100
its size and the body of water which accumulated there. It is
a vast cirque whose edges are steep, although accessible,
and keep on going under the water with the same slope value.
It seems, however, that this slope does not continue for a long
time, and, according to some soundings which were made,
we ensured that the bottom of the lake is almost fl at, as one
of a saucer would be despite the steep slope of its edges.
From the surface to the bottom of the water, the depth
reaches 280 feet; the diameter of the lake is seventeen times
as large; the water escapes by a notch cut in one of the edges,
and its level must drop continuously as it wears out the dam
which retains it. Around the lake on the edge of the water, we
easily see the springs feeding it and stemming from a lava
fl ow which seems hanging more than forty feet over the
water level. The nature of the rock and the scoriae which
accompany it lead us to assume that it is very likely a modern
lava fl ow, especially as the volcanic puy of Montchalme rises
immediately above and dominates Pavin, and presents
another very well preserved crater. The lava is accompanied
with an accumulation of pozzolana; it exhibits indications
for jointed structure and crops out in several sites around the
lake. In every site where we can observe the deposit which
supports this lava, we see that it is formed by trachytic tuffs
of pumiceous deposits, which contain fragments of trachyte,
and we often distinguish this rock through the crystal clear
water of Pavin, which seems laminated and in which, in
appearance, the crater is opened. ” ( Lecoq 1835 , p 80–82).
His detailed work prevented him from missing what
escaped to the previous authors: the ejecta associated with
the eruption of the lake Pavin. However he did not recognize
juvenile lava clasts as he considered them as formed only by
fragments of the basement:
“Après un examen approfondi, on ne peut donc refuser à Pavin
le nom de cratère qu’il mérite sous tous les rapports; mais
comme le lac de Servière, c’est un cratère d’explosion. Il a été
établi au milieu de ces dépôts trachytiques et ponceux qui cou-
vrent tous les environs du Mont-Dore.
Jusqu’ici on n’avait trouvé aucun fait géologique qui puisse
indiquer l’âge de ce cratère, et on le considérait comme antéri-
eur aux volcans modernes. En examinant le sol des environs, et
surtout la déchirure par laquelle s’échappent les eaux, on voit
que le terrain est formé de débris de trachyte, de fragmens de
lave, de sables ponceux, et qu’il présente enfi n toutes les appar-
ences d’un sol où se trouvent amoncelés les débris de plusieurs
couches superposées. Ce sol, en effet, n’est autre chose que celui
qui a été chassé du cratère lors de l’explosion qui l’a produit, et
qui est retombé tout autour de la bouche. / After a thorough
examination, we cannot thus refuse to Pavin the name of crater
which it deserves in every respect; but like the lake of Servière, it
is a crater of explosion. It was established in the middle of these
trachytic and pumiceous deposits which cover all the neighbor-
hood of the Mont-Dore volcano.
So far we had found no geological fact which can indicate the
age of this crater, and we considered that it preceded the modern
volcanoes. By examining the neighbouring ground, and espe-
cially the notch through which waters escape, we see that the
deposit is formed by fragments of trachyte, lava, and pumiceous
sand, and that it presents fi nally all the appearances of a soilwhere the fragments of several superimposed layers were accu-
mulated. This deposit, indeed, is not different from what was
driven out by the crater during the explosion which produced it,
and which fell back around the mouth.” (Lecoq 1835 , p 84–85).Putting his observations in perspective, he was therefore
able to propose the fi rst and accurate chronostratigraphy of
the Montchal-Pavin system:
“Si l’on suit, en remontant au delà de Besse, le cours de la lave
[du Montchal], elle disparaît avant qu’on ne soit à Pavin, sous
les débris dont nous avons déjà parlé, et sous la pelouse qui
recouvre tous les environs du lac. Cette superposition de frag-
mens et de tufs trachytiques au-dessus d’une lave moderne,
paraîtrait inexplicable, si elle ne donnait elle-même l’époque
relative de l’explosion du cratère. Ce n’est qu’après
l’épanchement de la coulée de Montchalme , qu’a eu lieu la for-
mation de Pavin. / If we follow up, by going back up beyond
Besse, the course of the lava [of Montchal], it disappears before
we reach Pavin under the fragments we spoke about already, and
under the meadows which cover all the surroundings of the lake.
This overlapping of fragments and trachytic tuffs overlying a
modern lava would seem inexplicable, if it did not provide the
relative time of the explosion of the crater. Hence, it is after the
effusion of the lava fl ow of Montchalme that the formation of
Pavin took place.” (Lecoq 1835 , p 83–84).Lecoq copied almost word for word these descriptions
and conclusions in a summary work (Lecoq 1867 ). At this
time, Alphonse Julien ( 1869 ) highlighted in his thesis the
action of the Quaternary glaciers in the area of Sancy. From
inquestionable observations made on the surfi cial deposits,
he generalized and proposed a glacial (moraine) origin for all
the trachytic agglomerates, in particular those of the valley
of the Couze Pavin. Just like the volcano Tartaret which, by
blocking the valley of the Couze Chambon dammed the lake
of the same name, Julien imagined that Montchal blocked a
glaciated valley dug out in an “ erratic ground ” and thus
formed the lake Pavin, while he attributed its fl at bottom due
the erosion of a glacier:
“ Leur profondeur [lac Pavin, Montcineyre, Bourdouze] est tout
juste égale à celle du terrain erratique dans lequel ils sont creu-
sés; ainsi Pavin, qui d’après des mesures récentes à [sic] 90 m de
profondeur. Si l’on supprimait par la pensée ce vaste manteau
erratique, ces lacs disparaîtraient avec lui. Il serait intéressant
d’explorer le fond du lac Pavin car des sondages ont démontré
que ce fond était parfaitement uni. Il ne peut avoir été dressé de
cette façon qu’à l’époque où le puissant glacier qui couvrait le
plateau exerçait son action sur le sous-sol. / Their depth [lake
Pavin, Montcineyre, Bourdouze] is just equal to that of the erratic
ground in which they were excavated; hence Pavin is 90 m [sic]
deep according to recent measurements. If we discard this vast
erratic soil cover, these lakes would disappear with it. It would be
interesting to explore the bottom of the lake Pavin because sound-
ings demonstrated that this bottom was perfectly uniform. It may
have been shaped this way only when the powerful glacier which
covered the plateau exerted its action on the basement. ”He observed this erratic formation in the walls of the out-
let and supposed that this formation kept going all the way
down to the bottom of the lake.P. Boivin and S. Jouhannel