Soils: Source-Stonybrook watershed Guide p. 10
The soils of Mercer county have developed much more recently, over a few million years mostly from underlying rocks. So they form a gradient from the rocky hills and valleys of the northwest to the relatively sandy plains and marshlands of the southeast.  Both diabase and shale tend to weather in two ways, by cracking into large fragments and by sloughing off a very fine powder that can accumulate into a silt or clay.  Except for the ridge tops and swamps, Mercer county soils have been quite fertile. This is particularly the case along route 1 corridor, where there is the best balanced mixture of water-holding  clay with aerating sand.  However, the quality of all Mercer County soils is precarious.  If over farming or careless landscaping displaces this organic matter, the soils readily revert to stone.