|Click on the image for a larger picture. An eastbound train rumbles through the fossil leaf-bearing railroad cut near Verdi, Washoe County, Nevada, in the upper Miocene Hunter Creek Sandstone, dated through sophisticated radiometric analyses (radioactive isotope measurements) at 5.8 million years old. Eastern Sierra Nevada "foothills" in the background. The 18 species of Miocene plants preserved here demonstrate that some six million years ago, this portion of the western edge of the Great Basin Desert would have resembled, floristically, the California Gold County, western foothills of the Sierra Nevada, from roughly Placerville south to Jackson. Photograph courtesy an individual who goes by the cyber-name lennycarl08.|
|Text: The Field Trip||Images: On-Site||Images: Fossils (my collection)||Images: Technical Document|
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Yosemite, Kings Canyon, Mount Whitney-each is an awesome example of nature's handiwork amidst the grandeur of California's central to southern Sierra Nevada. Yet, some six to five million years ago that mighty mountain range revealed far less precipitous extremes, and the great alpine altitudes so spectacular today there were in their youthful stages of development. At that early period of Sierran geomorphological creation, sluggish rivers wandered through extensive floodplains in a region of only moderate topographic relief; lakes and stagnant ponds lay scattered in the basins, and thickets of riparian vegetation associated with elevations dramatically lower than present grew along the watercourses.
Proof of this radically different scene can be found in rocks exposed near Verdi, Washoe County, Nevada, on the eastern slopes of the Sierra. Here occurs a wonderful fossil leaf locality in a sequence of sedimentary beds dated by sophisticated geophysical radiometric techniques at 5.8 million years old, a site where the numerous species of plants preserved provide a direct link with the climate and geography of the geologic past.
Verdi is a small gaming community that lies along Interstate 80 approximately 10 miles west of Reno, Nevada. The leaf-bearing beds occur in the vicinity of town along a railroad cut in what stratigraphers have assigned to the uppermost (youngest) layers of the upper Miocene Hunter Creek Sandstone.
This railroad cut exposes a 28-foot thick section of the Hunter Creek Sandstone, presently considered 5.8 million years old through accurate radioactive isotope analyses. The exposure extends in an east-west direction for a minimum of 125 feet and fossil leaves can be found throughout it--primarily in the bluish to gray andesitic sandstones that were derived during late Miocene times from an older volcanic formation, the Kate Peak Andesite. Interbedded with the sandstones are minor lenses of tan to white diatomaceous shales (composed mainly of diatoms, a microscopic photosynthesizing single-celled aquatic plant), within which occur nicely preserved leaves of a species of pond weed, referred to scientifically as Potamogeton verdiana--named specifically for its special occurrence here in the Hunter Creek Sandstone near Verdi.
The history of fossil plant collecting at Verdi goes back to the 19th Century. In his 1878 report regarding the famous Fortieth Parallel Survey--an exporatory expedition mandated by the US Congress, conducted from northeastern California through Nevada to eastern Wyoming during 1867 to 1872--geologist Clarence King (first Director of the United States Geological Survey) first mentions the occurrence of fossil plants near Verdi, Nevada. In 1909, in a report about oil and gas potential in the Reno region, R. A. Anderson provides a brief but detailed examination of leaf-bearing beds in the vicinity of Verdi, from what Clarence King had previously called the Truckee Formation (a geologic rock unit whose paleobotanical preservations are now understood to occur in the newly named Hunter Creek Sandstone). On page 483 of that same paper, Anderson also describes a "near Reno" fossil leaf collection secured from probable Truckee Formation beds by a professor J. P. Smith, who kept the specimens at Stanford University; taxa preliminarily identified included birch, serviceberry, manzanita, willow, and a pine cone. Some five years later, around 1914, an H. S. Gale submitted a small collection of Verdi area leaves to the US National Museum, but apparently published no formal documentation of the occurrence; sad, that--because, as later paleobotanists have noted, Gale found several species that have never since been observed from what's now called the Hunter Creek Sandstone. In 1916, vertebrate paleontologist J. C. Merriam used information supplied by paleobotanist F. H. Knowlton to help establish what Merriam believed, at that date, was a geologic correlation between Clarence King's Truckee Formation and the bone-bearing Esmeralda Formation in Nevada--a paleontologically rich deposit presently known to contain middle to early late Miocene faunas approximately 16 to 10 million years old; Knowlton compared a collection of Verdi vicinity leaves with plants already sampled from the Esmeralda Formation and concluded (quite erroneously, of course) that the two floras could be considered contemporaneous. Not until the early 1920s did a professional paleobotanist--that would be R. W. Chaney--finally amass the first large collections of fossil leaves from the Verdi locality. Supplemental collecting expeditions to Verdi by the late paleobotanist Daniel I. Axelrod in 1939, 1947, 1953, 1954, and 1956 increased the number of fossil specimens exponentially. In 1977, David A. Orson, a graduate student at the University Nevada, Reno, extracted pollen grains from lignite beds exposed in the upper half of what's now called the upper Miocene Hunter Creek Sandstone, Verdi Basin, Washoe County, Nevada, but his palynological specimens, recovered through maticulous dissolution of the low grade coal matrix with potently hazardous acids (among them, hydrofluoric acid--one of the most powerful acids known to exist--which is frequently used to break down rocks suspected to contain microscopic pollens) did not come from the Verdi fossil leaf locality.
All told, seekers of fossil leaves have recovered 18 species of plants from the Verdi exposures of upper Miocene Hunter Creek Sandstone. The most abundant leaves found belong to a Miocene variety of black cottonwood, Populus alexanderi--called affectionately by paleobotanists the "common Pliocene cottonwood." Also present are the leaves of Miocene analogs of Scouler's willow (Salix boisiensis), Goodding's black willow (Salix truckeana), quaking aspen (Populus pliotremuloides), Korean aspen (Populus subwashoensis), pinemat manzanita (Arctostaphylos verdiana), valley oak (Quercus prelobata), Engelmann oak (Quercus renoana), interior live oak (Quercus wislizenoides), sierra gooseberry (Ribes galeana), buckbrush (Ceanothus precuneatus); and a bitter cherry (Prunus moragensis). All of these leaf specimens occur exclusively in the sandstone strata, along with the needles, cones, and cone scales of such conifers as white fir (Abies concoloroides), ponderosa pine (Pinus florrisanti), sugar pine (Pinus prelambertiana), and Knobcone pine (Pinus pretubercula). In addition to yielding the slender leaves of pond weeds (Potamogeton verdiana), the lenticular seams of whitish diatomaceous shales also contain the remains of water lilies (Nymphaeites nevadensis) and stonewarts (Chara verdiana).
Supplemental palynological material, secured in 1977 by University Nevada, Reno, graduate student David A. Orson from two separate lignite horizons in the upper half of the Hunter Creek Sandstone, Verdi Basin, Washoe County, Nevada, included--from the older level--pollen grains of red fir (Abies magnifica), white fir (Abies concolor), Grand fir (Abies grandis), knobcone pine (Pinus attenuata), Western white pine (Pinus monticola), Jeffrey pine (Pinus jeffreyi), Douglas-fir (Pseudotsuga menziesii), willow (Salix sp.), mountain whitethorn (Ceanothus cordulatus), maple (Acer sp.), and golden chinquapin (Chrysolepis chrysophylla). The younger low grade coal layer produced pollens of Jeffrey pine (Pinus jeffreyi), ponderosa pine (Pinus ponderosa), Douglas-fir (Pseudotsuga menziesii), cottonwood (Populus sp.), Mountain mahogany (Cercocarpus betuloides), mountain whitethorn (Ceanothus cordulatus), maple (Acer sp.), walnut (Juglans sp.), Pacific dogwood (Cornus nuttallii), and questionably (contamination with recent pollen grains is highly suspected) Mormon Tea (Ephedra nevadensis). The older plant association resembles a modern pine forest community on both sides of the Sierra Nevada, with obvious similarities to lower elevation yellow pine habitats in southern California, as well. Plants in the younger lignite bed have modern analogs now living in the woodland communities on the west side of the Sierra Nevada--primarily in low altitude Sierran yellow pine associations and the Sierra foothill chaparral areas of central California.
At the Verdi fossil site, the fossil leaves are predominantly preserved as black to brownish carbonized impressions throughout the bluish to grayish, dense to poorly compacted fluviatile sandstones; several intervals in the sequence reveal leaves matted together in an almost coal-like tangle of preserved vegetable debris. Specimens of pond weeds, water lilies and stonewarts recovered from the diatomaceous shales appear lighter colored than their counterparts in the sandstones, but they too are the impressions of original leaf material--organic remains compressed through geologic time by forces of heat and pressure. This process has driven out all the volatile constituents from the leaves, except the irreducible carbon residues, which now outline the original shapes and forms of once-living plants.
Although footing at the fossil site tends to be somewhat treacherous in places, owing to the extreme steepness of much of the railroad cut, most of the plant-bearing beds can be comfortably scouted. The most accessible collecting is available along the lower talus slopes, where natural erosion, combined with periodic pickings by other fossil hunters, has produced an abundant source of sandstone blocks.
And, happily for collectors, the fossil leaves tend to lie along distinct bedding planes, fossiliferous layers that when not in plain view on an already exposed surface can be reliably spotted as thin black strips parallel to the bedding of the unweathered sandstones. Where such an occurrence is suspected, take a rock hammer and chisel (wear eye protection at all times while cracking rocks) and strike the probable fossil-bearing layer forcefully. If the strike has been true, the sandstone will invariably split with a clean break, revealing leaf imprints to their first light in nearly six million years. They tend to pop out at you in dramatic display--a cottonwood, a willow, an aspen--plants never before seen by human eyes until you investigated this tangible link with the geologic past.
A time-honored observation is that many of the the better-preserved paleobotanical specimens occur in the finer-grained rocks at the western end of the railway cut. Which virtually guarantees that innumerable folks over a period of several decades have already explored the most promising leaf-bearing areas. Still and all, even though the westernmost area has obviously been heavily fossil-prospected over the years, be sure to investigate as much of that specific geologic rock exposure as possible; excellent paleontological material still awaits discovery by dedicated paleobotany enthusiasts, in both the fine and coarser-grained sandstones.
What the rocks and their 18 species of fossil plants inform us of the ancestral late Miocene Verdi Basin is most illuminating. First off, the sandstones in the Hunter Creek Sandstone prove that a wide floodplain existed within this portion of the eastern Sierra Nevada area 5.8 million years ago, through which sluggish streams wandered. This helps corroborate the geological conclusion, based on a synthesis of successive scientific investigations, that no significant elevation barrier existed at present-day Donner Pass (altitude 7,057 feet, 30.3 miles west of Verdi) in the proto-Sierra Nevada region prior to approximately 2.6 million years ago, suggesting substantial uplift of this portion of the Sierra Nevada during Pliocene times. Nevertheless, analysis of sedimentary depositional patters indicates that watercourses in the ancestral Verdi Basin never flowed westward across the nascent Donner "divide;" neither did streams carry detritus beyond the localized basin. Thus, present evidence inescapably supports the postulation that the late Miocene Verdi Basin was one of many widely scattered endorheic (internally drained) centers of sedimentary accumulation that came into existence during a period of later Tertiary Period extensional forces, which beginning about 16 million years ago had already begun to create today's Great Basin physiographic province.
Within the late Miocene Verdi area some 5.8 million years ago, watercourses supported thick woodlands composed of cottonwoods and willows. Ponds, lakes and swamps in the basin--where layers of diatomaceous shale accumulated--held colonies of water lilies, stonewarts, and pond weeds. Dominating the higher elevations was a forest community of aspens, ponderosa pine, Knobcone pine, Sugar pine, and White fir. In addition to the trees, such brushes as manzanita, bitter cherry, and gooseberry were confined to moister valleys and slopes bordering the basin.
From the available geological evidence it appears that most members of the preserved Verdi Basin plant communities were transported into the floodplain during episodes of periodic flooding, although the aspens and willows could have extended down to the lowlands in moderate numbers. The well-drained drier slopes and flats supported a chaparral-style woodland of valley oaks, evergreen live oak, and buckbrush; a species of closed cone pine occupied the rocky, exposed slopes throughout the woodland.
Climatic conditions were much more moderate than those observed in the Verdi area today. Rainfall was in the neighborhood of 18 to 20 inches per year, but that figure could have increased to as much as 25 inches in the higher hills. Verdi today lies within a semi-arid region of sparse rainfall, receiving most of its 13 inches of effective annual precipitation as snow during wintertime. It seems that late Miocene summers were warm to hot, as the plants indicate that the average July highs stayed around 85 degrees. January low averages probably ranged near 45 degrees, which contrasts dramatically with the present January Verdi low normal of 21 degrees. Additionally, today, wintertime temperatures at Verdi can sometimes plummet to numerous degrees below zero, with windchill factors reminiscent of Antarctica meteorological conditions.
Within such a temperate late Miocene climate, the growing season was likely as long as 210 days; today in the Verdi district it is barely 114 days. Elevations at the site of deposition could not have much higher than 2,500 feet, an estimate based on the known habitats of living members of the plant groups identified in the fossil beds. Verdi today lies at an elevation of 4,905 feet.
Probably the closest modern-day comparison with the specific association of plants displayed in the late Miocene Hunter Creek Sandstone is the California Gold Country, western Sierra Nevada foothills, from Placerville south to Jackson. Here the lush Sierran forest of pine, aspen, white fir, and cottonwood interfingers with a vigorous chaparral community of Valley oak, evergreen live oak, and manzanita.
Today, by contrast, the Verdi fossil locality lies within a botanic transition zone from pinon-juniper woodlands to a classic Sierra Nevada conifer forest. Within proximity of the late Miocene leaf-bearing site, for example, occur such typical transition plants as pinon pine (Pinus monophylla), juniper (Juniperus utahensis), Basin sage (Artemisia tridentata), curl leaf mahogany (Cercocarpus ledifolius), rabbit brush (Chrysothamnus nauseosus), desert peach (Primus andcrsonii), antelope brush (Purshia tridentata), plateau gooseberry (Ribes velutinum), and horsebrush (Tetradymia glabrata). Only a slight increase in elevation here brings on a dominant Sierran forest community, characterized by three defined botanic associations as one gains altitude. The lowest zone contains Jeffrey pine (Pinus jeffreyi), white fir (Abies concolor), incense cedar (Libocedrus decurrens), sugar pine (Pinus lambertiana). yellow pine (Pinus ponderosa), green manzanita (Arctostaphylos patida), chinquipin (Castanopsis sempervirens), deer brush (Ceanotlius integerrimus), white thorn (C. cordidatus), bitter cherry (Prunus emarginata), alder (Alnus tenuifolia), serviceberry(Amelanchier alnifolia), dogwood (Cornus californica), aspen (Populus tremidoides), black cottonwood (Populus trichocarpa), chokecherry (Prunus demissa), rose (Rosa gymnocarpa), and several kinds of willow (Salix spp.). From 7,500 feet to about 8,500 feet, a middle forest is dominated by white fir (Abies concolor), red fir (A. magnifica), Jeffrey pine (Pinus jeffreyi), pine-mat manzanita (Arctostaphylos nevadensis), chinquipin (Castanopsis sempervirens), huckleberry oak (Quercus vaccinifolia), alder (Ainus tenuifolia), bitter cherry (Prunus emarginata), lodgepole pine (Pinus contorta) and aspen (Populus tremuloides). At about 8,500 feet elevation occurs a subalpine forest association with white-bark pine (Pinus albicaulis), white pine (Pinus monticola), and mountain hemlock (Tsuga mertensiana). Regionally, not far above 8,500 feet lies the Sierran timberline of barren arctic-alpine inclines and high summits.
A genuine field day can be experienced finding fossil leaves at Verdi. Not only are the paleobotanic preservations plentiful and easy to locate, but they're also scientifically valuable--exquisite carbonized evidence of an important geologic age, the latest Miocene of roughly 6 to 5.3 million years ago. And because fossil leaves remain relatively infrequent occurrences throughout the Sierra Nevada district, the Verdi locality assumes even greater paleontological significance. Hence, specimens that reveal especial excellence of preservation should be brought to the attention of a professional paleobotanist. Who knows, you might have discovered a species that is new to science.
Naturally speaking, some special words of caution are obviously in order here. The fossiliferous beds lie in close proximity to an occasionally busy railway line (read: you're practically right on top of the tracks). Fortunately for paleobotany adventurers here, approaching trains can be heard--and seen--from a distance, so you have ample opportunity to get completely clear of the tracks.
Of course, always conduct due diligence before visiting the area; check with the railroad folks to determine the most current collecting guidelines. If permission is still granted, be careful not to scatter rock debris on the tracks. Use good judgment at all times and this site will potentially remain a safe and accessible place to visit.
|Click on the images for larger pictures. Left to right--Viewing perspective is northeast. A long train passes close to a classic fossil leaf locality in the vicinity of Verdi, eastern side of the Sierra Nevada, Washoe County, Nevada. Elevation here is presently 4,800 feet, but the late Miocene plants preserved in a railroad cut nearby demonstrate that 5.8 million years ago this same scene could not have been any higher than around 2,500 feet, resembling in botanic associations the modern California Gold Country, from Placerville south to Jackson, western foothills of the Sierra Nevada. A Google street car image I edited and processed though photoshop. Right--Viewing perspective is due east. The footpath to the fossil leaf locality near Verdi, Nevada, used to take off just to the left of that street sign (right side of photograph); a Google street car image I edited and processed through photoshop.|
|Click on the images for larger pictures. Left and right--Two views to the fossil leaf locality near Verdi, Washoe County, Nevada, both looking roughly southwest. The carbonized leaves occur in the sandstones and diatomaceous shales (composed almost entirely of diatoms, a microscopic photosynthesizing single celled plant) of the upper Miocene Hunter Creek Sandstone, dated by radiometric analyses at 5.8 million years old. Both photographs originally taken with a Minolta 35mm camera.|
|Click on the images for larger pictures. Left to right--The directional view is roughly slightly south of due east. A seeker of paleobotanical adventure inspects leaf-bearing sandstones of the upper Miocene Hunter Creek Sandstone (5.8 million years old) at a railroad cut exposure near Verdi, Washoe County, Nevada. Photograph originally taken with a Minolta 35mm camera. Right--A viewing perspective roughly westward. An eastbound train rumbles through the leaf-bearing railroad cut near Verdi, Washoe County, Nevada, in the upper Miocene Hunter Creek Sandstone. Eastern Sierra Nevada "foothills" in background. Image courtesy an individual who goes by the cyber-name lennycarl08.|
|Click on the images for larger pictures. Left to right-- A viewing perspective roughly westward. An eastbound train thunders through the leaf-bearing railroad cut near Verdi, Washoe County, Nevada, in the upper Miocene Hunter Creek Sandstone. A sliver of the eastern Sierra Nevada "foothills" seen in background, at top. Image courtesy Don Bachman. Right--A vintage image of the fossil leaf locality near Verdi, Washoe County, Nevada, originally taken by the late paleobotanist Daniel I Axelrod during one of his fossil collecting expeditions in either 1939, 1947, 1953, 1954, or 1956. Viewing perspective is roughly southwest. The carbonized leaves occur in the sandstones and diatomaceous shales (composed almost entirely of diatoms, a microscopic photosynthesizing single celled plant) of the upper Miocene Hunter Creek Sandstone, dated by radiometric analyses at 5.8 million years old.|
|Click on the images for larger pictures. Left to right--Three fossil leaves from the upper Miocene Hunter Creek Sandstone near Verdi, Washoe County, Nevada. For size perspective, specimen at upper left is 60mm long--it is a quaking aspen, called scientifically Populus pliotremuloides; the other two belong to a Black cottonwood referred to Populus alexanderi. Right--A Black cottonwood leaf, Populus alexanderi, from the upper Miocene Hunter Creek Sandstone in the vicinity of Verdi, Nevada. The specimen is 65mm long. Both Photographs originally taken with a Minolta 35mm camera.|
|Click on the images for larger pictures. Left to right---An isolated look at the carbonized quaking aspen leaf--called scientifically Populus pliotremuloides--seen in image, above. The specimen is 60 millimeters long. Right--A mostly complete specimen (the tip is missing) of a carbonized leaf from the Miocene analog of today's Goodding's willow. The specimen is 58 millimeters long. Both specimens came from the upper Miocene Hunter Creek Sandstone near Verdi, Washoe County, Nevada, a geologic rock deposit dated by radiometric analyses at 5.8 million years old. Both photographs originally taken with a Minolta 35mm camera.|
|Click on the images for larger pictures. Left to right--Two leaves from the Miocene counterpart of the modern interior live oak, called scientifically Quercus wislizenoides. Right--Two leaves from the Miocene counterpart of the living Scouler's willow (Salix boisiensis). All four leaf specimens came from the upper Miocene Hunter Creek Sandstone (5.8 million years old) as exposed near Verdi, Washoe County, Nevada. Both photographs courtesy a specific technical document.|
|Click on the images for larger pictures. Left to right--A leaf from the Miocene version of the living Black cottonwood, called scientifically Populus alexanderi. Right--Two leaves from the Miocene counterpart of the modern quaking aspen (Populus pliotremuloides). All three leaf specimens came from the upper Miocene Hunter Creek Sandstone (5.8 million years old) as exposed near Verdi, Washoe County, Nevada. Both photographs courtesy a specific technical document.|