Walk 1: Volcano

View over Lough Nafooey and the Finny River from the start of walk 1

Walk 1: Volcano

Knockkilbride and the Finny River area are renowned as the site of an ancient volcano (490 million years old), formed as the Iapetus ocean closed to bring the two halves of Ireland together (see Continents Adrift). The actual landform of the volcano is long gone but some of its rocks are preserved. Eruptions at the "Finny Volcano" were of two types: runny basalt magma formed pillow lavas, stacks of interlocking lava balls, when blobs of lava were chilled by eruption into water: sticky andesite magma broke up to form breccia deposits of rock fragments. Quiet periods between eruptions allowed very fine sediment to settle and form red and green chert.

Park in the lay-by on the narrow road at Cummer, between Finny and Maumtrasna pass [GR 004 597], from where a beautiful view takes in Lough Nafooey. Walk downhill along the road. At the first bend to the right note the stream gully, which marks the trace of a major fault in the rocks that truncates the volcanic rocks and juxtaposes them against younger conglomerates. Continue downhill to the sharp zigzag bend just before the junction with the Finny road. The low cliff on the left is made of pillow lava. The most obvious rounded shapes of whole pillows (up to 50cm across) are on the right end of the cliff. Look for small holes in the rock: these were formed when gas bubbles in the molten lava were trapped as it cooled and solidified. Note how they are concentrated in rings concentric with the edges of pillows. Continue to the road junction and cross onto the track. The first outcrop on the left is of angular fragments (not rounded like the pillows) of the "sticky lava" held together by fine sediment. The large fragments (up to 30cm across) are obvious, but look closely and the edges of many smaller ones can also be distinguished. This outcrop also has a streak of red cherty sediment running through it.
When the Finny River is low, it can be crossed using the stepping stones at the end of the track. Several outcrops on the other side show further examples of the fragmentary "sticky lava" and layered red and green cherts. For the adventurous, the very best pillow lavas occur at the top of Bencorragh, where exposure to weathering has etched them (and from where there are beautiful views over Derry Bay and Lough Kilbride).