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Roger Weller, geology instructor

Flaming Gorge, Utah
Fred E. Thomas
GLG 101- Fall 2005




The area discussed is located between Vernal, Utah and the Wyoming-Utah border along US route 191.
Vernal is at coordinates N40 27.331; W109 31.726.



Flaming George is part of the Flaming Gorge National Recreation Area, a 207,363-acre parcel that is part in Utah and part in Wyoming.  Located within the area is a reservoir having the capacity of 3,788,900 acre feet (43 thousand square feet to a depth of one foot).  The Green River is the source for the reservoir.


The whole area is on the “Colorado Plateau” which has risen almost straight up to elevations as great as 10,000 feet.  The Flaming Gorge area lies on the very northern edge of the Colorado Plateau.  The Plateau appears to end in the Vernal area.  The Plateau is made up of many smaller plateaus each a few hundred feet higher or lower than others.  Faults and folds result in a rich geological history of the area.

Colorado Plateau from “Pages of Stone”, The Mountaineers Book by Halka and Lucy Chronic.


US Highway 191 runs from Douglas, Arizona through Moab, Price, Vernal and Flaming Gorge and on to the Canadian border.   The area from Vernal to Wyoming is the subject of this project.


Traveling north from Vernal, on US 191, there will be seen markers naming the various earth and rock formations as seen from the road.  They start with the Cretaceous Period. [The Cretaceous, Jurassic and Triassic are within in the Mesozoic Era of the Phanerozioc eon] consisting of the Mancos Formation,  the Frontier, the Mowry and Dakota formations.  The Mancos is the youngest and composed of clay that settled to the bottom of the sea which covered the area. Many fossils are found in these strata.   Next is the Frontier in which for millions of years tropical plants were deposited by a river delta and then buried.  The pressure and decomposition formed coal beds.  Calcite building around fossils formed rocks or “cannonballs.”  Next, the Mowry formation is about 100 million years old and is composed of shale (clay and volcanic ash) left by an ancient sea.  In the strata is found fossilized fish scales and bones.   The last formation seen here in the Cretaceous Period is the Dakota. This formation results from streams flowing into the ocean transporting rocks and soils to this area.  Trees and plants growing along these streams are now petrified and contained within the deposit.  The formation of the Dakota took place about 110 million years ago.


A few miles further north on US 191 the Jurassic period formations are found.  The Morrison, Curtis, Entrada, Carmel and Navajo formations make up this period .  Low-lying plains, streams and lakes laid the Morrison about 145 million years ago.   The area was home to dinosaurs, crocodiles and turtles who left fossils records and became extinct.  This formation has “rainbow” colored clays – red, maroon, purple, white and green which are different types of clay, shale and sandstone.  The Curtis Formation is sandstone shale and limestone that was deposited by the sea.  Investigation will show mud cracks, ripple marks with fossil remains of many now extinct fish and reptiles.


After the Curtis formation the seas left and the land became desolate.  Neither plants nor animals lived here.  Winds laid down layers of sand and dust.  This was about 150 million years ago and is the Entrada Formation.  During the next period, the Carmel, the sea returned.  The ground was soft and beaches were formed.  Dinosaurs walked along the beaches leaving footprints that filled with a plaster like cement.  A sign along US 191 points out where these footprints are found along The Red Fleet State Park.  Additionally, this formation contains evaporites of gypsum that are being presently mined.



GOOGLE EARTH – Along US 191 to Flaming Gorge from Vernal North



Red Fleet State Park looking toward mudstone where dinosaur tracks are located.
In the background is the Navaho Sandstone and the “ships” that resulted in the “Red Fleet” name.


In the same period, Jurassic is the Navajo Formation.  The red rock in the background of the above picture is Navajo sandstone.  This layer was formed by large sand dune where the sand was cemented by silica or lime.  This layer is said to have been formed over millions of years.


It should be understood that in different geographic areas – differing formations are found.   A few miles east of Vernal and the Flaming Gorge area is the Dinosaur National Monument.  A section of the Monument is in eastern Utah and on the western boundary of Colorado.  The formations found in the Cretaceous period in that local are: Mancos, Frontier, Mowry, Dakota and then the Cedar Mountain Formation.  The Jurassic also has different formations, as: Morrison, Stump, Entrada, Carmel and Glen Canyon Sandstone.



Continuing north along US 191 we leave the Jurassic period formations and enter the area of the Triassic.  This is still in the Phanerozoic Eon and the Mesozoic Era.  During the Triassic period the area was covered by lakes and low lands where a crocodile like reptile was living.  Only the teeth have been found.  The strata contain mud pebbles, mud cracks and ripple marks.  Geodes are found there with cavities lined with calcite or quartz crystals.  In the very early part of the Chinle formation there is the Shinarump member (this area had pine trees that grew up to three feet in diameter and forty feet tall).  The trees were of the Araucaria species, as found  in Australia and South America.  Also found here are uranium deposits.  Next is the Moenkopi Formation.  This formation ranges from five hundred feet to one thousand feet in thickness.  It was formed at a time the sea had invaded almost to the western border of Colorado.  Its sources are tidal flats, floodplains and lagoons.  It is mostly dark red siltstone and mudstone with some yellowish limestone.  Found within it are reptile tracks.


US 191 passes through a section of the Permian period and the Park City formation.  This formation is rich in phosphate.  A large mining operation can be seen from the highway looking westward.  The phosphate was formed by decomposition of marine animals. 



Upon leaving the Permian Period and the Park City Formation, still traveling northward on US 191, the Pennsylvanian Period appears with its Weber and Morgan Formations. The Pennsylvanian and Mississippi period are components of a more general period named the Carboniferous.  The time line is 360 to 286 million years old.   In this area are oil fields.  The formation is normally four to six thousand feet below the earth’s surface but in this locality (Ashley Valley) the Weber Formation is only one thousand feet below the surface.  The formation is composed of poorly cemented sand grains and has been eroded by water leaving deep, steep-walled gorges.  A second formation exists in this period being the Morgan.  The Morgan was formed from a sea rich in many types of life – coral reefs, sponges, horsetail plants and sea urchins.  From this life comes the oil found in the Weber Formation.  The Carboniferous\Mississippi period has only one formation; the Madison.  In this time period the area was covered by a sea (the area had twelve seas cover it since the known beginning) which deposited large layers of limestone from primitive sea life.


Next in time, going further into the past, would be the Devonian (408 to 360 million years ago), the Silurian (440 to 408 million years ago), and the Ordovician (505 to 440 years ago) .  These periods are missing.  It is called a “unconformity.”


Next we come to the Cambrian Period (540 to 505 million years ago) of the Phanerozoic  Eon – the Paleozoic Era.  The only formation left of this period is the Loddore.  Again a sea was involved and a mud and sand strata in which trilobite’s found their grave.  Evidence is the fossilized shells of the trilobite and other ancient marine animals.


Now the highway passes over the oldest part of this area.  It is called the Uinta Mountain Group formed during the Precambrian Period.  This is of an older Eon – Proterozoic extending from 2500 to 540 million years ago.  During the early part of this eon the first large continents appeared.  Toward the middle of the eon the continents were grouped closely together in almost one land mass.  Salt was washed off the lands, algae plants produced oxygen and in time more complex life began.  This Uinta Mountain Group of about a billion years old is three miles thick.  If you went back to Vernal (south on US 191) and dug down to reach this stratum the hole would be two and a half miles deep. The strata that were deposited in this thirty mile trip from Vernal to the beginning of the Uinta Mountain Group were lifted vertically while retaining a horizontal layering.  This mountain was then eroded away leaving a pathway traveling toward the center of the earth.  While continuing northward passing over the Flaming Gorge Dam and into the State of Wyoming many of the same strata periods will be visible again but in the reversed order. 





PAGES OF STONE, Geology of the Grand Canyon & Plateau Country National Parks & Monuments, second edition, The Mountaineers Books, by Halka and Lucy Chronic.


THE FIELD GUIDE TO GEOLOGY.  Facts On File, Inc/ Diagram Visual Informatiion Ltd. , 1998.


WILDLIFE THROUGH the AGES – FLAMING GORGE – Uintas National Scenic Byway.  Compiled by Dinosaur Nature Association and the Utah Division of Wildlife Resources.