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Bentonite in Central Montana Bentonite beds are found at many places in Central Montana. Bentonite is weathered volcanic ash. The dominent clay mineral in bentonite is called montmorillonite. This clay mineral gives bentonite its remarkable properties.
Vaughn Member of the Blackleaf Formation
Over geological time older ash deposits alter to clay. The plagioclase feldspar alters to the clay mineral, montmorillonite. Ash deposits that have turned into clay form layers of bentonite. Bentonite at the Ulm Pishkun State Park
Settling Rate of Clay Minerals
Additional Information Geological Features in the Geyser Photo (top photo on this webpage). The Arrow Creek Bed in the photo is also found just below the top of Belt Butte. However, on Belt Butte, it has been "cooked" by an igneous intrusion--a sill--to a porcellainite. Square Butte and Round Butte, shown in the photo, are two prominent igneous intrusions--laccoliths--on the east end of the Highwood Mountains. The Highwood Mountains formed as a stack of lava flows about 50 million years ago--one flow on top of another, on top of another. The stack of lava flows here was at least 2 times higher than the top of Square Butte seen in the photo. The lava flows have been removed by 50 million years of erosion, leaving the laccoliths as prominent topographic features. Bench No. 2 is the erosion/deposition surface that forms the gravel-capped Arrow Bench. This was the floor of the Judith Basin, probably in earliest Pleistocene time (about 1 million years ago). Montana has been undergoing slow uplift for the last 50 million years. Erosion tries to reduce the topography to a vast plain, but hard rocks are resistant to erosion and form topographic highs--such as the Highwood Mountains, the Square Butte laccolith, and the Arrow Bench. The benches form gigantic "stair steps" in Central Montana. Geological Features in the Floweree Photo (second photo on this webpage). The steep slopes that rise up from the Missouri River have 3 different geological units. The lower quarter of the slope consists of the white bentonite-rich beds in the Vaughn Member of the Blackleaf Formation. The grass-covered slopes of gray shale that form the middle half of the slopes is the Bootlegger Member of the Blackleaf Formation. This gray shale was deposited as mud in a shallow, salt-water sea about 95 million years ago. The tan or buff-colored slopes that form the top quarter of the hillside consist of glacial till deposited by continental glaciers that came down from Canada. These deposits are likely about 140,000 years old, as that is the age of the last major glaciation that covered this area. The flat surface at the top of the slope is a former position of the floor of the Missouri River Valley. It has a thin veneer of bentonite-rich clay deposited from Glacial Lake Great Falls. The Highwood Mountains are on the skyline. These mountains consist of a large stack of lava flows. Most of the lava flows are phonolite; but there are some latite flows. Geological Features in the Map of Muddy Creek by Vaughn (third image on this webpage). The Great Falls North 30' x 60' Quadrangle is available as Open File Report MBMG 459 from the Montana Bureau of Mines and Geology. A paper copy of the map can be purchased for $15 or the map can be downloaded from the Internet in Acrobat pdf format. The Montana Bureau of Mines and Geology is a major source of quality information about the geology of Montana. The Taft Member, the Vaughn Member, and the Bootlegger Member of the Blackleaf Formation are the bedrock units shown on this map. The yellow area labeled "QTatg" is the gravel-capped Bench No. 1 or the Fairfield Bench. This is an erosional/depositional surface that was the part of a vast plain during the Miocene (about 5 million years ago). With continued uplift, erosion works to try to reduce the land to the level of the Sun River. However, the gravel cap greatly slows the erosion, creating a large, flat-topped bench as an erosional remnant.Volcanic Ash (fourth image on this webpage). The U.S. Geological Survey has a Fact Sheet about volcanic ash from Mt. St. Helens: Volcanic Ash Fall: A "hard rain" of abrasive particles A glossary of rock names and terms used for volcanic eruptions can be found on the University of North Dakota Volcanology website. See their description of a recent eruption on a Pacific Island. Another discussion of volcanic and igneous rock names can be seen on Stephen Dutch's webpage.
Here is a link that describes the kind of
minerals that
occur in volcanic rocks. Clay Minerals A discussion of the various clay minerals can be found in the Wikipedia On-Line Encyclopedia. Clay particles suspended in water forms what is technically known as a "slurry". The clay particles have the shape of tiny flat disks or platelets. There are dispersed slurries in which the clay particles have similar electrical charges and repeal one another. Dispersed slurries take a long time to settle. In flocculated slurries the charges on edges of the clay platelets are different from the charges on the faces of the clay platelets. The edge of one platelet is attracted to the face of its neighbor. The clay particles tend to "clump together" or flocculate and settle quickly to the bottom. Adding common salt to the water in the settling bottle experiment changes the electrical charges and causes the clay to flocculate and settle quickly. Adding sodium hexametaphosphate, (NaPO3)6 --usually called calgon--to the water inhibits flocculation and keeps the slurry as a dispersed slurry. More details on analyzing clay slurries in the lab can be found on a University of Maryland webpage.
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The Arrow Creek Bed is a thick bed of bentonite in the Bootlegger Member of the 
The Vaughn Member of the Blackleaf Formation contains many bentonite beds. The photo on the right shows white bentonite beds in the Vaughn Member along the Missouri River near Floweree. Bentonite from this locality has a remarkably slow rate of settling in water. 
The type locality for the Vaughn Member--the
place where these beds were first described by a geologist--is near the town of
Vaughn, northwest of Great Falls, Montana. The map to the left is a small
portion of the Great Falls North Geological Map. The squares are
sections--1 mile by 1 mile. The map shows the various
geological formations, each with its own color. The geology is plotted on a topographic
base map. The white band
that goes from the upper left to the lower right in the map is Muddy Creek.
Muddy Creek flows through the town of Vaughn in the lower right and then enters
the Sun River in the lowermost right-hand corner of the map. The pale green pattern
labeled "Kbv" is the Vaughn Member of the Blackleaf Formation. It contains
a lot of bentonite. The map shows large areas (several square miles) of bentonite-bearing Vaughn Member
(Kbv) in the Muddy Creek
drainage upstream from Vaughn. The bentonite is washed into the creek and
is a primary reason why the creek has its name. If one uses the program
"Google Earth Plus-Version 4.0" on the Internet, scrolls to Vaughn at 47° 33'
33" North and 111° 32' 47" West, and then looks to the north of town and to the
southwest of town, then one can see the prominent white bands of bentonite beds
in the area. 
Dark-colored
volcanic rocks, such as basalt or phonolite, freeze at temperatures around 1000°
Celsius or 1800° Fahrenheit. Light-colored volcanic rocks, such as
rhyolite, trachyte or pumice, freeze at much lower temperatures in the range of
600° C. or 1000° F. The dark-colored volcanic magmas have low viscosity,
do not tend to hold dissolved gases. They form lava flows that can flow for long
distances. The light-colored volcanic magmas have high viscosity and tend
to hold the dissolved gases until the pressure builds up so high that there are
explosive eruptions. The large, explosive eruption of Mt. St.
Helens in 1980 carried dacitic pumice ash more than 20 km into the atmosphere.
The ash consisted of shards of volcanic glass, tiny crystals of quartz,
plagioclase feldspar, and minor amounts of several dark minerals. Winds carried the ash
more than 1500 km to the east, covering the land below with a blanket of fine
ash.
The photo on the left
shows a vertical surface on a thick bed of bentonite at the Ulm Pishkun State Park.
It has the "popcorn" appearance that is characteristic of bentonite.
Montmorillonite, the main clay mineral in bentonite, expands up to 5 times its
dry volume when it is wet. Conversely when it dries out, it shrinks.
The "popcorn" appearance of the surface is caused by the large shrinkage when
the bentonite dries.
The
gray horizontal bed at the top of the photo on the right is a mosaic of small
pieces of shale separated from one another by shrinkage cracks--characteristic
of bentonite. When there is heavy rain, water flows out of the hole
in the outcrop. Notice the slightly greenish tint to the bentonite where
the hole is.
Close-up
of the photo above showing the prominent shrinkage cracks. Bentonite has a
number of unusual properties. It has very low permeability. It
absorbs water slowly. However, once it is wet, it can take a long time to
dry out. In the hot summer sun of Central Montana bentonite can form a
hard, tough layer on the surface (with shrinkage cracks), but underneath still
be soft and "goopy". If the "goopy" stuff on the inside can find a way to
the outside, it can flow out, creating a tunnel. Small tunnels or "pipes"
through a bentonite bed are features frequently found in bentonite outcrops.
The feature is called "piping" as if the bentonite bed had a series of
underground "pipes" installed. About 30 or 40 feet upslope from the photo
is a hole in the bentonite that serves as the intake for this pipe. When
there is a hard rain, water flows in the the upper hole and comes out of the
hole shown on the left.
To
the left is a close-up of the second bottle from the left in the photo above.
It shows a thick brown layer of silt with a thin layer of white kaolinite clay
on top. If there are two or three kinds of clay minerals present, then the
different settling times for each clay mineral creates distinct layers that can
be seen in the bottom of the bottle. If montmorillonite is present, it is
always the top layer.