LIDAR REVEALS THOUSANDS OF 18TH AND 19TH CENTURY CHARCOAL HEARTHS ON SOUTH MOUNTAIN, SOUTH-CENTRAL PENNSYLVANIA

Pennsylvania’s statewide PAMAP LiDAR dataset allows many new analyses of the land surface of the state. We report on the presence of over 3,000 charcoal hearths in an approximately 40 km stretch of South Mountain between its northeastern terminus near Dillsburg, PA and Caledonia Gap east of Chambersburg, PA. From 1716 to 1840 charcoal was the primary fuel for the iron industry which was essential to early development and expansion of European settlement. The furnace stack at Pine Grove Furnace consumed the charcoal equivalent of one acre of hardwood forest per day, seven days a week, most of every year at full production. Charcoal hearths were constructed by leveling a circular plot of ground ~15 m in diameter. A pyramid of logs was erected, covered with leaves and earth and burned for several days to produce charcoal. The level circles remain and show prominently on LiDAR-derived shaded slope images.

We had earlier identified hearths in the vicinity of Pine Grove Furnace and demonstrated the presence of charcoal in soil in LiDAR–identified flat circles. This study expanded the study area to include now-forested slopes of South Mountain where several iron furnaces operated from the mid-1700s to late 1800s. Hearths were identified and marked using ArcGIS. We contoured the density of the hearths, and in general they are most abundant near five known furnaces. The presence of so many hearths in what is now forested area attests to the removal of nearly every tree on the mountain in the late-18^th and 19^th century, probably through several cycles of regrowth. Charcoal and slag from the iron furnace at Pine Grove Furnace are abundant in legacy sediments trapped behind a dam along Mountain Creek 10 km downstream. It is likely that the preservation of the hearths is enhanced by lack of development because much of the study area is State Forest or State Park land.