The rocks sampled include ?Permian, Upper Triassic, Lower Cretaceous, and Lower Tertiary, with rocks at 2 sites being of uncertain age, probably Mesozoic. A total of 137 orientated samples was collected at 25 sites from 13 localities in N and central Sumatra.
2 Lower Tertiary sites (Geunteut & Breueh Island) yield magnetic directions of D=8, I = l l and D=34, 1=3 respectively, equivalent to apparent palaeomagnetic poles at 82°N 183°E, and 56°N 190°E. An Upper Triassic site gives D=220, I=26, equivalent to a pole at 49°N 210°E, assuming, as seems likely, that the field was reversed.
These results indicate a probable northward drift of 12° since the late Triassic, accompanied by a clockwise rotation of 40°.
The results from remaining localities are less reliable because of uncertainties of age, low stability, or small number of samples, but all results except one confirm clockwise rotation, and low palaeolatitudes (within 26° of present latitude) since the Permian.
The clockwise rotation of Sumatra contrasts with anti-clockwise rotation of W Borneo, the Malay Peninsula, and SW Sulawesi indicated by palaeomagnetic evidence, and suggests that Sumatra, or at least the parts sampled, was not coupled to 'Sundaland' until the mid-Tertiary.