G. K. Gilbert’s major works relating to gravity and isostasy consisted of three separate efforts: (1) a study of the deformation of the Lake Bonneville shorelines, (2) participation in the first gravity profile across North America, and (3) a U.S. Geological Survey Professional Paper on the interpretation of gravity anomalies. With the Lake Bonneville data, Gilbert established that the strength of the Earth’s crust in the Great Basin could not support the load of water in Lake Bonneville and that the basin subsided and rebounded as the water load was first applied and then removed. As a participant in the first major gravity survey in the United States, he experimented with techniques of interpreting gravity observations and speculated on the significance of these sparse data. As one of his last major scientific undertakings, he wrote an essay calling for a more flexible approach to the interpretation of regional gravity variations and outlining his thoughts on the deep structure of the Earth. Although each of these three efforts made a significant scientific contribution, the conclusions he reached in each are not totally consistent with one another, and he attempted neither to reconcile the inconsistencies nor to develop a unified theory of isostasy. Apparently by hypothesizing in each instance, he hoped to stimulate the geodesists to a broader view in their examination of the data.
Gilbert’s extraordinary talents are apparent in his work in gravity and isostasy, but he does not appear to have had a great impact on the geodesists who had dominated the development of these disciplines for many years. However, his conclusions that major deformations of the crust reflect “horizontal movements of the upper rocks (lithosphere) without corresponding movements in the nucleus and thereby imply mobility in an intervening layer (asthenosphere)…” and that the driving tectonic forces stem from a “primordial heterogeneity of the earth which gives diversity to the flow of heat energy. . . .” (Gilbert, 1914, p. 34, 35) indicate a remarkable insight into large-scale tectonic processes.