TOM DIBBLEE: FIELD GEOLOGY'S GRAND MASTER

Thomas W. Dibblee, Jr. was the greatest of all field geologists in California and probably in America. Tom's vocation and avocation were singularly directed to mapping geology wherever he went. In his almost 80-year geology career Tom mapped some 565 quadrangle maps covering over 40,000 square miles, some fourth of the State of California. His first geologic map covered the family's historic Spanish land grant, Rancho San Julian, near Lompoc, California, which he completed while still in high school.

How could Tom Dibblee have mapped so much territory? He developed efficient techniques early in his career. Some of the techniques I observed when I was Tom's geologic field assistant in the early 1960's in the Mojave Desert, and later in the Palos Verdes Hills.

Tom kept his field gear simple. He carried his field maps and aerial photos for the day's mapping in a manila envelope, and that and a pencil in a light cloth bag. He folded each quadrangle map's borders so that the maps would fit together along mutual edges to extend contacts and faults onto an adjoining map. He always had his hand lens on a chain around his neck, and he used it occasionally to check rocks. He carried a canteen and a rock hammer, too.

Tom used a Brunton only if the beds or foliation had low dips. In the Mojave, the foliations in the granitic and metamorphic rocks were generally fairly high, and he could estimate the dips within a few degrees. He also estimated strikes by the topography and in the late afternoon, by the angle of the sun on the horizon, because he new at any given time of the year, how many degrees off of west the sun set. He always kept track of his location on the topographic map, but did not need to stop for triangulation measurements to locate. He kept note-making simple; his notes were made as an explanation on the border of the map, which was organized in order of the age of the geologic units. He did not use a separate notebook or take time for extensive note taking.

Tom made long traverses, many as long as 12 miles. He generally planned his traverses to map along ridge tops and canyons. He would stop and sketch in distant contacts from a good viewpoint, and later check them when he covered that area. In this way he covered a lot of territory expanding from the known to the unknown. Each evening he inked his geology on the field map and planned the traverse for the next day.