arkose - medium-grained poorly sorted arkosic sandstone from the Lower Eocene Wasatch Formation, Sweetwater County, Wyoming - hand/display specimen

Wasatch Formation main body  Sweetwater County, Wyoming

Lower Eocene 

An arkosic sandstone is immature, meaning that the grains are not only quartz, but feldspar, granitic rock fragments and often mica. As sediment is transported, the feldspar and micas weather to form clay, with the more resistant quartz becoming the dominant remaining constituent in a pure sandstone.

A sandstone is generally described as an arkose when feldspar makes up 25% or more of the sediment grains, though many geologists call any sandstone where feldspar is present an arkose. The brightly reflecting grains in this arkose are mica. The clasts (sediment grains) are are poorly sorted and not well rounded. Small dust grains form a matrix filling the space between the larger grains, and the orange color is typical of many arkoses.

A fluvial basin-edge depositional environment of this arkose, and weathering of iron bearing minerals to hematite, account for the crossbedding and orange color of the Wasatch at this locality. The field photo shows the crossbedded nature of the outcrop on Highway 191 south of Rock Springs, Wyoming, where this was collected.

Select a specimen: When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it, a blue or black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.

Making multiple purchases? Click on the "combine shipping" button in the shopping cart. We'll send an invoice with combined shipping. A link in that invoice will bring you back to checkout, no hassle.

Shipping: By Priority Mail or USPS Ground Advantage, whichever is cheaper, unless we find a better rate. Click > here < for shipping rates. See comment at Note! Use back button to return to this page.