Th e tufa bridges have weathered in time from nutrient-voracious plants as Burke expected, however, most of the stone cladding is unnaturally black. Mike Angle, a geologist from Ohio familiar with tufa, concurs in the belief that Pittsburgh’s problem with coal dust in the mid-twentieth century resulted in an abnormal absorption. Moss and lichens thoroughly covering the uppermost part of the bridges also result in darkening of the stone. Another plausible theory from Angle is that the stone’s composition results from a chemical change over time. Such as the reddish brown appearance of stones results from iron oxidation, a black appearance may result from manganese oxidation. Minor quantities of manganese are present in most sedimentary rocks in Ohio.