Line drawing of old soil layers
between basalt flows in Queensland,
Australia (from Willmott and Stevens 1988).
Trap terrain developed on dipping basalt lava flows on Whinny Hill
Detail of dip slope and low scarps developed on lava flows
Trap terrain developed on the lava flows of the Long Row, Arthur's Seat
Trap terrain
Definition: the stepped topography characteristic of hillslopes developed in basalt lava flows
"Trap" is an old term for basalt lava and related sills. "Trap terrain" is a now little-used term for the distinctive landscapes developed on horizontal or gently tilted lava flows.
Basalt lava flows often form stacked sequences. Each flow may include lava that has cooled at different rates to give variations in jointing, competence or porosity. Sometimes the basalt resembles loose slag from a blast furnace; sometimes the basalt is a hard rock with columnar jointing. These differences, together with contrasts between the lava flows and interbedded sediments are picked out by erosion.
Differential weathering and erosion leaves a characteristic stepped slope profile. Successive basalt steps rise onto flat or tilted tablelands, each one defined by basalt cliffs. Scree may accumulate at the base of the cliff or the step may be covered by till. Essentially, the hard layers in the sequence are acting as cap rocks to form cuestas.
These trap landscapes reach perhaps their finest development in western Scotland on the islands of Skye and Rhum. There are many examples in Edinburgh and East Lothian but these tend to be subdued due to the smoothing action of glacial erosion and a general plastering of till. Perhaps the best examples lie around the flanks of Arthur's Seat, where a mix of stacked basalt lava flows and intrusive sills give small-scale scarp and vale scenery.
