radic layers and lenses of silt.* The materials at
through fractures, but their locations and orienta-
depth were likely deposited by meandering or
tions are generally unknown and difficult to pre-
braided streams that were ancestral to the Chena
dict. Water within fractures that trend generally
and Tanana Rivers. In addition, a layer of wind-
northsouth will tend to flow in accord with the
blown silt (loess), less than 1 m thick, commonly
southerly slope of Birch Hill and enter unfrozen
lies just below a thin (less than 0.5 m) surface layer
alluvial sediments from below (e.g., Williams 1970,
of organic material.
Pw et al. 1976). If such an aquifer is confined by
Unconsolidated materials on Birch Hill have
permafrost, the resulting piezometric surface
also moved downslope to form colluvial deposits
could become artesian, which will maintain iso-
composed of weathered bedrock, silt, and sand.
lated zones of unfrozen materials beneath and in
These deposits extend up to several hundred
the permafrost (e.g., Hopkins et al. 1955).
meters south from the base of the hill and overlie
The migration pathways of contaminants will
the prevailing alluvial sediments (e.g., Pw et al.
follow the aquifers and, thus, be affected by all
1976). In addition, runoff from gullies cut into Birch
the factors that determine ground water flow.
Hill has locally deposited sand and silt with thick-
nesses of up to 24 m. These deposits form fans
Surface and bedrock geology
Unconsolidated materials with relief generally
that can extend several hundred meters south of
Birch Hill.*
less than a meter make up most of the surface
The cross-cutting of alluvial landforms south
south of Birch Hill (Pw et al. 1976). Alluvial
of Birch Hill reflects the migration of the Chena
deposits cover most of the area of the Chena River
River (Fig. 5). Meander scrolls with associated
and its tributaries, and are represented at the
channels or swales generally trend eastwest in
surface by various landforms, including point
bars, swales, channels, and sloughs (Fig. 4). Drill-
ing has revealed that most of the unconsolidated
* Unpublished borehole records, U.S. Army Engineer District,
Alaska.
sediments consist of gravels and sands, with spo-
Figure 5. Assemblages of alluvial deposits as distinguished by the trends of point bars, meander
scrolls, former channels, and levees north of the Chena River. Cross-cutting relationships of
these features show that the surface deposits are of different ages. Compare with the aerial photo-
graph in Figure 4.
5
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