Mortimer's fieldwork and recording compares favourably with his contemporaries. His investigations include the recording of crop marks, and made plaster casts of post holes on site. He made stratigraphic observations, but these often lacked detail, some of his reporting has been shown to contain errors by later investigations. Mortimer made good records of his work, he was aided in "Forty Years' Researches.." by his daughter Agnes Mortimer, an artist, who when she was still a teenager did the sketches of her father's antiquities later published in the book. Mortimer credits Agnes in the preface to his volume: "For the sketches of the specimens figured in this book, and for numerous illustrations used elsewhere, I am solely indebted to my daughter Agnes, who from the time she was thirteen years of age until she was nineteen, devoted many of her leisure hours to the completion of this, which at her age, must have been a tedious and irksome task.”