"Geologic Time" As Calculated By C. D. Walcott

Abstract

In 1893, Walcott contributed to the debate on the length of geologic time. He approached the problem by calculating average thickness of the Paleozoic rock column in the west and dividing by rates of erosion and by rates of deposition to arrive at a time interval. Although he concentrated on the Cordilleran area, Walcott produced a general paleogeographic scheme for the Paleozoic of North America. He was quite clear in differentiating between chemical and mechanical deposits, and devoted most of his attention to the Paleozoic carbonates. Walcott chose western North America as the source for data, in part because of the long sections and in part because of the large amount of limestone relative to sandstone and shale. Throughout the discussion he included pertinent comments on such subjects as size of source areas and relative speed of deposition; he was familiar with many of the issues that occupy present-day sedimentologists. After considering various aspects of the issue, Walcott estimated 17,500,000 years for the duration of the Paleozoic. Walcott also derived a Paleozoic: Mesozoic: Cenozoic ratio of 12: 5: 2, the same ratio obtained today from radiometric dates. He estimated that the Algonkian was as long as Paleozoic and guessed 10,000,000 years for the duration of the Archean. The greatest flaws in his chain of logic were assumption of an erosion rate of 1 foot in 200 years and assumption that deposition of limestone was more or less continuous. Had he chosen 1 foot per 3,000 years, one of his other two calculations, he would have been close to present-day age figures. Perhaps it was the episodic, rather than the average, nature of sedimentation that was the pitfall. Nevertheless, Walcott's estimates of thicknesses of western Paleozoic rocks and his resulting calculations were the most detailed made on erosion/sedimentation rates to indicate the length of geologic time. His study was published in three journals, plus other outlets, and it may have been the most widely distributed paper of the decade. It was little cited, perhaps because within several years the debate on age shifted to use of ocean salinity as a potentially more precise calender. That approach in turn ultimately succumbed to the new concept of radiometric dating.