It is worthy of note that there is no elevation 
between the lake plain and the Progo gorge. 
Therefore, this incision can only have started 
after the beginning of the emptying of the lake. 
The Progo river must have been able to (re-) estab 
lish its course at its present position, before the 
formation of this (antecedent) gorge. 
Further downstream, towards the debouchure of the 
K. Elo, the Progo river shows incised meanders, but 
still in a gorge-like setting. The uplift has 
affected at least the sector of the K.Progo from 
the K.Tangsi to the K.Sileng debouchures. As 
pointed out earlier, the ’bulge’ noted by Schmidt 
(1934) starts even further downstream, at the 
Krasak debouchure. 
From this discussion it will be clear that we 
consider the uplift leading to the incision into 
the lake plain, different from and younger than the 
uplift ascribed to slipfaulting of the Merapi 
deposits against the Menoreh Hills. 
As stated in para 5, also the Sileng had estab 
lished its course prior to the uplift. Its incised 
nature is clearest in the eastern part of the plain 
and decreases in the upstream direction. 
This uplift may also be responsible for the south 
ward bends in the lower reaches of the Blongkeng, 
Pabelan and Elo rivers (fig 1). 
These bends actually increase the length of water 
way to the local base level (K.Progo), to which the 
channels’ debouchures are not adjusted either. This 
anomaly is likewise explained by this uplift. 
7. PHASING 
Based on the aforegoing, the following mor 
phogenetic phasing in relation to the ’Borobudur 
Lake’ is arrived at: 
1. Merapi fluviovolcanics reach the andesites of 
the Menoreh Hills. As the deposit grows in 
extent and thickness, this leads at one stage to 
local folding-crumpling of the front of the 
mountainfoot of the Menoreh Hills, which were 
already buried under the Merapi fluviovolcanics. 
This may be considered the result of slipfault 
ing in the Merapi body (van Bemmelen, 
1949). 
2. Blocking of the drainage creates a lake at the 
place of the present Borobudur plain, limited to 
the north, ultimately, by the Sumbing deposits 
of that time. 
3. The lake has had various phases of expansion and 
retraction, and may at times have been quite 
extensive. 
4. The lake must have existed- at least south of 
the Borobudur- for a considerable period: 
borings near the Borobudur show more than 10 
metres of lake deposits ( Purbohadiwidjojo and 
Sukardi, 1966 ). 
5. Local faulting trending northeast followed at 
the place of the p esent sector of the K.Tangsi 
running northeastward. Relative uplift in the 
south-western block caused the straight incision 
of the Kali Progo. This was preceded by a reduc 
tion of the lake whereby K.Progo - thus far 
flowing into and out of the lake- could re 
establish its course. 
6. The late-lake phase, really a stage in its 
draining, gives rise to the wide drainageways 
shown as LD in figs 4. In places, two different 
levels can be discerned, slightly differing in 
height. The lowest is then indexed -1, the 
other, higher one, -2. These drainageways are 
very characteristic for the present Borobudur 
plain, and their pattern resembles that of 
flatfloored gullies in a lowland coastal plain 
as e.g. found in northern West-Java. 
7. The uplift noted in 5) took place in three major 
phases, causing incised meanders of K.Sileng and 
K.Progo upstream of their confluence: at least 
two major terrace levels can be distinguished, 
marking places of relative rest in the uplift. 
The uplift also caused the southward bends in 
the courses of the K. Pabelan and K. Blongkeng, 
from southwest to due south. 
8. The events as outlined above have led to the 
present morphological appearance of the area. 
8. DISCUSSION 
Van Bemmelen (1949) postulated that the Gendol 
Hills (fig.l) must have slid down from the Merapi 
crater area in a catastrophic eruption which he 
places in 1006 AD. He considered that the slip- 
faulting in the Merapi body might be of the same 
age. 
From what was said in para 7, it will be evident 
that we consider the whole train of events leading 
to the origin and later dwindling of the ’Borobudur 
Lake’ as much older. The whole sequence must, 
however, be of Quaternary, and probably late- 
Quaternary age, as it started only when the Merapi 
deposits had extended as far as the foot of the 
Menoreh Hills. 
Assuming a linear relationship between the height 
of the Merapi and the areal spread of its deposits 
in time (which may be debatable), the Merapi would 
have had a height of about 2100 metres a.s.l at the 
time when its deposits reached the foot of the 
Menoreh Hills (i.e. the position of the Gendol 
Hills), as against its present height of 2911 
metres. 
If a further assumption is made for parallel 
growth, in time, of all sections of cone, midslope 
and footslope of the volcano, then the total volume 
of the Merapi body, at that time, would have been 
roughly half its present volume. 
Accepting van Bemmelen’s postulation that the 
further growth in mass and weight has ultimately 
led to slipfaulting and to folding of the front of 
the Menoreh Hills (now buried) with associated 
uplift or upwarping, then this event has to be 
placed well into the second half of the lifespan of 
the Merapi, i.e. the second half of the Quaternary. 
The question of whether the Gendol hills have slid 
from the summit area in the catastrophe of 1006 AD, 
or much before that time, does not take a central 
position in the context of the present paper. 
However, events as reconstructed here point to a 
much older date for this event also. 
The draining of the lake, in its turn, was in 
itiated and/or followed by a phase of uplift with 
incision and terrace formation. In other words: a 
considerable length of time has elapsed after the 
draining of the lake, before the present morphology 
came into shape. 
These conclusions do not preclude the possibility 
that the political and socio-economic fabric of the 
ancient Central Javanese Mataram kingdom was 
weakened sufficiently by volcanic activity, com-