Bruneau Field Office


Mud Flat "Shoofly" Oolite

Designated 1992
Bruneau Management Framework
 Plan amendment
5 acres

Flora of the Great Basin and the Columbia Basin intermingle on southwestern Idaho's Owyhee Front. The unusual soils of the Shoofly Oolite along the Mud Flat Road west of Bruneau
 support a high density of rare plant species.

The hummocks, small arches and
 other intriguing land features are clues to the area's geologic past.

Mulford's milkvetch - CREDIT: Lisa Hahn, Idaho Dept. of Fish aand Game

 

LEFT to RIGHT: 
Mulford's milkvetch 
| Astragalus 
    mulfordiae  

Snake River milkvetch 
| Astragalus purshii 
var. ophiogenes 

 White-margined wax plant 
| Glyptopleura marginata 

Packard's cowpie buckwheat - CREDIT: Christopher L. Christie

Packard's cowpie buckwheat
| Eriogonum  shockleyi var.  packardiae

 


White eatonella (BLM photo)

White eatonella
| Eatonella nivea

 

 


Snake River milkvetch (BLM photo)   White-margined wax plant (BLM photo)

grains of oolite, shown in the palm of a hand to illustrate size

EXPLORE the area

The Big Picture

regional cross-section of geological events along the Owyhee Front

Extending across southwestern Idaho between the Owyhee Mountains and the Boise Front is the broad valley of the western Snake River Plain, which evidence indicates began as a continental rift about 12 million years ago.  The earth's crust began to be pulled apart, northeast to southwest, stretched thin like taffy.  The Owyhee Mountains and the Boise Front rose along faults bordering the rift, reaching their present heights between 9 and 11 million years ago.

The rift valley became a basin for Lake Idaho.

Some 200 miles long and 35 miles wide, Lake Idaho drained south, into what is now Nevada.  Thousands of feet of sediment were deposited on the Lake's bottom over its 6.5 million years of existence, interspersed with layers of basalt and volcanic ash, or tuffs, from adjacent volcanoes.

Geologists think that between 2 and 4 million years ago, water from melting glaciers caused Lake Idaho to overflow to the west in a massive flood that gouged Hell's Canyon, along the present-day Idaho-Oregon border.

Sediments left behind became the Chalk Hills and Glenns Ferry Formations.
They form the deep sandy slopes and ridges that plants like Mulford's milkvetch need to grow and which are found in only in three areas of Idaho: the Owyhee Front, the Boise Foothills and the area around Weiser.

This dry expanse was once the floor of Lake Idaho.

 


topo map showing the extent of Lake Idaho in prehistoric times


« The view today from the southern shore of Lake Idaho


Oolite limestone formations in the Flat


The Shoofly   Oolite 

 

 

 

The tiny round pieces of limestone found on the Flat are called ooids, known as oolite ("egg stone") when deposited in larger formations.
The oolite on the Mud Flat, known as the Shoofly Oolite, is part of the Glenns Ferry Formation and one of the world's largest freshwater oolites.

The Shoofly's physical and chemical properties are the foundation for a unique set of plants and fossils.

a graphic depicting the formation of oolite in Lake Idaho

  Ooids form when calcium carbonate precipitates around individual grains of sand.

Wave action in Lake Idaho that varied with the seasons, the weather and the type of sediment in the water at the time washed the ooids back and forth in the shallows, depositing them in thicknesses of 2 to 40 feet on the steeper benches near the shores.  The Shoofly Oolite is unique because it formed in fresh water.  Sea water is more likely to have the necessary wave action, and saltier lakes the proper chemistry.

Beach sands of varying thickness underlie the oolite.  Siltstone forms the stratum immediately above it.

After Lake Idaho drained, erosion carried away softer sediments but left the more resistant oolite to weather above the mud flats.  Small deposits are exposed here and there across the 40 miles that separate the flats and the town of Murphy.  In some places, the exposed oolite has been sculpted into hummocks, small arches or other intriguing shapes, forming a natural sculpture garden.