Figure 29. Richfield oil field to Yorba Linda oil field and the Chino Hills. Although wells along this segment of the Whittier fault are sparse, the structure of the footwall block near the fault is well delineated on this cross section by surface mapping and dip and paleontological data in several wells. In the hanging wall block, the Union Gaines 1 well penetrated a thick section of the La Vida Member that contains sandstone, siltstone and diabase in the lower part, which are characteristic of the strata in the inverted half-graben immediately north of the Whittier fault. In the Gaines well, Durham and Yerkes (1964) included the Diamond Bar sandstones in the Topanga Formation. This correlation results in a top of the Topanga Formation that is structurally too high to be a reasonable possibility when compared with surrounding wells (See Bjorklund and Burke, 2002, section 1.3.3.1 for a discussion of the Diamond Bar sandstone correlation problem.). In this publication, the base of the Diamond Bar sandstone (base of La Vida Member in this area) in the Gaines well has been picked at a depth of 5860 feet, the top of a volcanic unit. The top of the Topanga Formation is picked at the base of the volcanic unit. An alternative interpretation for the top of the Topanga Formation is the top of the sandstone bed overlying the volcanic unit. However, volcanic rocks in this area commonly overlie the Topanga Formation but are not known to occur within it. See Figure 20 for a large-scale cross section of the vicinity of the Whittier fault.