Quarry Hill Sandstone Formation
This lithostratigraphic unit succeeds the Tillybrachty Sandstone Formation. It occurs throughout much of the basin and is particularly distinctive in the south west of the area where it outcrops in the disused sandstone quarries on Quarry Hill (see inset below). This formation primarily comprises stacked, well bedded, current rippled, laminated and cross-stratified sandstones rich in feldspar grains. In this particular area these sandstone beds form large-scale channelised deposits commonly exhibiting erosive bases with mud rip-up clasts and are occasionally interbedded with scoured thin shale horizons.
In the Quarry Hill area the sandstones locally yield casts of transported plant debris, some of which are particularly large (see inset below).
Of these plant remains, and perhaps one of the most enigmatic, may tentatively be assigned to the genus Prototaxites (belonging to an extinct group of Palaeozoic plants called the nematophytes). When the area was actively being quarried in the late 19th and early 20th centuries, these plant fossils were occasionally found in the form of logs preserved as sandstone casts. One specimen, recorded by Newlands (1913) as "a plant fragment" and nearly 1.3 m in length, is now housed in the collection of the Department of Geology and Petroleum Geology, University of Aberdeen (see inset below).
The "White Sandstones Unit" encountered in Borehole 97/2, north west of Rhynie village, is a 50m interval of hydrothermally altered white, laminated, rippled and cross-stratified sandstones with minor shale interbeds. It is thought to represent at least part of the formation in this area (see insets below). Locally the sandstones are heavily cemented by calcite. In this borehole its basal contact is with a hydrothermal breccia above the main fault zone and, with increasing shale content, passes upwards into the more mud-rich beds of the Dryden Flags Formation.
Typical features of the White Sandstones Unit in Borehole 97/2:
The overall environment that seems to be represented by this unit is one dominated by shallow fluvial channels of a river system flowing towards the northeast along the axis of the basin. The occasional shale horizons may in part represent poorly preserved overbank and floodplain deposits.
The lateral change from the more stacked, well-bedded sand-rich lithofacies at Quarry Hill to the dominantly thinly bedded sands and minor shales in the area northwest of Rhynie village suggests the presence of larger-scale fluvial channels in the Quarry Hill area.