Data collected and processed by J.B. Sharpe, I.S. Seten, R.J. Huizinga, B.C. Rivers, J.L. High, G. Lasher, I.M. Halterman, R. Hammer-Lester, R. Pucket, and M.T. Whitehead of the U.S. Geological Survey, in cooperation with the Cook County Bureau of Technology.