Table 3. Description of geomorphic units within Ft. Wainwright, central Alaska, 1998.
Geomorphic unit
Description
Weathered
Highly fractured or poorly consolidated bedrock that can have soil-like properties, but has more evidence of primary
Bedrock (Bxw)
structures than residual soil. Ground surface has abundance of exposed rock blocks. In the study area, this unit was limited
to alpine areas where soil formation is minimal.
Residual Soil
Completely weathered material formed from underlying bedrock conditions that has soil-like properties and little or
over Weathered
none of the original primary structures remaining. Typically there is an increase in particle size at the base of the soil
Bedrock (Bxr)
as it grades into weathered bedrock below. Thin (<30-cm) deposits of colluvial, eolian, or slopewash deposits can be include
in this class. Permafrost generally is absent on south-facing slopes, whereas north-facing slopes are frozen. Most
residual soils in the study area are formed from Birch Creek Schist, a metamorphic rock dominated by micaceous minerals.
Lowland Loess
Windblown silt deposited on poorly drained lowland locations in complex depositional environments near large river
(Ell)
floodplains. The deposit may contain a mixture of eolian sand, retransported, and organic deposits in close association with
the deposits of massive silt. Small hills generally have a thin cover of loess over eolian sand, whereas swales often contain
retransported deposits with higher clay contents or thick organic deposits. It the flat and lowland portions of this unit, the soil
is normally frozen with a high ice content. Small collapse-scar bogs are common. This unit is limited to the flats between the
Tanana River floodplain and the ChenaSalcha Highlands.
Upland Loess/
Windblown silt deposited on well-drained upland slopes. Gully pattern associated with these easily eroded deposits is
Weathered
usually evident on airphotos. Massive silt deposit lacks horizontal stratification and coarse fragments associated with
Bedrock (El/Bxw) residual soil or retransported deposit. Deposit must be at least 40 cm thick. Permafrost is absent. This unit occurs on the Chena
Salcha Highlands near the Tanana River as elevations below 300 m.
Upland Loess/
Windblown silt capping eolian sand dunes deposited over floodplain deposits. Typical profile consists of 0.1 m of
Eolian Sand/
organic material, 0.51 m of massive silt, 2 m or more of fine to medium sand, over fluvial gravel. Permafrost generally
Floodplain
is absent. This unit occurs at scattered locations in the Crooked Creek Flats, Wood River Lower Fan, Dry Creek Fan,
(El/Es/Fp)
and FrenchMoose Creek Lowlands.
Lowland Eolian
Contains a distinct assemblage of lowland loess, eolian sand, retransported, and organic bog deposits that occur in a
Complex
mosiac where individual deposits are too small to map separately. The topography has small knobs associated with
(Ell+Es+Fs+Ob)
stabilized dunes, swales where more clay-rich retransported deposits have accumulated, and thermokarst collapse scars
where organic bog material has accumulated. See descriptions of individual deposits for more detail.
Braided Flood-
Floodplain materials deposited in a river where flow is separated by bars within channels and the stream possesses a
plain Riverbed
higher sediment load than the energy level the stream can support. Sediments generally are composed of coarse-grained
Deposit (Fbr)
detritus. This unit is found along the Tanana River near Eielson AFB.
Braided Active
Relatively fine-grained cover deposits associated with a braided floodplain that are subject to frequent (every 12 years)
Floodplain Cover
flooding and deposition. Due to frequent deposition, the surface usually is partially vegetated or has riverine shrubs,
Deposit (Fbca)
but organic material can not accumulate at the surface. A typical profile has stratified silty and sandy material deposited
during frequent overbank flooding events and lack interbedded organic horizons typical of inactive cover deposits. The cover
deposits typically are 0.51 m thick over gravelly riverbed deposits. This unit forms thin margins adjacent to riverbed deposits
along the Tanana River near Eielson AFB.
Braided Inactive
Relatively fine-grained cover or vertical accretion deposits formed from infrequent (every 35 years or less frequent)
Floodplain Cover
overbank flooding events. Due to the infrequent deposition organic matter accumulates at the surface and deposits
Deposit (Fbci)
have distinct interbedding of organic and mineral layers. A typical profile has 0.050.2 m of organic material, 0.51 m of
interbedded organic horizons and silty or sandy silt layers, 12 m of interbedded silts and sands representing old active cover
deposits, over gravelly riverbed deposits. Permafrost generally is absent. This unit forms extensive floodplain margins
adjacent to the Tanana River near Eielson AFB.
Meander Flood-
Includes both meadering and anastomosing channel patterns characterized by channels that wind freely in regular to
plain Riverbed
irregular, well-developed, S-shaped curves (meandering) or have sinuous anabranched channels (anastomosing). River
Deposit (Fmr)
bed material can range from gravel to gravelly sand.
Meander Active
Similar in material type and stratigraphy as braided active floodplain cover deposits but it occurs on meander flood
Floodplain Cover
plains.
Deposit (Fmca)
Meander Inactive
Similar in material type and stratigraphy as braided inactive floodplain cover deposits but it occurs on meander flood-
Floodplain Cover
plains.
Deposit (Fmci)
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