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Rock Island is a hydrothermal locality situated in Tertiary volcanics at the base of the Rock Island Dam. It was unearthed when dredging for repair of the dam's footings was undertaken in the early 1950s. Some of this material is accessable in tips along the margin of terraces adjacent to the access road just to the south of the dam on the west bank of the Columbia, including the fore- and middle-ground of this photo. Specimens occur in small cobbles through small boulders of vessicular basalt, welded tuff and ignumbrite. |
| Material can also be collected on the east side of the river along the beach below the dam at low water. The SEMs come from material collected on the east bank. As part of repair and maintenance, cores were taken accross the reservoir just in back of the dam. No hydothermal deposits were encountered, so the actual source of the deposit remains a mystery. Right - arrow marks the collecting area on the west bank. |  |
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Left - Upper arrow is the downstream extent of material on west bank. Lower arrow suggests the location of the collecting beach on east bank when the water is lower. |
