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Perhaps the richest producer of Miocene-age (22-to-5-million-year-old)
plants in the entire state of Nevada is a geologic rock deposit
known as the Middlegate Formation. It is exposed primarily in
the Middlegate Hills a number of miles from Fallon. In this area
some 64 species of fossil plants have been described, including
such diverse types as evergreen live oak, giant sequoia, willow,
fir, maple and spruce. The fossil specimens, which consist of
leaves, winged seeds (called samaras in technical botanical terminology),
acorn cups, seed pods and branchlets, occur as pale to dark brown
carbonized impressions on a cream-white to pale-brownish matrix
of opaline shale--many of them exhibiting such an exceptional
degree of preservation that the original delicate venation on
the leaves is clearly visible.
All of the remains are Middle Miocene in geologic age,
dated by radiometric methods at some 16 million years old. They
occur in the uppermost (the youngest layers of deposition) 30
feet of the Middlegate Formation, just below the overlying Middle
Miocene Monarch Mill Formation, whose basal sedimentary conglomerates
have yielded to paleontologists a large vertebrate fauna, including
the silicified bones of moles, rabbits, squirrels, beavers, mountain
beavers, mice, weasels, martins, rhinocetotids, oreodonts, camels,
llamas and pronghorns (Kent Smith has been one of the primary
scientific investigators of this extremely important Barstovian-age
mammalian fauna).
Such scientifically invaluable fossil vertebrate material
on Public Lands is of course off limits to all collectors who
do not possess a special use permit issued by the Bureau of Land
Management, a formal collecting status that is perhaps well understood
by most amateurs and professional paleontologists alike. At present,
there is no such legal restriction on the hobby gathering of
leaves, winged seeds and other paleobotanical remains at Middlegate--but
that, too, could change.
The troubling circumstance is that commercial collecting
interests have recently begun to concentrate on a select number
of fossil leaf-yielding fields in Nevada--obviously those sites
which happen to provide them with the greatest numbers of well-preserved
specimens. This is patently illegal activity, since no fossil
remains collected on Public Lands may be either sold or bartered.
And while there is certainly nothing criminal about selling fossil
specimens collected on private lands (with the land owner's unambiguous
permission, of course), any desecration of a fossil horizon on
Public Lands in an attempt to secure as many saleable remains
as possible is without question an offense punishable by law.
Also, such behavior is with sure consequence horribly counterproductive,
since it only invites officials with the Bureau of Land Management
and the Forest Service to close down popular fossil areas, preventing
conscientious amateurs from sampling places of significant paleontological
interest.
The Middlegate Hills locality certainly fits that description.
It is a remarkably productive fossil plant-yielding region situated
in the middle of the Great Basin Desert, amid what botanists
call a shadscale desert flora, or an association of low, rigid,
spinescent shrubs no more than two feet high. But 16 million
years ago the Middlegate Hills district was the site of a deep,
cool, clear-water lake, a great body of water into which creeks
and streams occasionally discharged loads of fine detritus, along
with abundant plant debris from the surrounding countryside--a
landscape rich with conifers, deciduous varieties and evergreen
live oaks which now lie preserved along the bedding planes of
fine-grained shales.
| Click on the
images for larger pictures. Here are two views of the famous
Middlegate Hills fossil flora locality. At left is a photograph
taken from a distance of about two miles from the Middlegate
Hills, which are composed of lacustrine (lake-deposited) cream-white
to pale-brownish opaline shales, within which a genuine bonanza
of beautifully preserved fossil leaves, seeds and branchlets
can be found. At right is an image of the prime fossil plant-bearing
site in the midst of the Middlegate Hills; here, the rounded
hills composed of siliceous opaline shales produce some 64 species
of ancient plants from the Middle Miocene epoch. |
The most efficient way to locate fossil plants in the Middlegate
Hills is to dig into the slabby-weathering siliceous shales,
exposing fresh sedimentary strata below the surface. Fortunately,
most of the shales within a few inches of the surface are severely
fractured; hence, little splitting of them is necessary, since
they tend to separate from the outcrops in thin sheet-like plates.
Watch for the fossil plant compressions and impressions along
the bedding planes of every shale fragment you remove from the
hillside exposures. The deeper you dig, though, the more thickly
bedded the opaline shales become, until at last it will become
necessary to begin splitting the extremely dense, concrete-like
rocks. When doing this, always remember to wear safety goggles,
or at least some kind of eye protection such as sunglasses. The
denser, thick-bedded opaline strata crack apart only with the
greatest of applied brute force, thus increasing the likelihood
that sharp fragments might launch off the matrix into your eyes.
Stand slabs of shale on end, then give them a sure whack with
the blunt end of a geology hammer. If you're fortunate, the sedimentary
layers will break apart along their original planes of deposition,
revealing perfect carbonized leaf and seed impressions and compressions
to their first light of day in approximately 16 million years.
| Click on the
images for larger pictures. Here are two fossil conifer winged
seed (samaras in botanical terminology) from the Middle Miocene
Middlegate Formation, Middlegate Hills, Nevada: specimen at left
is a seed from the Miocene species of Brewer spruce (now native
to the Klamath Mountains of northwestern California), Picea
sonomensis, 15mm long; at right is a seed from the
Miocene variety of California Red fir, Abies laticarpus, which
is 30mm in length. |
By far, the most common specimens found in the Middlegate
Formation are leaves and acorn cups belonging to an evergreen
live oak, Quercus pollardiana, which is identical to the
modern canyon live oak native to the western flanks of the Sierra
Nevada and the coastal ranges of central California.
The 18 next most frequently encountered remains in the
Middlegate shales include: interior live oak (leaves); Birchleaf
mountain-mahogany (leaves); tanbark oak (leaves); common cattail
(leaves); Bigleaf maple (leaves and samaras); Balsam poplar (leaves);
willow (leaves); Boxelder (samaras and leaflets); Brewer spruce
(samaras); Tigertail spruce (samaras); silver maple (samaras
and leaves); Arizona madrone (leaves); Catalina ironwood (leaves);
Giant Sequoia (branchlets); New Mexican locust (leaflets and
seed pods); California Red fir (samaras and needles); small-stemmed
Horsetail (stems); and Ponderosa Pine (samaras and needles).
Twenty of the rarest species reported from the Middlegate
Formation include: Golden Chinkapin (a brush-sized variety--leaves);
Paper birch (leaves); Hairy mountain-mahogany (leaves); quaking
aspen (leaves); Utah juniper (branchlets); an extinct water oak
(leaves); Narrowleaf cottonwood (leaves); a second species of
willow (leaves); White ash (samaras); Oregon grape (leaves);
Alaskan cedar (branchlets); water lily (leaves and rootstocks);
Rocky Mountain hawthorn (leaves); Rocky Mountain maple (samaras
and leaves); Douglas-fir (samaras); Mountain hemlock (samaras);
Golden chinkapin (tree variety--leaves); East Asian maple (samaras
and leaves); and Cedrella (samaras).
The Middlegate Flora was discovered in the Spring of 1949
by Laura Mills, an avid amateur fossil collector from Fallon
who at the time was searching for petrified wood. She brought
the rich fossil deposit to the attention of paleobotanist Daniel
I. Axelrod, who accompanied her to the Middlegate Hills in the
early summer of 1949. Several weeks later Axelrod made his first
substantial collection of plants from the Middlegate Formation--a
fantastic array of Miocene species preserved in superior detail;
here was certainly one of the most productive and important fossil
plant localities in all the Great Basin.
Two years later, during the Spring of 1951, Axelrod made
yet another visit to Middlegate, this time accompanied by his
long-time field assistant Robert E. Smith, who maintained accurate
records of the various plant taxa recovered from the shales.
They spent an entire week in the field, eventually amassing a
truly exhaustive selection of Miocene fossil plant material.
In all, Axelrod and Smith gathered some 3,458 specimens from
the Middlegate Hills, 2,917 of which belonged to the evergreen
live oak Quercus hannibalii.
Axelrod published his scientific examination of the Middlegate
Flora in 1956 in a formal scientific paper. In it, he
described 42 specimens of ancient plants, assigning them a transitional
Miocene-Pliocene geologic age, or what was then understood to
be roughly 12 to 10 million years old.
Axelrod later revised the Middlegate Flora in another formal
paleobotanical publication. This new scientific analysis was
based on supplemental collections of fossil plants supplied by
Axelrod's students in paleobotany at the University of California,
Davis, during the 1970s. The student collecting expeditions not
only increased the total number of specimens known from the Middlegate
Formation to 6,882, but also added some 22 new species of fossil
plants to the ever-expanding paleobotanical record.
In addition to the larger plant collections, Axelrod also
had at his disposal increasingly sophisticated and accurate radiometric
methods of dating volcanic rocks interbedded in a sedimentary
sequence. In the late 1960s geologist Harold Bonham of the Nevada
Bureau of Mines And Geology selected fresh samples from hornblende
rhyolite tuffs present near the middle of the Middlegate Formation.
When the volcanics were run through a series of radiometric-age
analyses, paleobotanists were shocked to learn that the fossil
plants could not be transitional Miocene-Pliocene as originally
determined (12 to 10 million years old, prior to the recalibration
of both the Miocene and Pliocene Epochs; for example, the Pliocene
now begins at about 5 million years ago), but rather more in
the range of 18.5 million years old, or Middle Miocene in geologic
age. More recent radiometric determinations, though, prove that
the Middlegate Formation is younger still--more in the range
of 16 million years ancient.
In his monographs dealing with fossil plants from Nevada,
paleobotanist Daniel I. Axelrod concluded that the Middlegate
Flora most closely resembles a modern sclerophyll forest, which
reaches its ultimate development in the western Sierra Nevada
and the coastal ranges of central California. Such a forest is
dominated by evergreen dicotyledons, principally madrone, Golden
chinkapin, tanbark oak and several specimens of oak; typical
evergreen shrubs include buckbrush, mountain mahogany, toyon,
sumac and manzanita--all of which contribute to a classical chaparral
botanic association. In addition to the evergreen species, a
typical sclerophyll forest may also include such deciduous types
as maple, ash, black walnut, sycamore, cottonwood, currant, rose
and willow, each of which prefers the moister sites around streams
and seepages.
The association of sclerophllous species in the Middlegate
Formation suggested to Axelrod that the fossil plants lived under
environmental conditions quite similar to those present today
in the Santa Cruz and Santa Lucia Mountains along the coast of
central California. There are also obvious relationships to the
modern groves of Giant Sequoia in both the northern and southern
portions of the Sierra Nevada, particularly the North Grove (southeast
of Auburn) in Placer County and the Tule River Grove near Belknap
Creek in Tulare County.
Based on the available geological evidence, the fossil
plants accumulated in waters of moderate depth roughly one-half
mile from the southern margin of the Miocene Lake, into which
creeks and streams discharged detritus from south-facing slopes
of volcanic origin during periods of intermittent storm runoff.
Surrounding the basin of deposition and reaching down to lake
level was a dominantly sclerophyllous forest consisting of madrone,
Golden chinkapin, tanbark oak, canyon live oak, interior live
oak and water oak.
Drier sites in the ancestral Middlegate Basin supported
such shrubs as buckbrush, mountain-mahogany, toyon, Lyontree
and locust. The stream banks were lined with species of maple,
cottonwood and willow, and at higher elevations along the distant
slopes there grew an impoverished conifer-hardwood forest of
fir, spruce, pine, Alaska-cedar, Douglas-fir and Giant Sequoia.
Associated with the conifers were such deciduous varieties as
alder, maple, hawthorn, willow and mountain ash.
Summer rain indicators in the Middlegate Flora include
types whose closest modern relatives live in the eastern United
States, eastern Asia and the southern Rocky Mountains--such deciduous
species as birch, persimmon, hawthorn, hydrangea, Oregon grape,
maple and cottonwood. All of these varieties prove that throughout
Middle Miocene times there must have been, at a minimum, some
two to three inches of precipitation during each of the three
summer months; this contrasts wildly with the extreme aridity
of the Middlegate Hills today, which receive on average only
5 inches of rain per year.
Yet, 16 million years ago the Middlegate Basin received
approximately 35 to 40 inches of rain on an annual basis, an
estimate based on the known environmental requirements of living
members of the fossil flora. The Middle Miocene climate was apparently
mild-temperate, with an average temperature reading for the month
of January of 40 degrees; today in the Middlegate Hills temperatures
remain below freezing throughout January, averaging a chilly
30 degrees. Summertime weather conditions were also more moderate
during Middle Miocene times, when a typical average for the entire
month of July was fully 10 degrees lower than that experienced
in the area today--68 degrees for the Miocene, compared with
78 in the present-day. Elevations at the site of deposition were
likely much higher than today's 5,000 to 5,300 feet--more in
the neighborhood of 9,000 feet. This estimate is based on rather
recent sophisticated geophysical and paleobotanical studies which
demonstrate that 16 million years ago the ancestral Great Basin
region stood appreciably higher than it does at present; by 13
million years ago, elevations had collapsed through extensional
geologic stresses to roughly the same as what we see today in
the Great Basin.
| Click on the
images for larger pictures. Fossil leaves from the Middle Miocene
Middlegate Formation, Middlegate Hills, Nevada. At left is a
complete leaf specimen from the extinct water oak, Quercus
simulata, 50 millimeters in length; at right is an essentially
complete leaf (stem is partially obscured by opaline shale matrix)
from a tanbark oak, Lithocarpus nevadensis, which is 65mm
long. |
Here is certainly one of the premiere fossil localities
in all of Nevada: the Middlegate Hills, a place where thousands
upon thousands of well-preserved leaves, seeds, acorn cups, seed
pods, conifer needles and branchlets have been recovered by professional
paleobotanists and amateur fossil enthusiasts alike over a period
of several decades. Today, the site lies within the brutal aridity
of a shadscale desert, a land of low spiny shrubs adapted to
harsh alkaline soils, great extremes in temperatures and scant
rainfall--as low as 5 inches per year. It is a scene radically
different from the one which existed throughout this portion
of the Great Basin some 16 million years ago, when vegetation
identical to that now living near the Giant Sequoia groves in
the western Sierra Nevada and in the coastal ranges of central
California thrived in a moist land of ample rainfall--where a
great pristine lake splashed the trunks of canyon live oak and
Giant Sequoia.
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