Geomorphologic features

Lagoni di Mercurago

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Morphology of the Park

The morphologic situation of this area is quite simple and it is characterised by two main elements. The first one is the presence of two series of hills lined in a NW-SE direction. They are made of morainic materials and they are part of the great morainic system of Lago Maggiore.
The second feature is represented by the presence, between these two circles, of a series of flat and depressed areas derived from fluvio-glacial debris. Humid zone and peat-bogs, which are located in this zone, have great importance thanks to the presence of water and marsh vegetation and fauna.

Notes about the glacial theory

The morainic system formation was caused by the interaction of different depositional and erosive events, during thousands of years. These events correspond to glacial and interglacial periods.

Every glacial period is divided in an expansion phase (anaglacial) and in a reduction one (cataglacial). The first phase is caused by relative uniform average seasonal temperatures; a decrease of average annual temperatures and an increase of precipitations. On the other hand, the cataglacial phase is due to great seasonal ranges, to an average annual temperature higher than normal and to precipitations concentrated only in short periods of the year. Between two successive glacial periods there is an interglacial one.
The glacier reduction phase is not regular and continuous, but it is characterised by breaks and limited expansion moments (pulsations) generally called "stages".

Usually a glacier can be considered like a watercourse where water moves much slower in the form of ice. All the materials transported by the glacier are placed along its edges in debris stratum which give origin to moraines. Landscape shapes deriving from this debris accumulation are called morainic circles and cordons.

Morainic deposits, as a consequence of the particular transport they undergo and of their deposition methods, are typically not stratified and not sorted, so they appear as chaotic mass of blocks and pebbles immersed in a silty matrix. Visitors can find all these features in a section of the abandoned quarry in the South-West slope of the Park.

On the contrary, fluvio-glacial deposits are more or less stratified and sorted: both these features are better defined from upstream to downstream. Fusion water, which transports further the solid material, continuously rielaborates the morainic debris giving origin to depositional shapes called "fluvio-glacial".

Glaciers leave morainic and fluvio-glacial deposits both in the expansion phase (anaglacial) and in the reduction one (cataglacial). Very often rapid renewals and sudden increase of solid load in the reduction phase morphologically turn into one or more marginal moraines which reproduce the terminal position shape of the glacial tongue at that moment.

Fluvio-glacial deposits usually give origin to flat and sub-horizontal areas both inside and outside the moraine. In the Park we find only the first.

Often we can find different circles flanked, divided by depressions which imitate the shape of the circles beside them. In the most depressed points water stagnates and creates small intermorainic lakes which, in Mercurago, have lasted until the post-glacial period, thanks to human activity that, as we will see, favoured their preservation.

So, in the area where the ice tongue meandered, inside the circle, we can find this deposition section (from bottom up): morainic deposits, fluvio-glacial deposits, glacial-lacustrine deposits and finally fluvio-lacustrine deposits.

Even though we still are not sure of their number, duration, size and geographical diffusion, we can affirm that the great part of researchers now recognizes five glaciations in the Quaternary period: Donau, Gunz, Mindel, Riss and Wurm. Their names derive from water courses in the North Slope of the Alps. These glaciations are included in the first period of the Quaternary: the Pleistocene, which, lasting about two million years, represents almost the whole Era. The second period of the Quaternary, the Holocene, starts at the end of the last glaciation (Wurm), about 10 thousand years ago.

Effects of glaciations on the "Lagoni di Marcurago"

In the morainic amphitheatre in the "Verbano" (NOVARESE, 19..; NANGERONI, 1954) morainic deposits belonging to the Donau and Gunz glaciations do not influence the landscape morphology. It is also difficult to reconstruct the state of the Mindelian circles. On the other hand, the cordons and circles morphology, with a general amphitheatre shape, are evident in the superficial moraines of the latest glaciations (Riss e Wurm).

In this area distinction between Rissian and Wurmian glaciations is made through different criteria, e.g. stratigraphical, alteration, morphological and idrological (usually in the external Rissian moraines the idrography is centrifugal, whereas in the internal Wurmian ones it is centripetal).

The most internal circles are considered Wurmian because they probably had originated during the reduction phases immediately after the greatest Wurmian expansion (here sandy and clay-sandy material is more abundant than in the Rissian moraines). These circles are not always continuous and even where they are continuous they are fringed by planimetric protrusions and indentations (centripetal idrography).

Thanks to bibliographical notes we can claim that in the "Verbano" morainic amphitheatre there are six Wurmian circles: only the fourth and the fifth are included in the Natural Park of the "Lagoni di Mercurago". The fourth circle starts in Oleggio Castello and it is composed by a series of lined hills from Motto Lagone to Costa del Pinino. The fifth one is composed by a series of humps from Oleggio Castello to Dormelletto. Between these two circles there are, as we have already mentioned, fluvio-glacial and lacustrine deposits contemporary with the internal morainic circle (the fifth).

translated by Katherine Marangio

This page has been updated on August 20th 2004
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