Brief outline of known documentation for Matthias in America:
Helen Black's letter would put Matthias' family on the South Fork (South Branch of the Potomac, West Virginia) shortly after 1738.
1742, Matthias and others appear on a petition to Orange County (later Augusta County, WV) (South Branch area) for a road.
1744, Matthias and others appointed overseers for the new wagon road to the South Branch along west side of Cacapon River.
1747-1749, Moravian missionaries visit Matthias Jochem family at their home on South Branch.
22 Nov 1750.Felty (Valentine Yockham) noted as first settler on Greenbrier.
24 April 1751, Mathias Yockham, purchased 330 acres along Muddy Creek in present Greenbrier Co. (Thomas Walker journal, Virginia History Magazine)
9 Aug 1755, survey of land for Mathias Yoakum, 267 acres on north side of Roanoke River
1755-1756, Matthias on tax rolls, "lower end of Augusta County" (VA)
1760, Mathias "M" Youcam, "now of Bedford County" power of attorney to William Thompson to convey 267 acres on Roanoke River.
1763, "Matthias Yoakum was living in Bedford County, Va on the James River" (History of Bath County, Va.)
Multiple records of Matthias is Botetourt/Bedford County from 1769 to death in 1780.
Several children remained in (West) Virginia (Hampshire/Greenbrier Counties) when Matthias went to Virginia, including George?, Valentine, Philip Paul, Jacob, John, Conrad.Apparently going with Matthias: Michael, Matthias Jr., John, Henry, George (the youngest).
Documentation can be found in the article "Yoakums of Virginia" by Dr. Peter Stebbins Craig.