REMOVE

1. railroads
2. freight cars
3. sky
4. utility poles
5. paths
6. buildings (structures)

WHAT

1. Visual Works (hierarchy n … (1)
2. black-and-white film (1)
3. bushes (1)
4. exterior views (1)

More 

WHERE

1. Killam, Alberta, Canada (1)

WHO

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

WHEN

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