The writings were only abridged by Moroni because of lost pages and needed information that the Lord Jesus Christ wanted in them. Other than that, the writings were all from prophets and leaders in ancient America from about 600 BC to 421 AD. When you study the "Translation of the Plates" you will find that there were times with the Urim and Thumim, the writings were translated by Joseph Smith. However, as mentioned, there was also information Moroni needed to convey in which Joseph put the stones in a hat and received revelation to write that information down. Can't translate words if the words aren't there. For instance, the first original 116 pages which was the Book or writings of Lehi, were stolen (lost) by Martin Harris's wife, who was against all of it. The Lord told Joseph not to re-translate it. And, there was the Brother of Jared long before Lehi and Nephi in 600 BC and all of those people That Moroni would abridge the information necessary to complete the stick of Joseph in the hands of Ephraim.
So, the short answer is yes. It's in the Book of Mormon.