Letters from George Creed to Colonel Garrick Mallery outlining Creed's progress gathering the petroglyphs
In these letters George Creed explains his classification system, the creation of the positives, and the difficulties of tracing the petroglyphs. There are some discussions concerning the identification of elements within specific classes and petroglyphs.
Reference: George Creed - Petroglyphs Nova Scotia Archives MG 15 Vol. 11