“The distinction [between natural and supernatural revelation] was made rather early in history, however, but was not intended as a designation of a two-fold origin of revelation. It was clearly understood that all revelation of God is supernatural in origin, since it comes from God. It served rather to discriminate between two different modes of revelation. Natural revelation is communicated through the media of natural phenomena, while supernatural revelation implies a divine intervention in the natural course of events; it is supernatural not only in origin, but also in mode.” (Berkhof, Introduction to Systematic Theology, 126)
Can be purchased (as part of his Systematic Theology) here.