Monahans Sandhills fulgurites, Texas

Monahans Sandhills State Park covers 3,840 acres of dunes in Ward and Winkler counties on the southern Llano Estacado of West Texas, some of them up to 70 feet high. The park opened in 1957 on ranch land the state leases from the Sealy-Smith Foundation, and it sits in the Permian Basin, with oil still produced in and around it.

The dunes belong to a sand sea about 70 miles long and 20 wide, running from Crane County to Andrews County and hemmed in on the east by the Llano Estacado escarpment, which stops the wind from carrying the sand any farther. The sand probably spread widest in the Late Pleistocene, about 16,000 years ago, after an earlier bout of dune building around 25,000 years ago. It is almost pure quartz in small, even grains, and the dunes take many forms, coppice, barchan and parabolic among them, as the wind swings from southeast in summer to northwest in winter and southwest in spring. The shapes shift, yet the dunes mostly hold their place from year to year. Groves of Havard oak, which often grow only 4 feet tall but send long roots down to groundwater, have stabilized many of them.

The same deep quartz sand keeps a record of lightning. Rangers at the park noticed strange lumps rising out of the sand and wondered whether they were fossils, rocks or the roots of an ancient tree. They turned out to be glass tubes called fulgurites. When lightning hits a dune, the bolt runs down through the sand until its energy is exhausted, and the sand along its path melts into glass. Texas Parks and Wildlife puts most strikes at up to 50,000 degrees Fahrenheit, well above the temperature needed to melt sand. Any strike that reaches the ground could make one, but finding one is rare, and here the depth of the dunes lets a strike travel a few inches before it is spent. Ground near home is denser, and lightning there tends to hit taller buildings and trees first.

The park asks anyone who finds a fulgurite in the dunes to study and enjoy it, then leave it in the sand. Sand discs can be rented at headquarters for sledding the slopes, there are no marked trails, and visitors are free to explore at will.