The clays of the Rolling Downs formation overlie a series of sands and drifts, saturated with water under high pressure, which discharges at the surface as a flowing well, when a borehole pierces the impermeable cover.
Kaolin, fire-clays and brick-clays are common to all the states.
The best soils are in the west section, where limestone clays or shell marls are common.
The oak grows most luxuriantly on deep strong clays, calcareous marl or stiff loam, but will flourish in nearly any deep well-drained soil, excepting peat or loose sand; in marshy or moist places the tree may grow well for a time, but the timber is rarely sound; on hard rocky ground and exposed hillsides.
The Cambrian is represented by blue clays, ungulite sandstones and bituminous slates in Esthonia and St Petersburg.
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