From reading old records in the cellars of churches and libraries in New England, I believe part of the progress of the writing of ð (eth) to y and then to "th"

From reading old records in the cellars of churches and libraries in New England, I believe part of the progress of the writing of ð (eth) to y and then to "th" involved writing the y as a superscript at the beginning of words. Any truth to this? Or was it just an oddball habit of New England scribes?