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An early-20th-century uplift has been recognized in southern California that compares with the 1959-76 cycle in areal distribution, magnitude, and history. Although the data set used in reconstruction of this earlier event forms but a small fraction of that used in reconstruction of the modern uplift, its distribution and density are such that a generalized representation of the earlier uplift is relatively easily developed. The earlier uplift was characterized by an elongate, double-lobed configuration extending from Point Arguello east-southeastward to the north edge of the Salton Sea. Height changes within the uplift locally exceeded 0.5 m but probably averaged 0.3 to 0.4 m over about half of its nearly 50,000-km2 areal extent. At or immediately after its culmination, the uplift included most or all of the Transverse Ranges province and parts of the Coast Ranges, San Joaquin Valley, Sierra Nevada, Basin and Range, Mojave Desert, Peninsular Ranges, and Salton Trough provinces. Although the two cycles clearly were similar, the earlier uplift differed from its modern counterpart in its significantly greater penetration southward into the Peninsular Ranges. Although nontectonic displacements may have accompanied the growth and subsequent collapse of the uplift, comparisons with areas of known or suspected fluid extraction indicate that compaction-induced deformation through 1935 was trivial outside of the Los Angeles Basin. Measured height changes used in the reconstruction of the early-20thcentury uplift are based on comparisons between the results of geodetic levelings of differing accuracy. Survey quality ranged upward from wooden-rod, single-run, single-rodded, third-order to Invar-rod, doublerun, first-order leveling. Errors associated with signals (height changes) derived from comparisons between the results of surveys of different standards of accuracy presumably were dominated by those identified with the lower order survey; had these signals been but small fractions of their actual values, they could easily have been obscured by the noise. Errors that contributed to inaccuracies in the height determinations and resultant signals can be categorized as blunders, systematic survey errors, random survey errors, errors in the evaluation of the orthometric corrections, and errors associated with intrasurvey movement. A number of blunders, all of which were associated with single-run, single-rodded, third-order surveys, were detected during our reconstruction of the observed-elevation differences identified with specific surveys. Unless both the source and the magnitude of these blunders could be reasonably assessed, those line segments including and extending beyond the blundered portions were discarded in our reconstruction. Comparisons between signal and terrain show that the two are uncorrelated in any significant way and, hence, that heightor slope-dependent systematic errors can be dismissed as equally insignificant. Although conventionally computed estimates indicate that random error could account for as much as 100-120 mm of the signal at the distant ends of several very long lines, redundancy and other evidence suggest that these estimates tend to be exaggerated. All of the orthometric corrections have been based on observed gravity, and the errors associated with this routine geodetic correction can be dismissed as negligible. Intrasurvey movement clearly contaminated the signal along only one line and probably led to a distorted tilt along the south flank of the eastern lobe of the uplift. Reconstruction of the uplift has been based chiefly on the results of sequentially developed observed-elevation differences referred directly to bench mark Tidal 8, San Pedro. Because Tidal 8 is located adjacent to a tide station that has operated intermittently since the mid-19th century and as a primary station since 1924, its stability is well known. In general, height changes referred to Tidal 8 are biased slightly toward the appearance of subsidence and against the appearance of uplift. Because all the available evidence suggests little, if any, regional deformation during the period 1897-1905, and because an accurate control line was established athwart the uplift in 1902, we have selected 1902 as our reference datum. The early-20th-century uplift probably evolved largely as two welldefined episodes, the second of which was followed by, or closely associated with, partial collapse. The timing of this second episode is so poorly defined that the partial collapse could have either preceded or accompanied the growth of the eastern lobe the second major episode of uplift. The first major spasm of uplift began no earlier than spring 1905, probably was largely over by the following spring, and is thought to have included nearly all of the central and western parts of the ultimately uplifted area. Although the evidence is equivocal, uplift may have continued within the central and western parts of the uplift through at least 1914. Partial collapse of the central and western parts of the western lobe began no later than 1925. Uplift of the eastern lobe exceeded 0.5 m; the survey data clearly indicate only that this uplift evolved sometime within the period 1902-28. Although a large section of the central and western parts of the uplift had sustained major tectonic subsidence by 1926, an additional episode of localized collapse, centering on Bakersfield, apparently occurred sometime between 1927 and 1931. Control on the distribution of this later event, which dropped Bakersfield well below its 1902 height, is virtually nonexistent; and we have no evidence indicating that it extended significantly into the already partially collapsed north flank of the uplift. The growth of the early-20th-century uplift, unlike that of its modern analog, is vaguely correlated with seismicity. A group of mediumto large-magnitude earthquakes just preceded or, less likely, accompanied the first major episode of uplift. Similarly, several relatively large magnitude earthquakes, notably the 1927 Point Arguello M= 7.5 shock, closely followed the collapse of the central and western parts of the uplift.

An early-20th-century uplift in Southern California