The Chalk Cliffs (above), located 25 miles north of Stanford are not chalk, but rather an especially light-colored sandstone. Chalk is a soft type of limestone that forms from the accumulation of microscopic marine organisms, such as foraminifera and coccolithophores, which have shells made of calcium carbonate (calcite). When these organisms die, their shells sink to the seafloor, piling up to form a soft, white ooze. Over millions of years, the layers of ooze are compacted and lithified under pressure, hardening into chalk. The White Cliffs of Dover (England) are a famous outcrop of chalk.
Some beach . . .
In contrast, sandstones are composed of tiny rock fragments, primarily quartz, mixed with fragments of other minerals like feldspar, etc. The grains are typically glued together by natural cements such as silica (quartz), calcite, or iron oxide. The color and characteristics vary greatly, depending on the type of grains and cementing material, with iron oxides often creating red, yellow, or brown hues. Sandstones form in a variety of environments, including beaches, rivers, lakes, and deserts. The Chalk (NOT) Cliffs shown above are made of sand eroded from ancient mountains then deposited along the shore of an ancient sea, whereas many of the iconic sandstones in the soutwestern USA tell of ancient deserts.
The Virgelle sandstone . . .
The cliffs shown above are composed of a colorless quartz sand that was deposited near the shore of the Western Interior Seaway during the late Cretaceous period, which ended 66 million years ago. Cross-bedding suggests the sand was transported by coastal streams draining eastward as the seaway retreated. The cliffs are made of Virgelle sandstone - technically it's the Virgelle member of the Eagle formation. It is named "Virgelle" because the distinct sandstone was first identified near the tiny town of Virgelle, located along the Missouri River near Big Sandy (several miles west of Highway 87). An even more impressive outcrop of the Virgelle can be found along the White Cliffs of the Missouri, from Coal Banks (near Virgelle) to Judith Landing - a great canoe trip! The Virgelle is a "lower" member of the Eagle formaton, which means the Virgelle layers are near the bottom (older, deposited before other layers in the Eagle formation). The Virgelle varies in thickness but generally ranges from around 100 to 125 feet, reaching over 200 feet in certain areas. The Eagle formation outcrops in many places in central and eastern Montana - The Rimrocks of Billings are Eagle sandstone.
Right: Sketch borrowed from Roadside Geology of Montana, showing the location of the Virgelle Sandstone beach adjacent to the Western Interior Seaway during Cretaceous time. - Hyndman and Thomas, 2020
Stratigraphy . . .
In this part Montana the Virgelle and a transitional layer below it (Telegraph Creek Formation) were deposited on top of sea floor mud that became the Marias River Shale. The sediment of the Marias is composed of finer grain sediments deposited when the sea was deeper here (farther from the shore). Another shale, the Claggett, sits on top of the Virgelle, indicating the area went from being beneath deeper water (Marias shale) to being near the shoreline (Virgelle sandstone), and then back to a deeper environment (Clagget shale). The Western Interior Seaway was a shallow sea whose shoreline was everchanging.
Term: stratigraphy
CLICK HERE to see my photo album from a hike onto the Chalk Cliffs (includes several drone photos).
Below: Google Earth view of the Chalk Cliffs - The "X" marks the area I explored.
Below: Here is how the Eagle formation (including the Virgelle member) is described on the MBMG Geology Map of Montana.
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