416 
the sea; it runs just to the south of the Hoorn-Islands and of 
Pajang-island, and may be traced thence over a distance of 70 km. 
in north-northeastern direction with an approximately uniform depth 
of 30 fathoms. 
Data are wanting as yet to determine the further course of this 
stream and its branches. 
e. Traces of revived erosion in pleistocene time. 
Among the large rivers of Borneo there are some which possess 
terraces there where the low land passes into the upland. These 
rivers have cut themselves a bed into gravel formerly deposited by 
themselves. This must have taken place at a time when the erosive 
power of the rivers was stronger than at present, for now they 
have filled up their beds again, for the greater part, with finer 
deposits, sand and silt. These ancient gravel terraces have been 
observed by me at the Kapuwas near Sintang, a little above the 
confluence of this river with the Melawi and at the Katingan along 
its right bank, at and somewhat downstream from the place where 
it receives the Samba’). As late as the year 1894 gold was washed 
near Sintang from the gravel of these terraces. I feel inclined to 
think that the gravel of these terraces has been deposited in the 
late pliocene time and even in the beginning of the Pleistocene, 
when in Borneo denudation was not nearly so far advanced as it 
is at the present day. The origin of the terraces may readily be 
accounted for if we assume that during the glacial period the base- 
level of denudation was lowered about 75 m; this caused the fall 
of the rivers to become greater and the erosive power to be increased, 
and enabled the rivers to cut deep gullies into their own gravel 
deposits, which later on became broad valleys during the alternate 
periods of increased and decreased erosion corresponding to the 
successive glacial and interglacial periods. 
At present the base-level of denudation is about as high as it was 
at the commencement of the Pleistocene just before the ice-age, but the 
island of Borneo having been meanwhile much denuded and eroded and 
thus having attained a stage of mature erosion, the rivers can only carry 
sand and silt at those places, where formerly gravel was deposited. The 
broad pleistocene valleys cut into the gravel terraces, consequently are 
now gradually filled up with sand and silt. Precisely such old 
gravel-terraces are found in the middle- and the upper-course of 
1) G.A. F. MOLENGRAAFF. Geological explorations in Central Borneo p. 17—20 
and p. 388, Leiden 1902. Geol. Verkenningstochten in Centraal Borneo p. 19—21 
and p. 409—410, Leiden 1900. 
