MICHIGAN CITY | The National Park Service has extended the closure of Indiana Dunes National Lakeshore’s Mount Baldy section indefinitely while park employees investigate whether more underground depressions could collapse and endanger visitors to this popular recreation area.
A cavity halfway up the sand dune, which towers 123 feet over the nearby Lake Michigan shoreline, swallowed Nathan Woessner, 6, of Sterling, Ill., on Friday afternoon. Rescuers eventually dug him out from a hole 11-feet deep.
The accident has launched an investigation and debate into whether forces of geology and weather caused this cavity or it was man-made.
Bruce Rowe, public information officer of the park service, said Monday they are looking into the possibility the hole was created by a long-buried tree that had decomposed and left a void in the sand.
The park service is bringing in ground-sensing equipment to look under the dune's surface "to see if voids or other potential hazards can be identified."
Rowe said there was no previously known occurrence of such hidden cavities, but park employees strictly will enforce the closure of the dune, its parking lot, trails and beachfront until the public's safety can be assured.
Professor Zoran Kilibarda, of the Indiana University Northwest's Geosciences Department, said he has studied the dune over the years and, "Sinkholes do not happen in these dunes, especially not on the lakeward side."
He said Mount Baldy began forming about 4,000 years ago from sand transported by lake currents and windblown inland. The same wind pushes the dune several feet each year, according to the park service. It has engulfed trees in its path over the centuries.
But Kilibarda doubts any of the buried trees decomposed and left voids in the dunes because tree stumps regularly uncovered by wind erosion have been preserved in the sand and remain relatively solid.
"Gravity and the north wind compact it. The sand is only loose maybe a foot or so below the surface," Kilibarda said
Roger D. Nanney, a retired district conservationist for the U.S. Department of Agriculture and liaison to the Great Lakes National Program, said he believes wind erosion, rain and decomposed trees could create and camouflage a small blowout.