White Sands is in southern New Mexico, 16 mi west of the town of Alamogordo. White Sands, which covers 275 square miles, was established as a National Monument in 1933. It recently became a National Park. The elevation of the park is 4,235 feet.
White Sands is geologically situated between the San Andres Mountains to the west and the Sacramento Mountains, just east of Alamogordo, to the east. [Google Earth photo, 2018].
Aerial view, looking northwest. White Sands is in the distance, below a cloud layer on the horizon and "between" the two
pointed-metal objects on the airplane wing.
White Sands consists of white gypsum sand dunes, the largest of their kind on Earth.
During the Late Permian Period, about 250 million years ago, shallow seas covered the area. Evaporating seas left behind deposits of white gypsum crystals, consisting of hydrous calcium sulfate. Gypsum is very soft: 2 on the Moh's Hardness Scale. Fingernails can scratch gypsum. See my previous post on "Some Varieties of Gypsum" (Sept. 30, 2017).
Subsequent tectonism uplifted the San Andreas and Sacramento Mountains, and, over time, rain dissolved the gypsum deposits, and rivers transported the dissolved material to the nearby Tularosa Basin, which had no outlet the sea. The trapped water evaporated and gypsum was deposited (once again). Over time, weathering and erosion broke down the gypsum crystals into sand-size grains. In the last million years or so, prevailing winds from the southwest transported these crystals and dunes formed. There are transverse, parabolic, and barchan dunes present.
It is quite a wonderful experience to visit White Sands. You can walk barefoot and not get burned by the sand, and you can roll around in the gypsum sands without getting abraded, like you would if the dunes were made of quartz grains (hardness 7).
Photography is a challenge because of the glare. It is like photographing a snow field.
The gastropod genus Maclurites Le Sueur, 1818 is characterized by large flat-spiral shells. It lived in shallow, subtidal, warm-marine waters and was widespread where carbonates were deposited. Locally, it can be abundant and associated with algal fossils.
Maclurites is restricted to Ordovician time, thus it is a guide [or index] fossil for the Ordovician Period. This genus belongs to family Macluritidae, which includes 10 genera.
Maclurites has a type of shell coiling called hyperstrophic, in which the animal is anatomically dextral (its genitalia are on right), but its shell is falsely sinistral, being actually ultradextral. If there is an associated operculum (the lid-like, partial or complete covering of the aperture), the operculum exhibits counter-clockwise coiling.
Most gastropods have dextral (right hand) clockwise coiling of the shell, and any associated operculum exhibits counter-clockwise coiling (see image immediately below). A few gastropods are mirror opposites and have sinistral (left hand) counter-clockwise coiling of the shell, and any associated operculum exhibits clockwise coiling.
In my Nov. 20, 2014 post, entitled "Gastropod operculum," I provided views of the dextrally coiled, gastropod shell Megastrea undosa. In this present post, I provide another view of the exterior side of its operculum (38.7 mm hight), which shows counter-clockwise coiling.
Although hyperstrophy is best determined using soft parts for anatomical study, in the case of extinct gastropods, like Maclurites, molluscan paleontologists have to rely on the coiling direction of its calcareous operculum. As shown below, the operculum of Maclurites is coiled counter-clockwise, thus it cannot be a sinistral gastropod.
The above image is a sketch of the apertural view of a complete specimen of Maclurites logani [86 mm diameter], of Middle Ordovician age from Quebec, Canada. [Fascimile of fig. 105 in The Treatise on Invertebrate Paleontology, Part 1, Mollusca (1964), p. I188].
The following four views are of a specimen of Maclurites sp. [68.4 mm diameter, 21 mm height] from the Lower Ordovician Lebanon Limestone, near Nolensville, Tennessee.
front or apertural view, aperture (poorly preserved) is to the right
back or abapertural view
basal view
dorsal view (the "tip" is missing)
The shell rested on its very flat base, which provided great stability for the shell, thereby resisting being flipped over by waves or currents. The flatness of one side of the shell is inferred to be related to its sessile (unmoving), filter-feeding mode of life.
