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Figure 2 - Geologic explorations prior to original 1960s construction [5]. 2.2. GEOLOGIC CONDITIONS DOCUMENTED DURING 1960S CONSTRUCTION During construction of the FCO Spillway chute, DWR geologists created as- excavated geologic foundation maps (Fig. 3). These maps denoted the general rock weathering at the foundation invert as well as the location of shears and shear zones. Each shear or shear zone greater than 3 cm (0.1 ft) wide was assigned a unique sequential number with a relatively detailed description given in the construction geology documentation. The shear descriptions included the thickness, composition or mineral infilling (e.g., breccia versus clay-filled), and other pertinent information [6]. The Emergency Spillway includes an ungated monolith/weir structure that is approximately 527 m (1730 ft) wide at the weir crest. The left 283 m (930 ft) consists of concrete-gravity ogee monoliths/weir sections that were keyed into slightly-weathered rock. The heights of these monoliths range up to 27 m (90 ft). The right 244 m (800 ft) is in a rock cut with a small 0.3 m (1 ft) high concrete ogee weir. As-excavated foundation geology maps were similarly prepared during original construction to document the foundation rock at each concrete monolith of the Emergency Spillway, as well as the foundation rock conditions in the foundation of the concrete weir. 106