REMOVE

1. horizon line
2. sky
3. threshing machines
4. farming

WHAT

1. Visual Works (hierarchy n … (1)
2. black-and-white film (1)
3. farmers (1)
4. farms (1)

More 

WHERE

1. Bow Island, Alberta, Cana … (1)

WHO

1. Clyfford Still; Still, Cl … (1)
2. Still, Clyfford (1)

WHEN

1. 1920 (1)
2. 1940 (1)
3. between 1920 - 1940 (1)