Beginning in 1870 to 1875, the Navajo weavers began to have access to commercially spun and dyed wool yarns. The earliest were often three ply yarns, which is one method of dating older weavings. After 1875 the yarns were mostly 4 ply and provided by the early traders to some of their better weavers. The majority of these commercial wool yarns were produced at the mills at Germantown, Pennsylvania. They were sent out by train to these early trading posts on the reservation. This introduction of colorful yarns, never before seen by the Navajos, caused a dramatic shift in Navajo weaving- and we see an explosion of colorful, eye-dazzling designs during this period.