The impending innundation of a section of the Susquehanna River by the Holtwood and Safe Harbor hydroelectric dams in south central Pennsylvania in the 1930s precipitated a 1930-1932 field survey of the affected archaeological resources. This survey recorded a fascinating variety of petroglyphs in the Safe Harbor area, many of which are now underwater. This book, first published in 1934 is considered a classic work on these "picture rocks."