0.0

Begin at junction of US highways 385 and 20 near west end of Chadron. Drive south on Highway 385.

For the next 12 miles we will rising up the slopes of the Pine Ridge. The Pine Ridge is a prominent topographic feature of the northwestern corner of Nebraska. Sedimentary rocks south of the Pine Ridge are mostly Miocene and younger in age. Those in and north of the Pine Ridge are Oligocene and older. The stack of young rocks to the south is gradually being eroded away by streams and gullies flowing north into the White River. The north-facing edge of the eroding stack is the escarpment we call the Pine Ridge.

5.5

Brule Formation outcrop West of highway.

The bluffs here along Chadron Creek are composed of soft clay-rich siltstone and sandstone of the Brule Formation, Oligocene in age. The Brule owes its color and character to its origin as fine volcanic ash. During the Oligocene, volcanoes in the Southwest, mainly Nevada, spewed enormous volumes of ash into the atmosphere. The prevailing winds carried this ash as far as Nebraska and beyond, clogging the creeks and rivers, which redistributed the ash and deposited these thick layers of sediment.

8.6

Cliff with cross, East of highway.

We have now climbed up the Pine Ridge and up the stratigraphic section to the Gering Formation. The Gering is the lowermost part of the Arikaree Group of Miocene Age. It is similar to the Brule in that volcanic ash still makes up a significant portion of the sediment, but the Gering typically contains more concretions.

Concretions are rounded features that have been cemented more completely than the surrounding sediment. They may be formed by cementation around a nucleus of organic material or by groundwater circulating through the rocks.

18.6

Slumping Hills

A slump is a form of mass wasting, or downslope movement. Slumps occur where unconsolidated sediments move downhill on a curved, scoop-shaped surface. The scar left at the top of the slump block is called a scarp. The scarp for this slump is covered with grass, indicating that it has not moved in some time.

The landscape here has been carved by gullies, which have cut through the layer of protective sod and exposed softer sedimentary rocks underneath. Gravity acting on these soft rocks, further weakened by water, causes intermittent slope failure (mass wasting).

20.0

Hillside with more erosion and slumping.

58.7

Junction - Highway 385/2 at eastern end of Alliance

0.0

Turn East (left) on Highway 2.

The road log now enters the area known as the Nebraska Sand Hills. The Sand Hills is an area of sand dunes, mostly stabilized by vegetation and therefore no longer active, or moving. The dunes were active about 8000 years ago, after glaciers that had covered much of northern North America and the Rocky Mountains melted. During the period after glaciation the climate was slightly warmer and drier than today.

Much of the sand for the Sand Hills probably blew out of river valleys and intermontane basins of Wyoming. Most Sand Hills dunes are elongated in a northwest-to-southeast direction, indicating that the prevailing wind was from the northwest, and that the source of sand was also in that direction.

8.2

Sandhills blowout South of highway.

The bare sandy area near the top of this hill is called a blowout. The name describes well how the feature forms. Wind literally picks up and blows the sand away by a process called deflation. Although the Sand Hills are mostly stabilized by grass and other vegetation, occasionally grazing or other activity disturbs the vegetative cover, exposing bare sand and making the dunes vulnerable to deflation and the formation of blowouts.

This blowout was formed prior to the mid 1970s and has since begun to stabilize with a cover of grass. Since the 1980s a second blowout has formed, and is now active, possibly due to changes in movement of cattle, which leave sharp trails in the soft sand.

10.2

Sand and railroad crossing.

The Sand Hills is a dynamic region, even where dunes have been "stabilized" by vegetation. Here is evidence of fairly recent movement of sand. Notice that some of the old fence posts and telephone poles have been partly buried in sand; others have been partly exhumed. Sand movement in the Sand Hills occurs both by blowing -- wind erosion, or deflation -- and in transport by running water.

This air photo was taken with film that records visible and near-infrared (invisible) light reflected from objects on the ground. Since colors are artificially assigned to both visible and infrared wavelengths, the image is referred to as "false-color infrared."

Return to Alliance

0.0

Junction of Highways 385 and 2 at West end of Alliance. Turn South on Highway 385.

24.8

Junction of Highways 385 and 26; turn West on Highway 26.

26.0

Hillside North of road

Arikaree Group

34.2

Turn South to Bayard

41.3

Junction of Highways 26 and 92; turn West on Highway 92.

41.6

Chimney Rock

Chimney Rock is a good example of relief caused by erosion. Look at the rocks in the side of Chimney Rock and imagine them extending as layers outward in all directions. Weathering and erosion by running water are slowly stripping away that rock and leaving behind isolated buttes and knobs known as erosional remnants. All of the buttes in western Nebraska were formed in a similar manner.

42.7

Chimney Rock Visitor Center

Bluff East of Chimney Rock.

Brule Formation—

The Brule Formation is the uppermost formation in the White River Group. It was named for the Brule Indians who lived in western Nebraska and South Dakota. The Brule is exposed over a wide area of the West, including Nebraska, North and South Dakota, Wyoming, and Colorado.

Arikaree Group—

The Arikaree Group is the stratigraphic unit containing (from lower to upper) the Gering, Monroe Creek, and Harrison formations.

43.8

Return to Highway 92.

0.0

Turn West on Highway 92 to Scottsbluff.

21.4

East Gering; junction of Highways 71 and 92; continue West.

22.2

West Gering Junction; continue West.

23.2

Scottsbluff National Monument

Scottsbluff National Monument was created to commemorate this butte, which was a landmark on the settlers’ westward trek. Like Chimney Rock, it is an erosional remnant of a much larger sheet of sedimentary rock and is slowly being reduced in size as erosion saps away from all directions.

The notch between the two buttes is the path the Oregon Trail took through here. This was an important route for the westward migration of settlers in the 19th Century.

24.3

Return to West Gering; turn North.

26.8

Scottsbluff; junction of Highways 71 and 26.

0.0

Turn North on Highway 71.

12.5

Hillside on West side of highway.

Ogallala Formation. The Ogallala Formation is widely exposed in west-central Nebraska, especially in the valley of the North Platte River. It is composed in part of coarse-grained sediments that are often important in their ability to hold groundwater. The Ogallala aquifer, as it is called, is an important supplier of water to the High Plains of the southern and central Great Plains region from Nebraska to the Texas Panhandle.

48.3

Junction of Highways 71 and 2; turn North on Highway 71.

68.2

Hillside on the East. Contact between the Brule Formation (White River Group) and Gering Formation (Arikaree Group).

74.8

Crawford; junction of Highways 71 and 20; continue North and then East on Highway 20.

90.1

Trunk Butte South of highway

Pierre Formation

The Pierre Formation, or Pierre Shale, is a dark gray shale that was deposited as mud in a shallow sea. The Pierre Sea was the last time this part of North America was covered by sea water.

The Pierre contains remains of marine animals such as mussels and ammonites (both (both mollusks) and the giant marine lizards called mosasaurs.

Brule Formation

Arikaree Group

96.2

Junction of US 385 to Hot Springs; continue East to Chadron.

98.3

Chadron

Route extension—Agate Fossil Beds National Monument

0.0

Junction of US highways 385 and 20; drive West toward Crawford.

7.5

Trunk Butte on the South side of the highway. Exposed in the steep sides of Trunk Butte is the Orella Member of the Brule Formation (Oligocene age).

23.5

Junction at Crawford; turn West on Highway 20/Route 71.

49.7

Junction at Harrison, turn South on Route 29.

71.8

Turn East to Agate Fossil Beds

72.8

Visitor Center

The Agate Springs area is a world-class natural repository of fossil mammals. Starting in the early 1900s, bone beds at Agate were mined for well-preserved remains of rhinoceroses, chalicotheres, horses, and other mammals and the fossils were mounted in museums around the world. Much of the bonebed remains untouched and still rests inside the hills behind the visitor center.

The fossil beds at Agate were part of a lake in a landscape that resembled modern-day Africa’s Kalahari Desert. Mass death of the fauna resulted from starvation induced by a drought. Grazing animals were forced to wander farther and farther away from the watering hole in search of grass. When a vast area around the watering hole was depleted of grass, the animals died and their remains were scattered around the lake. Some of their bones were preserved in the lake sediments and in subsequent deposits.

The Visitor Center at Agate Fossil Beds National Monument was completed in 1993 and had its grand opening in the summer of 1997.

Return to Chadron

End of Loop 1