Dames & Moore, 1999 - USDA Forest Service

8.2.2 Geology
8.2.2.1 Railroad Creek Watershed
The Railroad Creek geology reflects ongoing tectonic mountain building. Based on historic data, Railroad
Creek is located in an area of moderate seismicity due to the mountain building processes. The geology of
the'watershed consists of metamorphic and igneous rocks resulting from the compressive forces associated
with the mountain building. The bedrock is exposed at the ground surface throughout the Railroad Creek
watershed, but predominantly along the valley walls and ridge lines.
The bedrock was modified into a u-shaped valley by glaciation that occurred within the last 12,000 years
(Pleistocene period). Lyman Glacier, located approximately 10 miles to the west of the Site reflects the
remnant of the Pleistocene glacier. The glacier removed the bedrock to form a mixture of clay-sized silt,
sand, gravel, cobbles, and boulders. The soil was deposited on portions of the floor of the valley and the
lower valley walls. The soil mixture was compacted by the glacier into a dense glacial till. ,
During the most recent recession of the glaciation, the glacial till was covered in places with mixtures of
relatively loose sand and gravel with less silt, cobbles, and boulders. as noted at the existing Dan's Camp
gravel pit in the lower of Railroad Creek. This material is known to be more permeable than the
glacial till.. Since the glacial period, Railroad Creek has continued to rework the deposited exposed near the
ground surface, in some isolated places removing the glacial deposits to expose the underlying bedrock.
CE.
Localized areas throughout the Lake Chelan region contain deposits of economic minerals that have resulted
in natural enrichment or mineralization. Areas of mineralized zones in bedrock have been mapped by others
throughout the Railroad Creek watershed; the economic metals noted by others as being present in the
watershed include those extracted from the Holden Mine (copper, zinc, gold, and silver).
In addition to the Holden Mine, more than 15 other mineral prospects and several smaller mines were
developed in the Railroad Creek watershed. Two mines were located approximately 10 miles to the west of
the Site, and were reported to have been operated by others early in the 1900s. One prospect was reportedly
developed by Howe Sound Company above Holden Lake, northwest of the Site near Martin Ridge at an
elevation of approximately 7,500 feet. These workings reportedly included three primary adits, the most
extensive being 370 feet in length. A preliminary assessment of the Holden Lake prospect was conducted as
part of the RI. In addition, water quality sampling of Holden Creek (which flows from Holden Lake) was
completed during the RI. Based on the findings, additional evaluation of the Holden Lake prospect was
determined not to be necessary.
8.2.2.2 Site
General
Based on seismic refraction data collected as part of the RI, glacial till covers the bedrock underlying the
Site. However, the depth to bedrock near Railroad Creek between tailings piles 2 and 3 was relatively
shallow (less than 20 feet), and isolated exposures of bedrock appear to exist immediately downstream of
tailings pile 3 in the south bank of the Railroad Creek, and within the lower portion of Copper Creek; the
absence of glacial till in the areas is likely due to stream erosion.
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17693-005-019Uuly 28.1999;10:24 Ah4;DRAFT FMAL RI REPORT