This wonderfully dark chocolate coloured yarn is from a mixed flock which includes five pure and cross Ryland Zwartbles.  It is soft but there's also a body to it which makes it great to knit with.  The pure Zwartbles have wool is medium to fine with excellent crimp and fibre length, a Bradford count of 54-56 and a micron count of 27.  The Ryland mix doesn't detract from this.
 
The history of these sheep begins around the beginning of the 20th century with a particular type of sheep called the Schoonebecker. They were popular with farmers in northern Holland around that time for their natural abilities to fertilize pasture.   As commercial fertilizers became more readily available, their value was in producing cheeses and milk.
They are tall, quite leggy sheep who have some  goat-like traits (they don't like getting wet as well as being good milkers).  In the 1980's they went into rapid decline as commercial farming practices declined but have been saved largely because their fleeces are nice to hand spin.