In the 1830s, several purchases were made by trappers working out of Fort Hall, of a garment known as a Guernsey Frock- which was a sweater made from oiled wool.  Yes, at least a few western trappers were wearing hand knit wool sweaters in the mountains.

Following is a description of the history of this unusual yet present garment of the far west, and the diagram at left, showing the traditional design.
The Story of the Guernsey Frock (sweaters in the western fur trade)

The knitting industry in the British Channel Island of Guernsey dates back to the early 16th century, when licences were granted to import wool from England. After more than 400 years the traditional Guernsey is still going strong! In those early days, the Island’s exports were stockings and at the peak of this trade it is believed that 10,000 pairs left the Island each week. Both
Mary 1 and Queen Elizabeth (1558 to 1603) owned articles of Guernsey knitwear. Even the unfortunate Mary Queen of Scots wore Guernsey hose at her execution.

The Guernsey sweater came into being as a garment for seamen, who required a warm, hard wearing, yet comfortable item of clothing that would resist the sea spray. So was born the now famous oiled wool Guernsey! The hard twist given to the tightly packed wool fibers in the spinning
process and the tightly knitted stitches, produced a finish that would "turn water" and repel a surprising amount of rain and spray. As one knitter boasts "With a little bit of love and care a Guernsey will practically last forever!" During the era of Sir Walter Ralegh, Governor of the Channel Island of Jersey from 1600 to 1603, a substantial amount of trade was established
between the Channel Islands and the New World, in particular Newfoundland, where the Grand Banks were fished for Cod.

The Guernsey found favour with these intrepid seafarers and, as a result, most coastal communities in the British Isles have a "Gansey" which derives from the original Guernsey . In those days, the styles had the distinctive patterns of each parish to which the wearer belonged, so that it was possible to identify an unlucky sailor by his Guernsey, should he be drowned, Nowadays, the traditional, hand finished Guernsey still retains much of the original design patterns.

The rib at the top of the sleeve is said to represent a sailing ship’s rope ladder in the rigging, the raised seam across the shoulder a rope and the garter stitch panel waves breaking upon the beach. The gussets under the arm and at the neck are for ease of movement, as are the splits at the hem
This distinctive pattern has been handed down from generation to generation and the knitting skills passed from mother to daughter. Many of the expert knitters today come from old Guernsey families.

The traditional Guernseys were knitted by the womenfolk and are hand finished in their cottages to this day. However, in former times, knitting became such an industry and was so popular with the fishing and farming families, men, women and children included, that the harvesting of the "Vraic" began to be neglected. An edict was published forbidding the knitting of Guernseys during this season of collecting and spreading the seaweed (Vraic) on the land! During the time of the Napoleonic Wars, Admiral Lord Nelson recommended that the Guernsey be worn by the Royal Navy and it was at this time that the Guernsey, which until then had been knitted in the un-scoured natural wool, was dyed Navy Blue.
 
Although a common sailor's garment, the Guernsey Frock was carried as an inventory item at Fort Hall in the 1830s, and ledgers note the sale of them to men working out of that establishment.

To locate references to these items in the far west, see Book of Buckskinning VII, Chronister and Landry's chapter on clothing of the trappers, and Book of Buckskinning VIII, Clay Landry's chapter on the Fort Hall Trade Ledgers.