Sentinel Rock is a large sandstone outcrop whose staircase and cables were installed by the Civilian Conservation Corps sometime between 1933 and 1942, and it is now a popular climbing rock. In response to the Great Depression, the federal government created the CCC in 1933 and the Works Progress Administration in 1935 to build public works projects nationwide, and both made major contributions to the nation’s infrastructure, to state park facilities, and to the restoration and reconstruction of historic sites. The CCC was instrumental in building many of Mount Diablo State Park’s roads, hiking trails, park residences, picnic tables, camping facilities, and notable stonework, including culverts, retaining walls in the campground, and Diablo Stoves, with its main achievement there being the Summit Visitor Center. More than 70 years later, hundreds of CCC buildings, structures, and landscape features still remain in use in about 48 California State Parks. Sentinel Rock is made of the same sandstone that formed the rugged landscape of nearby Rock City. That sandstone consists of small grains of mineralized quartz and feldspar eroded from the ancestral Sierra Nevada, carried by rivers and ocean currents, and deposited in an ancient sea that covered the area 45 to 50 million years ago, where the deposits later consolidated into thick sandstone layers. During the uplift of Mount Diablo, rocks in this area that were originally horizontal were tilted upward 9 degrees. From this overlook, the rock layers can be seen across the landscape, becoming younger from right to left, while erosion has left the hardest layers standing upright as lone ridges and isolated pinnacles such as Sentinel Rock.