The dates don't add up for Bertha. Her parents were married in 1875 so possibly the stone was misread and incorrectly recorded in Warren County Genealogical Society, Cemetery and Death Records of Warren County, Iowa, (Walsworth Publishing Company, Marceline, MO, 1980). "Gray, Bertha C., d Oct 20, 1870, age 6m 21d, dau of T.& E." Both parents would have been about fifteen years old.
Possibly the stone should have read 1876 rather than 1870??