Dames & Moore, 1999 - USDA Forest Service

silver, and included 34,000 tons of removed rock. However, no indications of the mine workings were
discovered on aerial photographs or in the field during the RI.
Several other prospects andlor mines were developed at Crown Point Falls, approximately 10 miles west of
the Site, as well as above Lyman Lake. The mineral deposits included antimony, arsenic, cadmium, copper,
gold, lead, molybdenum, silver, and zinc. Based on observations during the RI, the Crown Point prospect
included several workings in the cliff face to the south of Crown Point Falls, as well as a single drifi to the
north of the falls, with no apparent seepage flowing from the openings of the workings.
4.2.3 Site Geology
4.2.3.1 Near-Surface Geology
Summary of Historical Data
Referring to Figure 4.2.6a, the surticial geology in the Site area consists primarily of alluvial, colluvial, and
glacial deposits overlying bedrock. The alluvial soil is deposited by water-related action and, therefore,
found near the bottom of the Railroad Creek valley. The alluvium is normally composed of rounded sandy
gravel- to cobble-size material. These alluvial deposits are considered to be normally very well drained, with
localized areas of poor drainage.
The colluvium includes those materials which are deposited by gravity and are, therefore, generally found
near the base of relatively steep slopes. The colluvium is generally characterized by a mixture of silt- to
boulder-size, angular soil and rock fragments. These soils can Age from poor to well drained.
The glacial soil in the area generally consists of glacial till, a dense mixture of rounded silt- to cobble-size
materials. These materials are anticipated to underlie the alluvium and colluvium, and have been
observed extending up the valley sidewalls. The glacial soil is normally more than 10 feet thick (ORB,
1975). During the construction of the main mine portal, the glacial materials were found to be
approximately 65 feet thick (Adams, 1981), and were observed to thin with increased elevation above the
valley floor, as would be expected from such a deposit. The mapping indicates that the glacial till
terminates above the 1100 level in the Honeymoon Heights area; the glacial till appears absent, with
bedrock exposed at the surface at the 300- through 1000-level portals (Figure 4.2.6a).
RI Findings
The geology of the Site was hrther characterized during the RI utilizing the results of: (I) a seismic
refraction survey and downhole geophysical evaluation completed across selected portions of the Site for
the RI (Appendix A); (2) borings drilled by others during the installation of the groundwater monitoring
wells (Appendix B); (3) test pits completed for the RI (Appendix C); and (4) borings and test pits completed
by others as part of a previous geotechnical engineering evaluation (Appendix E). The locations of the
explorations are presented on Figures 4.2-6b and in Appendix K.
The Site geology was disclosed to consist of the following principal units: (1) native soil and fill soil, (2)
colluvium, (3) alluvium, (4) tailings material, (5) waste rock, (6) alluvium/reworked' glacial till, (7) glacial
till, and (8) bedrock. A brief description of each unit follows.
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17693-005-019Uuly 19. 1999;4:51 PM-DRAFT FINAL RI REPORT