G. K. GILBERT, PLANETARY GEOLOGIST

That Grove Karl Gilbert is the only geologist to have served twice - in 1892 and again in 1909 - as president of the Geological Society of America is a fitting introduction to his stature in the history of American geology. Hardly a branch of the field does not owe some part of its development to Gilbert’s prolific and thorough assessment of the processes that shape Earth’s surface, from geomorphology, to geodesy, to palaeontology. One aspect of his work, however, is rarely emphasized, though it forms a consistent thread throughout his distinguished career and publications: his insistence on thinking of the Earth as a planetary body - unusual against the backdrop of geology at the turn of the twentieth century. Gilbert’s 1892 discussion of “The Moon’s Face,” in which he lays out his theory for the impact origin of lunar craters, is perhaps the most explicit expression of this planetary perspective, and - although his contemporaries in geology and astronomy were not quick to embrace his transgression of disciplinary boundaries - he is recognized today as an early champion of cross-disciplinary planetary science. Gilbert’s placement of the Earth in a planetary context is not confined to discussions of moons and craters, but rather appears throughout many of his more terrestrial publications. Here, we examine these instances - including his treatment of geologic time and his search for a “chronometer” in orbital and climate cycles, his demonstration that the Coriolis force affects how streams erode their banks, and his analysis of gravity anomalies and their implications for the mass distribution within the Earth - in relation to Gilbert’s methodological principles and the reception of his work by his colleagues at home and abroad. Whereas his investigations into the impact hypothesis for lunar and terrestrial craters, which included both experiments and telescopic observations, were considered a distraction from more serious topics by his fellow geologists, to Gilbert they may have seemed only a natural extension of his interest in the Earth as a planet, subject to astronomical influences. Gilbert may be known first and foremost as a field geologist, but he was also a planetary geologist through and through.