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The Great Basin wilds of west-central Nevada are rich in
productive fossil plant localities. While they are probably not
as well known to amateur fossil plant hunters as the classic
Paleocene through late Miocene (roughly 64 to 5 million years
ago) leaf-bearing sites of Oregon, Idaho, Washington, Montana,
Colorado and Wyoming, the Nevada fossil plant deposits continue
to yield many excellently preserved paleobotanical remains. One
of the more interesting and paleontologically rewarding leaf
and seed-yielding areas lies near Yerington (the county seat
of Lyon County) at Aldrich Hill. Here can be collected some 35
species of ancient plants from what geologists call the Middle
Miocene Aldrich Station Formation, a geologic rock unit dated
at roughly 13 to 12.5 million years old. Among the many fossil
plant remains found at Aldrich Hill are complete, carbonized
leaves from an evergreen live oak, in addition to many conifer
winged seeds and even giant sequoia foliage. It is indeed a special
place to visit, an isolated region in the Great Basin "outback"
where the Bureau of Land Management still permits the hobby collecting
of fossil plant remains--a situation that could change literally
overnight, by the way, should commercial collecting interests
begin to raid the stratigraphic section with power equipment,
desecrating the integrity of the exposed geologic section and
destroying in the process the great scientific value of the locality.
Fortunately, Aldrich Hill remains accessible to the general public,
and folks interested in collecting fossil plants there for personal
use only may continue to visit, remembering of course that such
specimens gathered must be neither sold nor bartered--activities
which would constitute a clear violation of the rules and regulations
established by the Bureau of Land Management for visitors to
America's public lands.
All of the fossil plants--including evergreen live oak
leaves, spruce winged seeds, conifer needles, alder cones and
giant sequoia/big tree foliage--occur in the tan to reddish-brown
and cream-colored diatomaceous to tuffaceous mudstones and shales
of the Middle Miocene Aldrich Station Formation exposed on the
north side of Aldrich Hill. Excellent outcrops of the plant-bearing
strata can be examined along the main wash which trends generally
east-west across the northern side of Aldrich Hill. Additional
productive fossiliferous exposures can also be found in the minor
erosion gullies that dissect the north slope of the hill. It
should also be pointed out that virtually every outcrop of diatomaceous
mudstones and shales in the Aldrich Hill district yields fossil
plant material in varying degrees of relative abundance, from
very rare to common, although the prominent and accessible exposures
along the north side of the hill have in a historical sense provided
collectors with the majority of paleobotanical remains.
When fossil hunting at Aldrich Hill, as at most other fossil
leaf and seed-yielding localities, try to cover as much terrain
as possible in search of the most productive layers. Split heaps
of the shales with the blunt end of a geology hammer (some folks
prefer to use the pick end, obviously, though this technique
actually decreases the likelihood of splitting with precision
the blocky diatomaceous mudstones and shales. A number of collectors
prefer a roofer's or brick-layer's-style hammer, with a wide
narrow blade, which theoretically splits shales with great effectiveness;
such a hammer probably works well with very soft, classically
fissile shales, but the tool lacks any kind of "punch,"
or heft, for cleaving bulkier, more compacted mudstones and shales.
The upshot: the blunt end of a traditional geology hammer splits
the Aldrich Station Formation diatomaceous shales and mudstones
quite nicely.) whenever you stop for a "look-see."
Remember, of course, to wear safety goggles, or some manner of
eye protection while splitting the mudstones and shales. Although
nowhere abundant, the fossil plant impressions in the Aldrich
Station Formation are nevertheless common and even obvious at
several horizons in the diatomaceous material. Watch for their
pale to dark-brown, carbonized coloration on the tan to reddish-brown
and cream-colored rocks. Associated with the leaves, winged seeds
and twigs are conspicuous oval specimens roughly one-half to
one inch in diameter. These fossils represent the internal molds
of fresh water clam shells; the actual shell substance has long
since been dissolved away, as the siliceous mudstones and shales
were evidently a poor medium of preservation for the tests of
pelecypods.
If a microscope is available, you can, in addition to finding
the plants and clams, examine the remains of an especially prolific
fossil type at Aldrich Hill--the diatom. This is a microscopic
single-celled plant which during the geologic past, particularly
in west-central Nevada in rocks of Middle through Late Miocene
age (roughly 17 to 5 million years ago), contributed its resistant
siliceous remains in vast numbers to the plethora of paleohistory
in the rocks. The scientific extraction of diatoms for paleobotanical
study is a dangerous operation, involving as it does the use
of several powerful acids, among them hydrochloric, sulfuric
and hydroflouric--potent brews that if not handled properly can
cause frightful, life-threatening burns. It is a process only
an expert should attempt. Fortunately, though, you can get an
adequate, general view of diatoms simply by powdering a small
amount of diatomaceous matrix on a glass microscope slide and
then examining the residues under moderate to high powers of
magnification. Most of the diatoms from the Aldrich Hill district
resemble minute boxcars and discs.
Excluding the diatoms, among which numerous species can
be identified from the Aldrich Station Formation, some 35 species
of fossil plants have been described from the exposures at Aldrich
Hill. This particular fossil deposit was first investigated scientifically
by the late American paleobotanist Daniel I. Axelrod during one
of his geological reconnaissance investigations in western Nevada
in the early 1940s. Dr. Axelrod later published his paleobotanical,
paleoecological and geological conclusions concerning the paleoflora
from Aldrich Hill in a formal paleobotanical report--a publication
in which he describes in detailed scientific fashion four important
fossil floras in Churchill and Mineral Counties east and south
of Reno (the Middlegate, Chloropagus, Fallon and Aldrich Station
paleofloras). All of the fossils from Aldrich Hill occur in the
Aldrich Station Formation, as named by Axelrod in his monograph,
a geologic rock formation originally believed to be transitional
Miocene-Pliocene (about 10 million years old by the geologic
time scale then in fashion--as recalibrated, the Miocene-Pliocene
transition on the geologic time scale today is roughly 5 million
years old), but now considered Middle Miocene in geologic, or
approximately 13 to 12.5 million years old.
The Aldrich Station paleoflora shows quite a variety in
its composition. The five most commonly collected specimens are,
in decreasing order of relative abundance: (1) winged seeds from
three varieties of spruce--Picea sonomensis (by far the
most abundant representative of the paleoflora), whose seeds
are similar to the modern Brewer Spruce (Picea breweriana)
of the Klamath region of northwestern California and adjacent
Oregon; an extinct spruce that Dr. Axelrod called Picea lahontense--the
seeds from this conifer cannot be compared with any known living
spruce, although they do show a general resemblance to those
produced by a few modern "larger-coned" spruces of
eastern Asia, chiefly China; and a presumed extinct spruce called
Picea magna, whose winged seeds resemble those produced
by the living tiger-tail spruce, Picea polita, a conifer
now native to the volcanic soils of Japan; (2) fragmental and
occasional complete, intact leaves from an evergreen live oak
called Quercus pollardiana, a species that is practically
identical to the modern maul oak, also called canyon live oak,
Quercus chrysolepis, presently native to the moist western
slopes of the Sierra Nevada and the Coastal Ranges of California;
(3) leaves from a species of cottonwood, Populus payettensis,
whose fossil foliage matches leaves produced by the living
Narrowleaf Cottonwood, Populus augustifolia; (4) foliage/twigs
of giant sequoia, Sequoidendron chaneyi, that match very
well with those produced by the living Sierra Redwood/Big Tree,
Sequoiadendron giganteum, which is now restricted solely
in its natural habitat to a narrow, moist belt along the western
slopes of the Sierra Nevada in California; and (5) leaves from
a species of willow referred to as Salix payettensis--its
foliage appears very similar, if not identical to leaves produced
by the modern willow Salix exigua, a rather widespread
variety that goes by many common names, such as Coyote Willow,
sandbar willow, or even Narrowleaf willow. The nine next most-common
specimens encountered are: Catalina ironwood (recently, Dr. Diane
Erwin, who is the Collections Manager of fossil plants at the
University California Museum of Paleontology, and Howard Schorn--retired
Collections Manager of fossil plants at UCMP--have revised the
fossil species of Catalina ironwood, genus Lyonothamnus, in
their paleobotanical paper, Revision Of Lyonothamnus A.
Gray (Rosacea) From The Neogene Of Western North America, Int.
J. Plant Sci 161(1): 179-193. 2000. @ 2000 by The University
of Chicago.), Mountain alder, an extinct water oak, California
buckeye, black cottonwood, an extinct cottonwood, zelkova, Arizona
ash and Cedar elm. Rarer occurrences include such varieties as
sugar pine, white fir, Ponderosa pine, Douglas-fir, Western Red
Cedar, Western hemlock, California sycamore, serviceberry, Oregon
grape, California coffeeberry, coralberry, japanese pagodatree,
birch-leaf mountain mahogany and a Horsetail.
The vast majority of fossil plants recovered from the Middle
Miocene Aldrich Station Formation at Aldrich Hill show a demonstrable
resemblance to their modern analogs still living today. For example,
the humid, cooler-weather conifer elements in the paleoflora
(spruce, fir, pine, giant sequoia) have their closest contemporary
counterparts in the Sierra Nevada and Cascade Mountains regions
of western North America. As a matter of fact, there is a direct
relationship postulated between the overall composition of the
Aldrich Station Flora and modernday plant associations present
in the giant sequoia forests of the western slopes of the Sierra
Nevada in California--specifically, the impressive, awe-inspiring
Sierra Redwood groves at Calaveras Big Trees State Park (Calaveras
County east of Angels Camp) and Sequoia National Park east of
Fresno.
Based on the known climatic requirements of present day
plant counterparts of the fossil flora, the Aldrich Hill district
some 13 to 12.5 million years ago had quite a different environment
than the arid juniper-sage-Pinyon pine associations observed
there today. For one thing, rainfall was approximately 25 to
30 inches per year, distributed in both winter and summer months.
This is fully 15 inches more than the area receives today, with
almost all of the effective precipitation now occurring during
wintertime as snow and rain. Temperatures today also show much
greater extremes than what can be inferred for the moderate Middle
Miocene days, when freezing conditions were extremely rare and
summer highs normally ranged around 85 degrees--this contrasts
dramatically with our regular winter weather patterns that mimic
Arctic-style extremes and summer peaks to over 100 degrees Fahrenheit.
Elevations at the sites of deposition were in all likelihood
slightly higher than the 6,000 to 6,700 feet we observe today
in the Aldrich Hill district--probably an elevation between 7,000
and 7,500 feet for those Middle Miocene times is not out of the
question.
In addition to telling us something of the climate of the
geologic past, the Aldrich Hill fossil plant bonanza also reveals
a striking gradual change in both the local paleogeography and
the associated plant communities. In the earliest phases of their
deposition, the fossil plants reveal that the ancestral Aldrich
Hill region was a broad valley within which sprawled one or more
fresh water lakes of perhaps moderate to large size; the diatoms
preserved in the Aldrich Station Formation suggest that the lakes
were rather cool and deep and well within the range of normal
mineral content. Occasionally, though, volcanic activity from
distant areas contributed layers of ash and pumice to the accumulating
diatomaceous sediments. The adjacent slopes supported a thick
mixed conifer forest consisting of white fir, Ponderosa pine,
Brewer spruce, Douglas-fir, Western Red Cedar, Western hemlock
and giant sequoia, along with a subordinate deciduous grouping
of alder, poplar, cottonwood, willow, elm, zelkova, japanese
pagodatree and coralberry. Living on the more exposed slopes
were evergreen live oak, serviceberry, california buckeye, cottonwood
and ash. Yet, higher in the geologic section at Aldrich Hill,
in rocks younger by hundreds of thousands of years, the fossil
plants prove that the geography had changed significantly. In
place of a widespread conifer forest with only minor areas of
woodlands surrounding a great lake basin, there had developed
a vast hilly woodland where only a few interfingers of forest
penetrated from the higher slopes surrounding the lake basin
of deposition. Replacing big tree, spruce, pine and other conifers
as dominants were evergreen live oak, mock locust, california
buckeye, coralberry, mountain mahogany and serviceberry--a paleoenvironment
that was much more xeric in nature than that suggested by plant
communities which had preceded it. Here rare forest elements
were white fir, Ponderosa pine, western hemlock and giant sequoia.
Also present in the forest association were such species as alder,
hollygrape, Catalina ironwood, cottonwood, poplar, elm, willow
and zelkova. The once thriving forest grouping of conifers and
deciduous varieties appears to have survived in the upland regions,
only descending into the dominant evergreen live oak territory
under especially favorable conditions.
Today, the sedimentary rocks at Aldrich Hill provide proof
that roughly 13 to 12.5 million years ago an extensive giant
sequoia forest reigned over what is presently an arid Great Basin
land of sage and juniper and Pinyon pine. Along with the big
trees grew such plant varieties as Brewer spruce, Ponderosa pine,
white fir, Western hemlock, evergreen live oak and an array of
deciduous kinds--a scene that closely resembles the modernday
humid western slopes of the Sierra Nevada in the vicinity of
Calaveras Big Trees State Park east of Angels Camp, California,
where two groves of mighty Sierra Redwood continue to thrive
in a setting of pristine grandeur. Within the diatomaceous mudstones
and shales of the Middle Miocene Aldrich Station Formation can
be found direct evidence of that ancient Big Tree forest, the
beautifully preserved carbonized impressions of plant remains
covered over by the sediments in a long-vanished lake--leaves
and seeds and conifer foliage which only await a patient, dedicated
fossil hunter to bring to their first light of day in many millions
of years.
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