ventifact.com

Ventifacts, unlike artifacts, which are fashioned by humans, are shaped by the wind. The term ventifact is derived from the latin "ventus" meaning wind. Ventifacts can be found in sizes that range from centimeter-sized pebbles to huge boulders many meters across. Sometimes the term ventifact is used to describe a wind-abraided rock exposure still attached to bedrock but most geologists consider ventifacts clasts such as pebbles and cobbles. Ventifacts are usually in the form of small rocks which have become naturally "sandblasted" by wind-borne sand and sometimes finer sediments such as silt and clay. In arctic climates ice particles are also involved in the shaping of the rock. As the particles collide with the rock, the rock is smoothed and flattened on the windward side. Ventifacts are defined by their smoothed, flattened or faceted surface(s) and sharp ridges or edges. A ventifact may have one or more scoured facets. Classic faceted types are described as einkanter, zweikanter, and dreikanter for one-, two-, and three-ridged forms sometimes seen in fine-grained rocks. Three-ridged dreikanters appear somewhat pyramidal. Some of the very large specimens can include pits, flutes or grooves on the surface.

While it is not completely understood how ventifacts form multiple facets, it is believed that either the rock moves (perhaps as wind removes sediment from underneath) and another side of the rock is positioned toward the prevailing wind or else the rock remains in place and the wind changes direction. It is thought that at least some ventifacts have been created in environments where the prevailing winds blow in one direction during the summer and another during the winter. Ventifacts have been seen on the surface of Mars, in the dry deserts of Antarctica and in desert regions of the Amercan Southwest. We do know that the processes that interact to form ventifacts are very complex. They are formed in environments that have large amounts of fine particles (usually sand, silt and ice) which act as the abraisive, low precipitation, sparse vegetation and strong winds that blow in the same direction for a very long time. The terraine must offer ample exposure to the wind. Local topography such as a gently sloping windward upgrade has been shown to enhance the abraision of the rock by wind-blown sand. Even the spatial distribution of the rocks can strongly influence the formation of each individual ventifact according to researchers studying ventifact formation in the Mojave Valley of California.

Over the years Ken Ettlinger and his son, Zak collected hundreds of Pleistocene age ventifacts from their farm in Long Island, New York. The specimens varied from boulder to pebble sizes and were sold and donated to museums and universities around the world for collections and teaching specimens. Their farm in Flanders, NY is located midway between two glacial features, the Ronkonkoma Moraine and the Harbor Hill Moraine. The rocks are unearthed in the normal process of tilling the land. Professor Ettlinger discovered the ventifacts on the farm in the early 1980's shortly after purchasing the land. He later found that the ventifacts were derived from a thin layer of pebbles and cobbles about three to four feet beneath the surface of his farm.

Wind carrying suspended particles like sand can polish and smooth a wide variety of rock types but Long Island Pleistocene ventifacts are mostly formed in fine quartzite and occasionally coarse quartzite, granitic rock and quartz pebbles. The origin of these Pleistocene ventifacts began when the last continental glacier which covered much of North America about 60,000 years ago reached Long Island. The ice sheet transported crushed rock down to Long Island from New England and Canada in the form of boulders, cobbles and pebbles rounded due to their transport to Long Island by the Wisconsin glacier and glacial meltwater streams. Glacial transport was harsh and for the most part and softer rock materials such as limestone and marble did not survive the trip. On Long Island, the surface rock is, for the most part, unconsolidated sediment, mostly rounded quartz pebbles and cobbles and an enormous amount of sand. In the moraines the glacial till includes considerable sand and quartz pebbles as well as silt, clay and boulders. After transport to Long Island and deposition as stratified drift or glacial till some of these rocks were faceted by the intense winds of the Pleistocene age to produce ventifacts.

Professor Ettlinger believes that the ventifact layer on his farm represents a buried surface formed during the last ice age when Long Island was a cold desert. Because wind moves very light sand particles, leaving behind larger pebbles and cobbles, a "desert pavement" was formed paved by pebbles and cobbles exhumed as wind swept away the fine sand around them. The pleistocene ventifacts of Long Island are relict to times of a greater loose sand source and probably, persistent, higher velocity winds. According to Ettlinger, the larger rocks remained as fine sand was swept away by strong persistent winds. "It's easy to imagine that the rocks exposed on this flat glaciated desert surface were probably partly imbedded in permafrost since ventifacts today seem to form on hard surfaces. Without vegetation and precipitation, the winds picked up sand and perhaps ice particles and buffeted the exposed portion of the rocks relentlessly," explained Ettlinger. The Pleistocene is believed to have had strong winds that changed directions seasonally so that first one side of the rock became smoothed and flattened and then another. Where the two sides met a ridge or sharp edge formed. Pleistocene ventifacts may have one or more of these ridges depending on how many sand faceted surfaces were produced. The desert surface at Ettlinger's farm was later buried under fine sands probably carried by meltwater runoff from the retreating glaciers.

Ken Ettlinger teaches Natural Sciences at Suffolk County Community College in Riverhead, New York. For questions and comments he can be reached at: ettlink@sunysuffolk.edu