IN THE FIELD WITH
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
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.