Geographical and geological introduction
The Mountain of Montserrat, which geographically is part of the Pre-Coastal mountain range, is a unique relief for its originality. Its highest point is the summit of St. Jeroni, of 1236 metres. The massif has a WNW-ESE direction and extends along approximately 8 km long and 5 Km wide.
Broadly speaking, its morphology is quite asymmetric. In the northern sector, where the villages of Marganell and Monistrol are located, it forms a very vertical wall while in the south where populations are Collbató and Bruc, the descent to the plain is softer and there are less important reliefs.
It is noteworthy that the Mountain of Montserrat is a unique relief because of its originality, characterized by rounded needles isolated by channels that confer this characteristic look to the mountain that gave him the name (Mount-sawn).
Montserrat is composed of materials of the Cenozoic or Tertiary Era, ie, rocks between 65 and 35 million years old.
The formation of the relief of Montserrat
To understand how the relief that we observe nowadays was formed, we must go back in time to some 45 million years (Eocene), when the mass of conglomerates, marls and clays we see in the mountain of Montserrat was an accumulation of gravel sands and clays that were deposited in a delta at the end of a set of rivers.
These rivers were born in the Catalano-Balearic Massif, which was located where the Mediterranean Sea is currently ahead of the Catalan coast (see Fig 1), and involved an inland sea that occupied the present Depression of the Ebro river. The rivers carried material from the massive erosion that lay along its route and deposited when flowed into the inland sea. These materials were mainly dragged boulders and gravel as the rivers had a lot of energy because of the marked drop it saved. When they reached the sea the rivers lost strength and transporting materials were deposited to form a delta.
Subsequently to the formation of this delta, during the Oligocene, about 25 million years, there was a collapse of the Catalano-Balearic massif and the Coastal and Prelitoral Range were formed as well as the Prelitoral Depression. This new structure resulted in the lifting of the relief of conglomerates, marls and clays of Montserrat at the same time they developed, for relaxation, a series of vertical joints striking mainly in two perpendicular directions (see Fig 2 a and b).
Since then over millions of years, rainwater used to infiltrate these fractures inside the mountain. The water, when combined with atmospheric CO2 and soil with acids is converted into an acid solution capable of dissolving carbonate cement conglomerate and causing the widening of these diaclases and forming potholes and caves inside. While the joints widens, the needles were getting isolated and rounded (see Fig 2 c and d). The genesis of the needles of Montserrat was due therefore to the existence of fractures or joints striking and dissolution and erosion processes that were later developed.
Components of clusters of Montserrat
Montserrat Mountain is mainly formed by clusters though in a smaller proportion levels of marl and red clay are also found. Conglomerates are sedimentary rocks composed of detrital components in three sizes:
The clasts are fragments of minerals or rocks measuring more than 2 millimeters in the case of Montserrat which were eroded and transported from the Catalano-Balearic massif and sedimented in the river mouths in the inland sea.
The clasts may be more or less rounded depending on the transport they have suffered until being sedimented. Depending on the degree of rounding we can speak of conglomerates (case of Montserrat) or gaps in the event that the clasts are angular. If the clasts are larger than 25 inches we call them bolques. The predominant size clasts in the southern sector of Montserrat is between 10 and 15 centimeters although in some mountain areas clasts up to 1 meter can be found.
The matrix is a smaller fraction, measuring less than 2 mm that fills the spaces between the clasts. The matrix which forms the clusters of Montserrat is sandy, as its size corresponds to the sand.
Finally a chemical cement, which occupies the empty spaces that remain between the clasts and matrix, is what gives consistency to the rock. The cement is formed by chemical precipitation of minerals that are dissolved in the water circulating through the clasts and matrix after its deposition.
For the valid "cache found" log:
1) you have to upload a photo of you with GPS close to the cache coordinates
2) and send me via e-mail answers to these three questions:
- Is it easy to find fossils in Montserrat?
- Can you see vertical or horizontal fractures from here?
- Which medium size are the clasts in coordinates N 41° 35.636 E 1° 50.474?
Waiting for my reply before doing the log.