Value Stepping for Depth

Das Geroldsauer Tal bei Baden-Baden
by Johann Wilhelm Schirmer (1807–1863) 

This painting by Johann Wilhelm Schirmer uses value stepping to achieve depth and atmosphere.

Detail of Schirmer

Here's a detail of the same painting. In the foreground there's a full range of values used to model the foliage. The leaves and branches are painted individually, with considerable variation of value.  

In the middle distance, all foliage is divided into masses of light and shadow, with the shadow rising to the mid-range. The warm greens in the light side are grayed down as the distance increases. Shadows get cooler as you go back, and detail in the shadow is greatly reduced. 

In the far distance the values step back even further. Light and dark values become very close. In the last range of hills, they merge into a single tone just a shade darker than the sky color.

When painting landscapes in oil, it helps to mix batches of each of these value steps on the palette and make sure they progress evenly.
-----
There's also a double gradation going on in the sky. More on Sky Gradations on a previous GurneyJourney post
and more of this kind of stuff in my book Color and Light: A Guide for the Realist Painter