The Geological History of Fossil Butte National Monument and Fossil Basin — Themes and Context

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Authors: McGrew, Paul O. (Paul Orman), 1909-1983, Casilliano, Michael Digital Source: Project Gutenberg Gutenberg Release Year: 2015 First Publication Year: Not confirmed

Key Word: Geology -- Wyoming -- Fossil Butte National Monument; Geology, Stratigraphic -- Eocene; Paleontology -- Eocene; Fossil Butte National Monument (Wyo.) Target Reader: Readers of public-domain and historical texts

Book Edition: Project Gutenberg digital edition Book Language: en

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Words 22,261

Reading time 97 min

Text sections 3

The source record for The Geological History of Fossil Butte National Monument and Fossil Basin — Themes and Context measures this digital text at 22,261 words, 1 hr 37 min estimated reading time, and 3 detected text sections.

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This editorial note examines the structure and recurring images in McGrew and Casilliano's geological study of Fossil Basin, focusing on how the text moves between lake systems, fossil types, and time scales.

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faulting but it is somewhat disturbed and rests under the Wasatch Formation with angular uncomformity. The Evanston Formation bridges the time boundary between the Cretaceous and Tertiary periods. In the lower part of the unit are found many fossil leaves, pollen, and spores and a jaw of the horned dinosaur _Triceratops_ that prove its Cretaceous age, and in the upper part are found fossil mammals of Paleocene age.

The Evanston Formation has been studied in detail by Oriel and Tracey (1970). These authors divide the formation into three members. The lowest, which they called the Lower Member, is predominantly “gray to very dark gray mudstone, siltstone, claystone and gray carbonaceous sandstone.” The Lower Member reaches a thickness of 500 ft in some places. Above and in part interfingering with the Lower Member is a 1000 ft thick unit that was named the Hams Fork Conglomerate Member. This unit consists of beds of boulder conglomerate interstratified with thick beds of coarse, partly conglomeratic brown sandstone and gray mudstone. Where the Lower Member of the Evanston is missing, the Hams Fork Conglomerate forms the base of the formation.

The Upper Member of the Evanston Formation is termed by Oriel and Tracey (1970) the Main Body. It is more than a thousand feet thick and the lower part intertongues with the Hams Fork Conglomerate. The Main Body is “light to dark gray carbonaceous sandy to clayey siltstone interbedded with gray, tan, yellow and brown sandstone and conglomerate and carbonaceous to lignitic claystone.” It is this Main Body that can be seen along the highway just southeast of the monument.

The types of sediments and fossils found in the Evanston indicate that the formation was deposited by streams on flood plains and in marshes and ponds. A subtropical climate is indicated and the area was heavily wooded.

The term Wasatch was first used by Hayden (1869:91) as follows:

Immediately west of Fort Bridger commences one of the most remarkable and extensive groups of Tertiary beds seen in the West. They are wonderfully variegated, some shade of red predominating. This group, to which I have given the name of Wasatch group, is composed of variegated sands and clays. Very little calcareous matter is found in these beds.

In Echo and Weber Canyons are wonderful displays of conglomerates, fifteen hundred to two thousand feet in thickness. Although this group occupies a vast area, and attains a thickness of three to five thousand feet, yet I have never known any remains of animals to be found in it. I regard it, however, as of middle Tertiary age.

The Wasatch is well exposed in Fossil Basin. There the unit was regarded by Veatch (1907) as a group and divided by him into three formations: the Almy, Fowkes, and Knight. He wrote (1907:88):

In the Wasatch group as thus defined by Hayden the field work of the season 1905 showed three divisions: 1) a basal member composed of reddish-yellow sandy clays, in many places containing pronounced conglomerate beds, which has been named the Almy Formation; 2) a great thickness of light-colored rhyolitic ash beds containing intercalated lenses of white limestones with fresh-water shells and leaves—the Fowkes Formation; and 3) a group of reddish-yellow sandy clays with irregular sandstone beds (the Knight Formation) closely resembling 1) lithologically and separated from 1) & 2) by a pronounced period of folding and erosion.

Veatch, however, erred in his field work and did not realize that the Fowkes Formation had been downfaulted into the position in which he saw it (Tracey and Oriel 1959; Eardley 1959). The Fowkes is actually the youngest of the three formations of Veatch and is considerably later in age than the true Wasatch.

The Almy and Knight formations are not separable (Oriel and Tracey 1970:16), and can be seen to grade into each other at the basin edges.

Recent, Pleistocene Pleistocene and Recent gravel and alluvium Pliocene Gooseberry Member of Fowkes Formation Oligocene to Pliocene Hiatus Eocene Bridgerian Fowkes Fm. Bulldog Hollow Member Sillem Member Unconformity Wasatchian: Wasatch Formation, Tunp Member Bullpen Member Green River Formation Angelo Member Mudstone Tongue Fossil Butte Member Sandstone Tongue Main Body Lower Member Basal Conglomerate Member Paleocene Possible hiatus or unconformity Evanston Formation Main Body Hams Fork Conglomerate Member Latest Cretaceous Lower Member UNCONFORMITY Late Cretaceous Adaville and older formations

Eocene Late Eocene Uintan Mid Eocene Bridgerian Early Eocene Wasatchian Lostcabinian Lysitean Graybullian Paleocene Late Paleocene Tiffanian Mid Paleocene Torrejonian Early Paleocene Dragonian Puercan

The Almy and Knight, as defined by Veatch, are probably different facies of Wasatch, the Almy being a more peripheral facies and the Knight a more basinal facies (Oriel and Tracey 1970). These authors proposed that the terms Almy and Knight be dropped and that the name Wasatch be applied to all these rocks to avoid confusion.

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The study opens with a precise geographic and temporal frame: fifty million years ago, southwestern Wyoming held a system of three freshwater lakes—Flagstaff, Gosiute, Uinta, and Fossil Lake. This initial image of interconnected lakes recurs throughout the text as a structural anchor, with each formation tied to a specific lake phase. The authors then shift abruptly to the present-day monument, creating a telescoping effect that compresses deep time into a single landscape.

The book’s organization mirrors this movement: it proceeds from location and historical research through stratigraphy, structure, paleontology, and finally paleoecology. Each section builds on the previous one, but the authors frequently interrupt the chronological flow with cross-references to earlier or later formations, reinforcing the idea that the basin’s history is a layered, interconnected system.

Stratigraphic Layers as Narrative Structure

The table of contents reveals a deliberate stacking of formations—Thaynes Limestone, Evanston, Wasatch, Green River, Fowkes—each with its own members and depositional environments. This vertical arrangement is not merely descriptive; it becomes the book’s primary organizational device. The authors treat each formation as a chapter in the basin’s story, but they avoid a simple linear narrative. Instead, they repeatedly loop back to earlier units, noting how the Wasatch Formation’s basal conglomerate member relates to the Green River’s Fossil Butte Member. This recursive structure mirrors the geological principle of superposition while acknowledging that rock layers are not isolated events but parts of a dynamic, shifting system. The reader is thus trained to think in three dimensions: depth, time, and lateral variation.

Recurring Images of Water and Sediment

Water appears as a persistent motif, from the initial lakes to the shoreline deposits where turtles and crocodiles are found. The text frequently describes sediments as “shore-line deposits” or “lake sediments,” emphasizing the aqueous origin of the fossils. Even when discussing terrestrial mammals, the authors note that their remains are often found in “wash and screening” processes, linking them back to water transport. Another recurring image is the “complete skeleton” or “complete shell”—the authors highlight exceptional preservation (e.g., the boa Boavus idelmani, the bird Gallinuloides) as evidence of the unique conditions in Fossil Lake. These images of wholeness contrast with the fragmentary nature of most fossils (jaws, vertebrae, isolated plates), creating a tension between the ideal of complete preservation and the reality of taphonomic loss.

Movement Between Scales: From Basin to Bone

The text constantly shifts between macroscopic and microscopic views. A paragraph might begin with the entire Fossil Basin, then zoom in to a specific formation, then to a single fossil jaw. For example, the discussion of mammals starts with the broad statement that mammal fossils are used for correlation and dating, then narrows to the index fossil Haplomylus, and finally to a jaw measuring 1.3 cm. This rapid scaling is a deliberate rhetorical strategy: it keeps the reader aware that every small fragment carries implications for the larger geological picture. The authors also move between temporal scales, juxtaposing the 50-million-year-old lakes with the “Quaternary” section that covers only the last 2.6 million years. This asymmetry underscores how much of the basin’s history is compressed into the Eocene, while later events are treated as a brief coda.

The Role of Absence and Inference

Throughout the paleontology section, the authors acknowledge gaps in the fossil record. They note that the absence of certain bird or mammal species “is more likely due to nondiscovery than nonexistence,” and that bird fossils are rare because hollow bones are “especially susceptible to breakage.” These admissions of uncertainty are not weaknesses but structural features: they remind the reader that the geological history is an interpretation based on incomplete evidence. The text also uses inference to bridge gaps, as when the presence of crocodiles is taken as “further evidence for a prevailing warm and humid climate.” This interplay between what is found, what is missing, and what can be deduced gives the book a cautious, scientific tone. The authors rarely claim certainty; instead, they build a case from multiple lines of evidence, often citing other researchers (Schaeffer and Mangus 1965; Brodkorb 1970) to support their conclusions.

Readers will benefit from paying attention to the book’s layered structure and its habit of moving between scales. The stratigraphic table of contents is not just a list but a map of the authors’ thinking. When you encounter a fossil description, consider how it fits into the larger basin history—and what the authors leave unsaid. The gaps are as instructive as the finds.

I recall evenings with The Geological History of Fossil Butte, how it layers lake upon lake, each era patiently waiting beneath the next. There is a similar patience in Child Versus Parent: Some Chapters on the Irrepressible Conflict in the Home — A Reader’s Guide, where old tensions settle into quiet strata, neither buried nor forgotten, just deeply understood.