in Anchorage and going south of Juneau.
Hemlocks have only been in the area 3000-5000 years. Pollen that falls on lake surfaces
settle to the bottom and is preserved in the mud. Carbon dating can then determine
the age if the sediment the pollen is found in.
Other trees in the forest include black and white spruce. Black spruce is found
in marshy areas and swamps. They are out-competed in dry areas. White spruce are
found in dryer areas and grow to large sizes. Starting in Portage and found to Seward
is the Sitka spruce. These are early colonizers and indicate the coastal forest
is very young. The plant revegetation sequence is shrub birch -> alder -> willow
-> spruce and currently hemlock.
During the last ice age, southern Alaska was covered with ice. The ocean was the
source of water and the sea level was 400 feet lower than today. Fairbanks and central
Asia were not covered with ice - moisture was pulled out of the air by the Chugach
mountains and was dried out by the time it reached the interior. A land bridge was
exposed and animals roamed between the continents, grazing on the tundra step in
Alaska. All trees were extinct in Alaska and the coastal forest survived in the
Oregon/Washington area.
Stop 4: Looking up the valley, many terrain features resulting from glaciers are
apparent:
- A hanging valley comes in from the east. It was formed where a glacier in the
adjacent valley connected with the glacier in Hiland Valley.
- Lateral features on the sides and in the center of the valley. Along the slopes
of the mountains, linear features sloping slightly towards the head of Hiland Valley
are present. These are the remains of moraines pushed out the side of the glaciers
and up the mountain slopes. Several different glacial episodes can be identified
due to the presence of distinct bands on the slopes.
- A horn could be seen at the head of the valley. Horns result when 2 or more glaciers
carve away the sides of a mountain and leave a pyramidal shaped hill.
- Lakes from buried ice pieces melting then having the tops fall are seen.
- In the valley bottom, terminal moraines that result in hummocky topography are
present.
- Far up valley, cirques could be seen at the head of the glaciers. These are deep,
semicircular basins carved into the mountain by an alpine glacier.
The hummocky terrain is due to a terminal moraine and other moraine material left
by a glacier. The flatter area located down valley is boggy and has silt layers.
A large lake is thought to have formed in front of the glacier, depositing silt.
This would result in poorer drainage and could cause problems when installing septic
fields or house foundations.
Stop 7 - Eagle River Overlook
Location: Park at corner of Hiland Road and Wild Mountain Drive.
Description: A cleared out area by the road. The north side is a steep drop, down
to Eagle River. Very overgrown, difficult to see over the edge. There is a boulder
by the road. The boulder has a green colored conglomerate on top of a lighter colored
sandstone.
Interpretation: The rock is from the McHugh formation. This location is near the
Eagle River fault and it was probably brought here for landscaping.
A water well dispute occurred in the Eagle River valley several years ago. Early
settlers drilled shallow water wells. When larger neighborhoods went in, they used
community wells. These wells were much deeper than the private wells and produced
at much higher rates than the individual house wells. the community wells pulled
the water table down and shallower wells went dry.
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
Chugach Gem and Mineral Society, 1986, Alaska ... A Guidebook for Rockhounds, Glacier
House Publications
Zimmerman, J., 1993, A Naturalists Guide to Chugach State Park, A.T. Publishing
and Printing