15 Sutton Knoll

Grid reference [TM 304 440]

Rockhall Wood caps the low hill known as Sutton Knoll. Four old Crag quarries cluster around its flanks, providing geologists with a window into the late Pliocene Suffolk of around 2 to 3 million years ago. Information panels have been erected by the GeoSuffolk group beside the footpath which passes the site.

Sutton Knoll has been researched since the 1830s when the pioneering geologist Charles Lyell visited, and it is now a geological SSSI. This is a key site for understanding the Coralline Crag. Quarries show a succession of beds, from shelly, silty sands up into cross-bedded, sandy limestone. Fossil pollen from the upper unit reveals information about local forests, about 3.4 million years ago. Our familiar pine, spruce, oak and elm trees were present, but also others typical of warmer conditions in the late Caenozoic era, including hemlock, umbrella pine, sweet gum and wingnut. GeoSuffolk has created a small arboretum of species from Pliocene genera on the site.

The Coralline Crag is an isolated outcrop surrounded by Red Crag: it formed an island in the sea in Red Crag times, some 2.55 million years ago. Shelly Red Crag sands and gravels are banked up against the remains of a degraded cliff of Coralline Crag resting on London Clay. Fossil shells have been found in their original life positions among the boulders at the foot of the cliff. Mussels and barnacles were attached to rocks, with clams, whelks and winkles living between them, while piddocks bored into the soft clay at the cliff base. Thus Sutton Knoll offers a unique opportunity to study aspects of marine ecology around a Pliocene island.

Figure

(Figure 42) A Coralline Crag exposure, showing sand martin holes in sandy limestones of the Sudbourne Member. © Roger Dixon; artist Louis Wood.

(Figure 43) ‘Suttona Antiquior’ — an artist’s impression of the «Pliocene Island’ at Sutton Knoll.