In places, these are interbedded with volcanic rocks such as tuff and there is also the odd limestone outcrop, which can contain ancient fossils. Soils in these areas tend to be red podzolic soils, grading to yellow podzolic soils on the lower slopes, and then through to yellow solodic soils on lower footslopes and in drainage lines. Many of these lower footslope soils are considered to present a high salinity hazard. Significant parts of the Shire have terrain developed on acid igneous rocks such as porphyry and dacite. These are the better quality lands in the district.
Soils are generally more fertile and less susceptible to erosion.
It is in these parts of the Shire that higher value enterprises such as vineyards have been located. Along with the neighbouring Gunning Shire, lands in parts of Yass Shire are amongst the more severely affected by dryland salinity in NSW, with obvious deleterious impacts on water quality. This is particularly the case in areas of the Yass River Valley underlain by Ordovician metasediments. The Shire was cleared of much of the original perennial native vegetation in the late 1900 s, with the arrival of pastoralists, and much of the mapped soil degradation, such as sheet and gully erosion, was initiated at that time. An ongoing consequence has been increasing groundwater levels with the subsequent development of dryland salinity.