brought hordes of adventurers westward. They poured across the Colorado River at two points. One crossing was near Yuma, Ariz., and the other at Needles, Calif.

With the acquiring of the territory, sentiment grew in the United States that the area should be explored at all costs. In 1857 the War Department dispatched Lieutenant J. C. Ives to the task, instructing him to proceed up the river by boat as far as practicable. He succeeded in going 400 miles up the river to the Black Canyon, present site of Hoover Dam. He reported the region to be valueless.

In 1869 Major J. W. Powell of the Geological Survey succeeded in leading a river expedition down the canyon of the Colorado. His expedition traveled from Green River in Utah to the Virgin River in Nevada, a few miles above where Lieutenant Ives stopped. Powell covered a thousand miles of unknown rapids and treacherous canyons, and became the first white man to gaze up at the sheer walls of the Grand Canyon and live to describe the adventure. Further discussion of Major Powell's investigations is given in section 9.

Following this, efforts were made to investigate methods whereby the river might be used beneficially. The river, annually fed by melting snows in the Rockies, swelled to a raging flood in the spring, then dried to a trickle in the late summer and fall, so that crops were frequently destroyed. Farmers built levees

to keep out the river. Even when the levees held, however, crops withered and died during the months when the river ran too low to be diverted into the canals.

The flow of the Colorado River is extremely erratic, ranging from 4 to 22 million acre-feet annually at Lee Ferry. There is a tendency for the high years or the low years to be grouped, thus accentuating problems of river regulation and use. Faced with constantly recurring cycles of flood and drought, the people of the Southwest appealed to the Federal Government to solve the problem. With the establishment in 1902 of the Reclamation Service, engineers began extensive studies of the river in search of a feasible plan for its control. The completion of Hoover Dam in 1936 undoubtedly did more to regulate the erratic flow pattern of the mighty Colorado than any other single item of construction. However, the upper reaches of this river were still more or less uncontrolled. The Glen Canyon Dam was one of the key structures depended upon for additional regulation of this river system.

7. COST SUMMARY. The following tabulation summarizes the total estimated cost of construction of Glen Canyon Dam and Powerplant and appurtenant structures based on information contained in the Final Construction Report. The costs are as of June 30, 1967. A listing of contracts and purchase orders is contained in appendix A.

(b) Structure.-The Glen Canyon Area is a part of the Kaiparowits region which Gregory1 describes as follows: "As suggested by the wide, sensibly flat plateaus and long, even crested escarpments, the rock beds throughout most of the Kaiparowits region are gently inclined or nearly horizontal. This simple general attitude is interrupted in places by sharp monoclinal flexures, which trend in a general northerly direction and subdivide the region into large gently tilted blocks. In places minor undulations interrupt the otherwise regularly inclined beds between the monoclines. West and northwest of the Kaiparowits region northward-trending faults divide the plateaus into blocks not unlike those produced by the monoclines. In each of the three monoclinal folds that traverse central southern Utah the dip of the beds is eastward and the rocks on the west are elevated and those on the east depressed. Along each of the faults the movement is in the opposite direction, the rocks on the east being elevated and those on the west dropped. The monoclinal folds affect all the rocks from the uppermost Cretaceous downward but do not involve the Tertiary, whereas the faults displace the Tertiary beds as well. The displacements of the two types are thus of different geologic age.

"Aside from the deflections that are due to the monoclinal folds, the general inclination of the beds in the southern part of the Kaiparowits region is northward, for the rocks here constitute the north flank of the broad Grand Canyon upwarp in Arizona."

Local uplifts, due to igneous intrusions, are represented in the Glen Canyon area by the Navajo and Henry Mountains. Evidence of other igneous activity in the area is found only in the recent basalt flows which cover parts of the higher plateaus.

All of the large faults and monoclinal folds are located a considerable distance from Glen Canyon Dam, so the beds in the latter area have been only slightly disturbed. At the damsite, the massive Navajo sandstone beds lie essentially horizontal with only a slight dip (10 to 2°) upstream and into the left abutment. This is in harmony with the regional dip which is northeastward away from the Grand Canyon uplift.

There are no faults in the immediate vicinity of the damsite. The area is characterized by relatively few joints of two distinct types: (1) Steeply dipping subparallel joints restricted principally to a single set trending NE-SW diagonally across the axis of the dam,

and (2) stress-relief joints roughly paralleling the canyon walls. The joints of the former type resulted from regional warping. They are generally tight and relatively clean and continuous; individual joints can be traced across the extensive bedrock exposures for distances of up to 800 to 1,000 feet. The latter type of joints reflect the reduction of loading due to the erosion of the deep canyon; they are discontinuous and spaced from 1 to 5 feet apart, commonly open from 1/16 to 1/4 of an inch. The resulting "sheeted" structure parallels major natural rock faces; it diminishes with depth back of the rock face and disappears within 20 to 50 feet.

(c) Stratigraphy.-The Navajo sandstone forms the canyon walls at the damsite and throughout most of the reservoir basin. It has a larger outcrop than any of the other Jurassic formations in the area. At the damsite, the Navajo sandstone, over 1,400 feet thick, extends from approximately 1,000 feet above river level to more than 400 feet below river level.

The Navajo sandstone is buff to reddish, medium to fine grained, and moderately hard to soft. It is massive with pronounced crossbedding and commonly indistinct horizontal bedding. It is composed essentially of quartz grains with a minor amount of feldspar and is poorly to moderately well cemented principally by secondary quartz, chalcedony, and to a much lesser extent by calcite and hematite. The sandstone is moderately porous and highly absorptive, owing to the high capillarity created by the small size of intergrain pore spaces.

The Navajo sandstone is remarkably uniform and homogeneous over wide areas and nearly identical samples can be obtained from areas separated by many miles. Two thin, shaly layers, encountered at elevations 3065 and 3115 in the right abutment keyway excavation were the only changes in the lithology in the entire excavation area.

9. INVESTIGATIONS. (a) Early History.-Perhaps the first explorer to traverse the Glen Canyon area was Father Silvestre Velez de Escalante. Father Escalante and his party crossed the Colorado River on September 26, 1776, at a point which has since become known as the Crossing of the Fathers. This crossing has been inundated by Lake Powell, as it was located about 10 miles upstream of Glen Canyon Dam.

Major John Wesley Powell, working under the sponsorship of the Smithsonian Institution, made the first ecologic traverse of the unknown canyons of the Colorado River during the summer of 1869. The section of the Colorado River from the mouth of the

Green River to the mouth of the Virgin River, a distance of 539 miles, was covered in 23 days. This first trip was hurried due to the loss of supplies at the beginning of the trip; as a result, Major Powell was able to make only a fraction of his intended studies.

Not satisfied with the results of his first survey, Powell determined that he would once more attempt to pass through the canyon in boats, devoting 2 to 3 years to the trip. Supplies were taken by pack train to several strategic points on the river where they would be available as the boat party progressed. The second expedition left Green River, Wyo., in May 1871. During this second trip and the years following, the parties of the Powell survey completed topographic maps of the area from the Henry Mountains to Kanab, Utah. These maps for many years constituted the sole available information regarding the topography and drainage of this region.

Valuable as were these data collected by the Powell survey, they were not sufficiently detailed for definite planning for the development of the Colorado River. Consequently, in the years 1921 and 1922, the U.S. Geological Survey undertook, in cooperation with power companies, a comprehensive survey of the potential power and water resources of the Colorado River. This survey consisted of taking strip topography of the canyon with more detailed survey of potential damsites. A hydrographic survey, with stream gaging at key locations, constituted an important part of these studies. Six potential damsites were studied from the mouth of the San Juan River to Lee Ferry. The site most favored was the Glen Canyon No. 1, located at RM4 (4 miles upstream from Lee Ferry, Ariz.). Six core holes were drilled at this site with the assistance of the Southern California Edison Co.

Concurrent with the drilling and other surveys at the Glen Canyon No. 1 site, the Bureau of Reclamation employed a Board of Consultants to examine the Glen Canyon damsite along with other sites on the lower Colorado River at Boulder and Black Canyons. This Board pointed out certain undesirable features of site No. 1 and, in particular, one set of rather closely spaced joints which crossed the proposed axis in a diagonal fashion. The Board suggested that the axis be moved downstream where the foundation would be improved without materially increasing the width of section. Work at Glen Canyon was not resumed until 1946 when the Bureau began a reconnaissance survey, thus initiating the studies that led to authorization of the project.

(b) Choice Of Site.-A number of damsite locations in Glen Canyon were considered, but

planning investigations resulted in the elimination of all but two. These were located at RM4 and RM15, respectively, above Lee Ferry.

Reconnaissance studies of the two sites in 1947 indicated that the RM15 damsite was geologically superior to the RM4 damsite. Although the site at RM4 had serious geologic deficiencies, it had an apparent topographic advantage so it was felt that it should not be discarded without further preliminary testing. The completion of three core holes in the river revealed that soft sandstone with thin shale beds existed at an unfavorable elevation in the foundation; accordingly, it was decided to forego further exploration until some comparable drilling had been done at the RM15 site. Initial phases of the drilling revealed no geologic defects; and, before the initial drilling program was completed, comparative estimates showed that the cost of a dam at this site would be less than at the geologically inferior RM4 site. As a result, the RM4 site was eliminated and the investigation program at the RM15 site expanded to provide sufficient data on foundation conditions and concrete aggregate sources for feasibility design of a concrete dam.

(c) Construction Materials Investigation.-Concrete aggregate sources available in the vicinity of Glen Canyon Dam consisted of a high-level terrace deposit on both sides of the Colorado River and one alluvial deposit on Wahweap Creek. A careful search for 60 to 70 miles in all directions-involving extensive study of air photos and ground checks-did not reveal any other important aggregate deposits.

The high-level terrace on the west side of the Colorado River opposite the mouth of Wahweap Creek was the most obvious deposit of sand and gravel in the area, so it was the first discovered. Preliminary test pitting and sampling of this deposit indicated that it was far too small to supply enough aggregate for construction of the dam and appurtenant works. The search was then expanded into the surrounding area.

The Wahweap deposit, a rather unimpressive looking stream deposit, was not at first given serious consideration as a possible source of aggregate. Later, more careful investigation indicated that this deposit contained considerable gravel, and it ultimately became the prime source of aggregate for the dam.

The Manson Mesa deposit is in a high-level terrace located on the east side of the Colorado River. This deposit was covered with blow sand and was so well hidden that it was discovered only by chance and was developed as the aggregate source for the construction of the townsite. Had it not been for this

deposit, all of the aggregate required for the construction of the town of Page would have had to be ferried by highline from the west side of the river. The highway bridge over the Colorado River had not been completed at that time (construction on the bridge commenced in 1957 and was completed in 1959).

(d) Investigations For Final Design And Specifications.-Preliminary investigations at the mile 15 site were initiated in October 1947, and a total of 28 holes and 2 test drifts was completed. All but five were vertical holes drilled in the bottom of the canyon to outline the bedrock profile and to determine condition of the rock in the foundation area. One horizontal hole and one exploration drift was completed near the base of each abutment. Three shallow holes were drilled near the base of the left abutment to secure 6-inch-diameter core for laboratory testing. Additional field investigations in 1949 included Nx (3-1/2-inch-diameter core hole) and 6-inch-diameter core holes, grouting, and load-bearing tests in and near the left abutment drift.

In the latter part of 1956, additional drilling was initiated to supply supplementary data on the bedrock profile in the river section and to provide samples of the abutment rock for laboratory testing. A total of 49 Nx-Bx (3-1/2- and 2-7/8-inch) holes and two 6-inch-diameter holes were completed. Six were vertical holes about 500 feet deep. They were drilled from the top of and a short distance back from the face of each abutment.

Twelve of the Nx-Bx holes were angle holes (DH A through J, Ln and Kn) drilled in the approximate direction of the maximum principal stresses in the abutments of the dam except DH56-J which (owing to an overhang interfering with the original location) had to be drilled in the direction of the horizontal component of the principal stresses at that elevation. The two 6-inch-diameter holes (La and Ka) were drilled parallel to and a few feet from the lowermost Nx angle holes (Ln and Kn) near the base of each abutment. Figure 6 is a drawing showing the areal geology and location of all but a few of the exploration holes. Figures 7 through 11 are typical geologic cross sections through the damsite.

The test drifts in each abutment were later utilized to perform in situ jacking tests to determine the modulus of elasticity and deformation characteristics of the sandstone. Tests were also performed at the Bureau's Denver laboratories on core samples to determine compressive strength, modulus of elasticity, Poisson's ratio, percent porosity, and percent set. A graphical portrayal of the principal elements of these tests is shown on figures 12 and 13. Geophysical