Hagerman Fossil Beds
The Snake River Plain is a major late Cenozoic tectonic/volcanic feature at the north end of the Basin and Range Physiographic Province (Bonnichsen and Breckenridge, 1982). The plain extends in a crescent shape across southern Idaho for roughly 300 miles and it is divided into two main regions identified as the western and eastern Snake River Plain. The town of Hagerman is located on the eastern plain while the Monument lies in the western plain. The western Snake River Plain is about 40 miles wide, bounded by normal faults, and has a northwest-southeast trend. Displacement started about 17 million years ago by rifting and down warping of the plain. The subsequent stretching of the crust produced a basin that began filling with sedimentary and volcanic rocks of considerable thickness during the Miocene, Pliocene and Pleistocene.
Strata at the monument are typically characterized by a series of sediments named the Idaho and Snake River Groups (Figure 3). Deposition of the Idaho Group began eleven million years ago on the silicic Idavada Volcanics. Cope (1883) identified and named these sediments “The Idaho Group” and the body of water where these sediments collected “Lake Idaho”. The Idaho Group is composed of seven formations identified by Malde and Powers (1962), which include the Glenns Ferry and Tuana Formations. These Cenozoic sediments crop-out (Figure 4) on the steep bluffs west of the Snake River and are composed of clastic deposits inter-bedded by occasional basalt flows, silicic volcanic ash, and basaltic pyroclastic deposits. Most of the sediments are poorly consolidated and range in texture from clays to gravels.
The age of the Glenns Ferry Formation is broadly constrained from Pliocene to early Pleistocene, or 5 to 1.5 MA. (Malde, 1991). Deltaic, fluvial and flood plain environments are the primary constituents of the Glenns Ferry Formation exposed on the bluffs of the monument (Figure 4).
Malde (1972) suggests the depositional setting as a highly sinuous meandering stream, its flood plain, and its delta near the east end of Lake Idaho. The climate was predominately humid but also semi-arid at times. These deposits are commonly characterized by monotonous finegrained, graded, calcareous, pale-olive silt beds from one to three feet thick and capped with a dark, carbonaceous clay from one to several inches thick (Malde, 1965).
The Tuana Gravel Formation rests unconformably on the Glenns Ferry Formation. Saddler (1997) describes the composition of the Tuana gravels as coarser grained sediments in the silt, sand and gravel fractions. The base of the Tuana exhibits cut and fill stream channels in the underlying silts and clays of the Glenns Ferry Formation. These stream channels are commonly filled with fine sand. A caliche layer has formed several feet below the surface on the bluff tops, although no significant deposits exist on the hillsides. The caliche covers most of the plateau and reflects a climatic change from the Tuana environment and is considered to have formed during arid interglacial periods of the Pleistocene (Bjork, 1968). Outcrop observations indicate the caliche is a very dense layer averaging several feet thick, which thins to less than one foot in thickness in some locations. The caliche is resistant to weathering and forms a cap rock near the top of the bluff in most of the Monument and surrounding area.
The Monument contains ten soil series described by the Natural Resources Conservation Service (NRCS) and listed in Table 2. Soil development is a function of the geology, landforms, relief, climate and natural vegetation, and each type is associated with a particular element of the landscape. The permeability of these soils has been divided into seven textural classes. The Purdam and Rakane-Blacknest series is moderately slow with 0.2 to 0.6 inches per hour. The Sluka, Bahem and Dolman series are moderate with 0.6 to 2.0 inches per hour.
All series are well drained and described by the SCS as having intermediate water holding capacity, which retain optimum amounts of moisture, but are not wet close enough to the surface or long enough during the growing season to adversely affect yields. All series, except the Bahem, have a hardpan which starts about two feet down and continues to four foot depth. The Bahem had no hardpan down to six feet which is the maximum depth of survey data (NRCS, 1996). A common name for the ‘hardpan’ soil classification is caliche.
As shown in Figure 1, the Monument is located along the west bank of the Snake River. A broad, flat agricultural area known as the Hagerman Valley lies east of the Snake River. Sand and gravel deposits and thin, poorly developed soils cover basalt flows in the valley. Geologic and geomorphic features within the valley were created or influenced by the Bonneville Flood, which produced scoured surfaces of basalt, known as scablands, and enormous sand and gravel bars (Maley, 1987).
Nearly the entire monument is located on the arid slopes of the Snake River Plain between the west bank of the Snake River and the Bruneau Plateau. Malde (1965) described the western Snake River Plain as containing a considerable thickness of primarily clastic sediments. Bluffs that rise 600 feet above the Snake River contain rich fossil-bearing sediments deposited 3 – 4 million years ago. The Monument boundary generally follows the top edge of the bluffs to the west, and the mid channel of the Snake River to the east (Figure 1). The river is held at a nearly constant elevation of 2,800 feet by the Lower Salmon Falls hydroelectric dam. Closure of the dam flooded falls used by Native Americans to intercept salmon migrating along the main stem of the Snake River.
The bluff face consists of sedimentary and volcanic layers that are deeply incised by arroyos and gullies that range from a half to one mile in length. Slope angles between the gullies commonly exceed 350 with some as steep as 700.
Bjork, P.R., 1968, The carnivora of the Hagerman local fauna (late Pliocene) of southwestern Idaho , University of Michigan , Ph.D. dissertation, 165 p.
Bonnichsen, B., and Breckenridge, R.M., 1982, Cenozoic Geology of Idaho , Idaho Geological Survey, Moscow , Idaho , Bulletin 26, 725 p.
Malde, H.E., 1991, Quaternary geology and structural history of the Snake River Plain, Idaho and Oregon in Morrison, R.B., ed., Quaternary nonglacial geology; conterminous U.S.: Boulder, CO, Geological Society of America, The Geology of North America, v. K-2.
Malde, H.E., 1972, Geologic Map of the Glenns Ferry-Hagerman Area, west-central Snake River Plain, Idaho, U.S. Geological Survey Miscellaneous Investigations Map I-169, scale 1:48,000, 2 sheets.
Malde, H.E., 1972, Stratigraphy of the Glenns Ferry Formation from Hammett to Hagerman , Idaho : U.S. Geological Survey Bulletin 1331-D, 19 p.
Malde, H.E., and Powers, H.A., 1962, Upper Cenozoic stratigraphy of western Snake River Plain , Idaho : Geological Society of America Bulletin, v. 73. no. 10, 1197-1220 p.
Malde, H.E., 1965, Snake river plain, in The Quaternary of the United States , A review volume for the VII congress of the international association for Quaternary research, Wright, H.E. and Frey D.G., editors, Princeton University Press, Princeton , New Jersey.
Maley, T., 1987, Exploring Idaho geology: Mineral Land Publications, 232 p.
Natural Resources Conservation Service, 1996, Soil survey of Hagerman fossil beds national monument, Twin Falls county, Idaho , by Ames , D., U.S. Department of Agriculture.
Saddler, J.L., 1997, Sedimentology and stratigraphy of the Pleistocene Tuana gravel at Hagerman Fossil Beds National Monument, Idaho : Idaho State University , M.S. thesis, 62 p.Hagerman Fossil Beds National Monument is most famous for the horse that is Idaho's state fossil. It is most significant for it's variety, quantity, and quality of fossils.
What the Scientists Found Here
No other fossil beds preserve such varied land and aquatic species from the time period called the Pliocene Epoch. More than 140 animal species of both vertebrates and invertebrates have been found in hundreds of individual fossil sites. Eight species are found nowhere else, and 44 were found here first. The Hagerman Horse, Equus simplicidens, exemplifies the quality of fossils. From these fossil beds have come both complete and partial skeletons of this zebra-like ancestor of today's horse.
In 1929, paleontologists from the Smithsonian Institution in Washington, D.C., made the first scientific excavations at Hagerman Fossil Beds. A local rancher, Elmer Cook, had shown the fossil beds to a government geologist, Dr. Harold Stearns. The Smithsonian finds led to more expeditions in the 1930s. Its National Museum of Natural History excavated 120 horse skulls and 20 complete skeletons from an area called the Horse Quarry. The Smithsonian exchanged some of these Hagerman Horse skeletons with other museums, which has resulted in their display around the world. Additional scientific expeditions have been conducted over the years by other museums and universities. More than 200 published scientific papers focus on the Hagerman fossil species.
Clues in the Landscape
The 600-foot-high bluffs rising above the Snake River and comprising the Hagerman Fossil Beds reveal the environment at the end of the Pliocene Epoch. Grassy plains dotted with ponds and forest stands then received over twice today's ten inches of yearly precipitation. Mastodons, sabre-tooth cats, beavers, muskrats, otters, camels, antelope, deer, ground sloths, hyena-like dogs, and fish, frogs, snakes, and waterfowl lived here. The sediment layers from river level to bluff tops span some 550,000 years; from 3.7 million years old at river level to 3.15 million years old atop the bluff. These layers were deposited when rivers flowing into ancient Lake Idaho flooded the countryside. The much later Bonneville Flood (15,000 years ago) carved the high bluffs, exposing the layers and fossils. This flood also deposited fields of so-called melon gravel (lava boulders ranging in size from a compact car to watermelons) from today's river level to gravel bars 225 feet higher.
The sediments in the bluffs include river sands, thin shale layers deposited in ponds, clay flood deposits, and occasional volcanic deposits such as ash and basalt. It is the radioactive elements such as potassium 40 in the volcanic ashes that allowed scientists to determine the age of the fossils by measuring the rate at which one radioactive element breaks down into another.
Adapt, Migrate, or Become Extinct
When significant environmental change occurs, most plants and animals have three options: adapt, migrate, or become extinct. The ancient ecosystem represented by fossil plants and animals illustrates each response as the region changed from a wetter grassland savanna to the drier high-desert conditions of today.
- Adapted: Hagerman's beaver and muskrat and many birds are similar or ancestral to today's species.
- Migrated: Llamas migrated to South America, while camels and horses traveled across the Bering Land Bridge to Eurasia.
- Extinct: Ground sloths became extinct, along with mastodons and other large herbivores. With the disappearance of their primary prey, sabre-tooth cats and hyena-like dogs also became extinct.
Hagerman Fossil Beds is one of the few sites that preserves the necessary variety and quantity of fossil evidence to study past climates and ancient ecosystems. Fossil studies also add to contemporary research on biodiversity, wetlands ecology, and evolutionary patterns.
Epochs of the Cenozoic
- Horses reintroduced into North America by Spanish 1500s
- Extinction of North American megafauna, including horses 11,000-10,000 BP
- Bonneville Flood 15,000 BP
- Damming of Snake River by McKinney Butte Basalt 50,000 BP
- Immigration of bison into North America from Eurasia 400,000 BP
- Lake Idaho drains 1.7 mya
- Immigration of mammoth into North America from Eurasia 1.9 mya
Pleistocene 2 mya
- first appearance of modern horse Equus at Hagerman 3.2 mya
- volcanic eruption at Yellowstone deposits Peters Gulch Ash at Hagerman 3.7 mya
- Ancestral Snake River begins depositing sediments at Hagerman 4 mya
- First appearance of modern beaver, Castor 4.8 mya
- extinction of rhinos in North America 4.8 mya
Pliocene 5 mya
- Banbury Basalt forms floor of what is now the Hagerman Valley 8-11 mya
- Bruneau-Jarbridge eruption south of Hagerman deposits ash as far east as Nebraska 11 mya
- first elephants (gomphotheres) immigrate into North America from Eurasia 14.5 mya
Miocene 25 mya
- gap in the record (unconformity)
- first appearance of beavers 35 mya
Oligocene 38 mya
- Volcanism in the Challis area begins 51 mya
Eocene 55 mya
- first horse, Hyracotherium 57.5 mya
Paleocene 65 mya
- extinction of dinosaurs 65 mya
- gap in the record (unconformity)
The General park map handed out at the visitor center is available on the park's map webpage.For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.
A geology photo album has not been prepared for this park.For information on other photo collections featuring National Park geology, please see the Image Sources page.
Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.
Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
9" x 10.75", paperback, 550 pages, full color throughout
The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!
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NPS Geology and Soils PartnersAssociation of American State Geologists
Geological Society of America
Natural Resource Conservation Service - Soils
U.S. Geological Survey
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