The pastoral ideal involves removing the complicated imagination of the urban dweller to the pleasant settings of rural life and an aesthetic of simplicity in the embrace of nature and human relationships.
This is utter escapist fantasy, of course, and has been so since Egyptian scribes wrote poetry in this style, assuming the narrative voices of unsophisticated, charming people to speak of romance and leisure in small villages along the Nile. The shepherd in the fields represents the archetype of such a life: therefore the intimate association with the style. "New Age" music often relates to pastoral settings and is meant to aid or induce relaxation, meditation, the alleviation of stress, and so on.
In contrast, the song "Grantchester Meadows" by Roger Waters reflects a more direct perception of such a landscape-- and soundscape-- as do numerous works by Pink Floyd from around the time it was written. It is this directness that distinguishes works reflecting the ambient mode from the generic pastoral style, as listening to a track like "Dunwich Beach, Autumn, 1960" by Brian Eno aptly demonstrates.
Nor is this direct, impressionistic experience of aural atmospheres limited to environments outside the urban; cities and their architectural layouts offer as many interesting and aesthetically appealing settings as fields, mountains and deserts, and imaginary landscapes, though likely more abstract, are equally fitting for treatment in the ambient manner.
The links between a transformational ambient work or sketch-- and even a purely generative one-- to the pastoral are tenuous, but notable. The contrast is not between the complex and the simple, or the urban and the rural, but between the perceptually mundane and the elevated. And there is no shepherd, unless it is the composer who facilitates the journey.
To the extent that the Black Mountain School albums reflect such considerations and ideas, they are successful in concept and design for listening.