The Collbató caves are located in the pre-coastal mountain
range on the lower southern slopes of the mountain of Montserrat, west of the
river Llobregat. The caves coincide with a progressive unconformity on the north
bank of the Torrent de la Salut.
Geological Interest
The Collbató caves are an excellent example of karst scenery
located in the iconic Montserrat Natural Park with close proximity to the
Barcelona metropolitan area.
Montserrat is one of the Eocene coastal ranges bordering the
southern edge of the Ebro basin, being the product of erosion of the Catalan
Coastal Ranges. These ranges are small units thrust northward over the Ebro
basin, creating progressive unconformities. This deformation occurred during the
Alpine orogeny.
There are a series of outcrops in the Collbató area that
clearly show the development of tectonic structures with simultaneous deposition
of alluvial fans on the edge of the basin.
Historical Significance
The area has been inhabited since at least 4000 BC, as shown by Neolithic
remains found in the caves. In the sixteenth century, the monastery received 12
pounds per year from the lease of the caves for the extraction of sodium nitrate
(saltpetre). There are numerous inscriptions inside the cave with dates ranging
from 1511 until 1780.
Montserrat karst scenery: broadened joints lead to limestone
pillar formations
Entrance to the caves protected from rock falls by steel
mesh.
South slope of Montserrat, with the lower level cut by an
unconformity
Definitions
Unconformity
An unconformity is a contact between two rock units in which
the upper unit is usually much younger than the lower unit. In general, the
older layer was exposed to erosion for an interval of time before deposition of
the younger, but the term is used to describe any break in the sedimentary
geologic record.
An unconformity represents time during which no sediments were preserved in a
region. The local record for that time interval is missing and geologists must
use other clues to discover that part of the geologic history of that area. The
interval of geologic time not represented is called a hiatus.
Conglomerate
A conglomerate is a rock consisting of individual rounded
clasts within a finer-grained matrix that have become cemented together.
Conglomerates are usually deposited by high energy processes
since it takes a strong water current to transport particles large particles.
The environment of deposition might be along a swiftly flowing stream or a beach
with strong waves. A smaller clast or a thinner clast is usually associated with
a beach or shallow marine environment. There must also be a source of
large-sized sediment particles somewhere up current. The rounded shape of the
clasts reveal that they were tumbled by running water or moving waves.
