Experts Surveying Proposed Dam Site fjfflwj'ijg mi m. Read a rock lately? A geologic task force operating north of Heber City has read the equivalent of several rock books recently near the site of the proposed Jordanelle Dam, a major feature of the Bonneville Unit's Municipal and Industrial (M&I) System of the Central Utah Project. Engineers selected the damsite in an upper reach of the Provo River channel because of the safety and economy of the natural "narrows! '-a tapering, V-shaped closure 400 feet high and 1,000 feet wide at the flood plain level. The narrows would require a minimum of fill material, and the impermeable roack abutments abut-ments would create a solid foundation for the dam. Motorists can see the narrows nar-rows on either side of the U.S. Highway 40 about 6 miles north of Heber City near a point where the highway crosses the Provo River. The drilling team, working this past winter in temperatures tempera-tures down to -35F., uses standard coring drill rigs that bore to depths of several hundred- feet. As core samples are brought up, they v are labeled, boxed, and logged. The team continually studies plans for the dam, spillway, outlet works, and powerplant as they drill on the foundations of these major features. What do these rock pages of history reveal? Core samples provide technical data important to designing the dam; such as, rock type and strength, possible faulting fault-ing and fractures that would permit moisture seepage, and ability of the rock to withstand erosion. A. typical core sample at the Jordanelle site would show sedimentary rock below be-low the 3f 0-foot level. This highly compacted rock dates , back to 60 million years ago when dinosaurs j . made their final exit and dog-sized horses and the ancestors of modern primates were beginning be-ginning to appear. During this period, erosion and faulting determined the water drainage development of the area. These early drainage patterns played an important role in sculpturing the landscape to form the foundation upon which the next rock layers would be deposited.' .Core samples directly a-bove a-bove the , sedimentary rock date back to about 30 million years ago. These rocks record the violent volcanic action that spewed ash, rock Mocks, and other extrusive ruddle over many square miles in the Ketley-Kamas-Heber area. This volcanic ? formation , comprises the-: lower foundation of the proposed damsite. Continued volcanic action frosted the area with thick, viscous flows of andesite, like syrup over an ice cream sundae. These andesite flows form the solid, impermeable imper-meable abutments of the proposed dam. The volcanic phase with its associated faulting ended 18 to 20 million years ago, about the Jordanelle Reservoir bite into the Weber River, were diverted into the Provo River system to flow into Utah Lake. Large Volumes of Glacial melt waters began draining from the upper Uinta Mountain valleys throught the Jordanelle Damsite area. This flow cut a now buried channel about 100 feet below the present flood plain surface at the damsite. As the glacial water slowed, part of the sediment was redeposited, filling the channel and forming a narrow flood plain between outcrops of resistant andesite. ande-site. THis buried channel will be excavated to bedrock for the placement of the cut-off trench for Jordanelle Dam. " Thus the geologic setting at Jordanelle Damsite has been formed ove rthe past 60 ' million years by processes of deep-seated earth actions, glaciation, and stream erosion. ero-sion. The hardness of the rock at the damsite and its ability to withstand erosion has created the narrow gorge suitable for the placement of Jordanelle Dam. time ancestors of modern bears, dogs, and cats were becoming common on the earth. The proposed Jordanelle Damsite was thus prepared for the next major geologic development-the glacial , period. During this time, a deep gorge would be cut throught the andesite to form a convenient course for the Provo River. .Less than a million years ago, as our present landscape land-scape began to appear, rivers in the upper Uinta Moutains became swollen with glacial melt waters, which caused drainage patterns pat-terns to change drastically. The headwaters of the Provo River, which had drained