Calculation of the total area of bedrock exposed in a geologically mapped strip, about 23 miles long and averaging about 1.6 miles wide, over the Harold D. Roberts Tunnel in the Front Range of Colorado indicates that 95 percent of the bedrock is concealed by surface cover. Exact correlation of surface geology with tunnel geology generally was unsuccessful, which suggests that geologic cross sections constructed by subjective downward extrapolation of scattered bits of information in extensively covered areas of complex bedrock geology are likely to be grossly in error. When cross sections are prepared, a clear statement should be made regarding the purpose of the cross section and the amounts and kinds of data that were employed in preparing the cross sections.

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