Helmut E. Ehrenspeck

Most California geologists are familiar with Thomas legendary achievements in mapping vast areas of California's complex geology. However, few have had the opportunity to routinely accompany him in the field and to see how Tom Dibblee's individual field maps come together into coherent mosaic of regional geology.

Only in recent years has Tom Dibblee preferred to be accompanied on the long drives to field areas and the logistics of remote field work. Having assisted him on weekly treks since 1987, I have had many opportunities to see the geology alongside him, to have him point out what he observes and records, and to witness his thorough and efficient methods, developed in over 70 years of geologic field work.

Why, at age 88, does Tom Dibblee continue to actively map geology? As he himself best states it, "I feel driven by a sustained curiosity to see as much of California geology as possible within my lifetime, and to better understand the genesis and interrelationships of the many uplifted areas I have mapped."

In whatever region Tom Dibblee has chosen to work in recent years, he has found that available geologic mapping often consists of an incomplete patchwork of maps at various scales and levels of detail, with differently defined sedimentary, igneous or metamorphic rock units, and diverse structural interpretations. Faced with such inconsistencies, Dibblee feels that he just has to go out there and see for himself what the rocks and structures really are, and to record them as accurately and consistently as possible.

Before initiating field work in a given area, Dibblee pencils any available geologic data onto his 1:24,000-scale topographic base maps. he then systematically and critically field checks all accessible areas. His field work may range from just adding a few of his own observations or measurements, to completely mapping or mapping that area.

A typical field day involves long walks preferably as 'loop' hikes along roads, trails and firebreaks, so that he can observe the rock units, structures and landforms from many different perspectives. He continually seeks out overlooks from which he can check or revise the geologic contacts, bedding attitudes, and structures both near and afar. This is how he covers a lot of territory in a short time.

Whenever necessary, Dibblee will also scramble through the brush and streambottoms to examine critical outcrops, look for diagnostic fossils or to study contacts and stratigraphic relationships. His amazing memory for the many rock types, structures and geologic conditions that he has encountered over the decades ensures consistency in his mapping throughout vast and diverse regions.

In mapping stratified rocks, Dibblee's first priority is to systematically map out the individual lithologic units or facies within a formation, such as shale, sandstone, conglomerate members, etc. This mapping method often reveals abrupt facies changes that might otherwise be mistaken as faults. It also reveals important folds, unconformities, overlaps, time-transgressive strata and other stratigraphic or structural features that would otherwise be missed or misidentified.

In metamorphic or igneous terranes, Dibblee carefully delineates the contacts of the major rock units and records regional flow banding or foliation trends wherever possible. In this manner he can identify important structural discontinuities that would otherwise be difficult to recognize.

Dibblee indicates only those faults that are clearly mappable or can be reasonably proven to exist; that is, where rock units, fold axes or physiographic feature are definitely displaced, juxtaposed, or truncated Any faults that must be inferred from questionable or only suggestive evidence are indicated as such or are not shown.

As he explained recently; "I find it most important to record all my geologic observations onto a base map whenever I go out into the field, especially in areas of difficult access that I may never be able to return to. To me, this is the most logical field procedure, to record only basic geologic data, in an unbiased way, and with a minimum of interpretive inferences. My mapping is not based on or influenced by favorite theories or models. I prefer to let other geologists express their interpretations or ideas in whatever ways they wish, rather than to include these on my maps."

Over these past six decades, Tom Dibblee has seen and mapped more of California's geology than virtually anyone else. His geologic maps provide an unsurpassed geological overview of a given area, while at the same time contributing a wealth of reliable, locally important information.

The Dibblee Geological Foundation's purpose in published Dibblee's geologic mapping is to ensure that the scientific, societal and educational values of his remarkable work will endure for the benefit of generations to come.


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