REMOVE

1. rivers
2. soil
3. rock
4. barriers

WHAT

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

More 

WHERE

1. Bow Island, Alberta, Cana … (1)

WHO

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

WHEN

1. 1938 (1)
2. 1942 (1)
3. circa 1940 (1)