They used a mortar mixture:
hog bristles or excelsior
Mix A (Donald A. Hutsler, "Log Cabin Restoration: Guidelines for the Historical Society," American Association for State and Local History, Technical Leaflet No. 74, "History News," Vol. 29, No. 5 (May 1974.)
Mix B and C are reprinted from "Log Structures: Preservation and Problem-Solving," by Harrison Goodall and Renee Friedman, Nashville, TN: American Association for State and Local History, 1980.