The Ivrea Morainic Amphitheatre (sometimes abbreviated as AMI) is a moraine relief of glacial origin located in the Canavese region.The northeastern strip of the AMI, centered on the municipal territories of Sala Biellese and Zubiena, is generally not considered part of Canavese but of Biella. Administratively, it encompasses the metropolitan city of Turin and, more marginally, the province of Biella and the province of Vercelli. It dates back to the Quaternary period and was created by the transport of sediment to the Po Valley that took place during the glaciations by the great glacier that ran through the Dora Baltea valley. With an area of more than 500 km2, it is one of the best-preserved geomorphological units of this type in the world. As an extension, it is surpassed in Italy only by the similar formation surrounding Lake Garda. The name amphitheater, usually given to these geomorphological structures, refers to their characteristic elliptical shape that is noticeable when it is shown as a plan on a map.
Orography Throughout the area concerned, the various glacial pulsations that have produced impressive moraine accumulations over time are clearly evident. Of particular note among these is the left lateral moraine of the ancient glacier, known as the Serra d'Ivrea: this is the largest formation of its kind existing in Europe. The Serra originates on the southern slopes of Mombarone (2371 m a.s.l.) and heads in an almost straight path southeastward for almost 20 km, then fraying into the heights surrounding Lake Viverone. It consists of a series of sub-parallel ridges, the highest of which reaches a maximum elevation difference of 600 meters from the AMI inner plain in the Andrate area. This elevation difference gradually decreases eastward until it reaches about 250 meters near Zimone.
Its right-hand counterpart, less regular in shape, is represented by the elevations located between Bairo and the outlet of the Chiusella stream on the plain. Here, as well, the highest altitudes are reached in the weld zone with the alpine chain (about 800 m a.s.l. near Brosso); between Strambinello and Baldissero Canavese the continuity of the hill chain is then interrupted by the gorge by which the Chiusella veers eastward heading toward the confluence with the Dora Baltea.Carta Tecnica Regionale raster 1:10.000 (vers.3.0) della Regione Piemonte - 2007
The frontal moraine, on the other hand, consists of a succession of hills that extend between Agliè and Viverone and are interrupted between Mazzè and Villareggia by the gap opened by the Dora Baltea. The high point of this moraine sector is Bric Vignadoma (520 m a.s.l.), near Vialfrè.
Within the amphitheater lies the vast flat area, whose elevation is generally between 210 and 270 m above sea level, in which numerous population centers, including the city of Ivrea, are located. The continuity of this plain is interrupted here and there by isolated reliefs and a few minor hilly cordons; one of these defines within it the Small Morainic Amphitheater, centered on the towns of Strambino and Scarmagno. The catchment area of Lake Viverone is also defined not only by the outer moraine circle of the AMI, but also by smaller internal deposits; the municipalities in this area have grouped into the hill community around the lake.
According to the SOIUSA orographic classification, the elevations located on the hydrographic right of the Dora belong to the Graian Alps and, more specifically, to the Rosa dei Banchi alpine group, while the moraines on the hydrographic left are part of the Biellese Alps and therefore also of the Pennine Alps.
Hydrography
The AMI is traversed in a north-south direction by the Dora Baltea, which also collects water from the Chiusella and other minor streams. Some of the water carried by the Dora is collected by the Naviglio di Ivrea and, after providing water for Vercelli's rice-growing industry, is diverted to the Sesia basin. The outer slopes of the hills that make up the AMI are tributaries of the Orco (to the west) and Elvo (and east) basins.
Nestled among the various moraine cordons that make up the amphitheater are numerous lakes whose formation is closely related to the geological history of the AMI. While Lake Viverone is quite large (in terms of surface area it is the third largest body of water in Piedmont) the others are small to medium in size. Just north of Ivrea are located the 5 lakes, the largest of which is Sirio; the right side moraine, on the other hand, hosts Alice Lake and Meugliano Lake, while between the hills that make up the frontal moraine are Candia Lake and the smaller ones of Maglione and Moncrivello.
The largest of these bodies of water belong to the category of moraine-dammed lakes, that is, they were trapped between moraine cords deposited during the various glacial pulsations that affected the area. In other cases, however, the geological origin is referred more directly to glacier action: indeed, Lake Alice and the 5 lakes of Ivrea are considered by geologists to be glacial erosion lakes. LakeSup. body of water (km2)Sup. basin (km2)Altitude (m a.s.l.)OriginLago di Viverone5,7225,7229Moraine-dammed lakeLago di Candia1,357,5227Moraine-dammed lakeLake Sirio0,291,4266Glacial lakeLago di Bertignano0,090,6377Moraine-dammed lakeLake Campagna0,14,1238Glacial lakeLake Nero0,11,3299Glacial lakeLago di Alice0,0961,05575Glacial lakeLago di Moncrivello0,030,3263Moraine-dammed lakeLake San Michele0,070,7239Glacial lakeLake Pistono0,122,8280Glacial lakeLago di Meugliano0,0290,18717Moraine-dammed lakeLago di Maglione0,052,7251Moraine-dammed lake
Geology
Even before the birth of modern geology, some legends widespread in the Canavese area told of the existence in the AMI area of a vast lake (the existence of which is denied by geologists)The existence of a single lake basin extending over the entire area within the AMI, at least as far as the period following the Upper Pleistocene glacial expansion is concerned, is denied by modern geologists and, in particular, by studies published by Francesco Carraro in the 1980s. which Ypa, the mythical queen-priestess who led the Salassi people, is said to have reclaimed by having a tunnel dug near Mazzè so as to discharge its waters outside the circle of hills that acted as an embankment to the south. Traces of this legend may also be found in the chronicle De bello canepiciano, compiled by Pietro Azario in the 14th century, in which the ancient presence of a large lake in the area is reported as certain news. Beginning in the second half of the 19th century, the origin of the AMI was studied by various Piedmontese geologists; the first classical studies can be credited to Luigi Bruno (1877), Carlo Marco (1892, 1893) and the geographer Giovanni De Agostini (1894, 1895). These studies were later deepened culminating in the synthesis works of the 1970s by geologist Francesco Carraro.
The substrate The rocky substrate on which the Ivrea Morainic Amphitheatre stands today belongs to three distinct geological units, separated from each other by the Insubric Line. This major tectonic discontinuity divides in the Biella and Canavese area into two faults with an almost parallel trend: the Inner Canavese Line, more southerly, and the Outer Canavese Line, more northerly. In the AMI zone north of the Outer Canavese Line is the Sesia-Lanzo Zone, composed mainly of mica schists and, in general, rocks that underwent metamorphic processes at depth; it includes Mombarone on the hydrographic left and Mount Gregorio on the opposite side of the Dora River. Between the two Canavese lines is the Canavese Zone, an area geologically characterized by rather heterogeneous lithological types and emerging near Montalto Dora and the 5 lakes. South of the Inner Canavese Line, on the other hand, the bedrock belongs to the Ivrea zone. Among the various types of rocks that make up this geological unit in the AMI area are particularly rare basic granulites, which, according to geological studies, originated from the deepest portions of the continental crust near its boundary with the Earth's mantle. Part of the city of Ivrea was built on this rocky substrate, which surfaces prominently near the sanctuary of Monte Stella.
Glacial phases
According to geologists, in the final phase of the Pliocene, the geological period that preceded the formation of the morainic amphitheater, in the Canavese area the sea that occupied the Po River basin at that time and reached into the interior of the Aosta and Orco valleys was gradually filled in by sediments originating from the erosion of the Alpine range.
The AMI, on the other hand, formed during the Pleistocene when, due to decreasing average temperatures and increased precipitation over the Alpine arc, a considerable mass of ice began to accumulate and was carried downstream by large glaciers. In particular, the valley floor of what is now Aosta Valley was on several occasions totally occupied by the Balteo Glacier, which, with a path similar to that of today's Dora Baltea, exited onto the Canavese plain and then widened in a fan-like pattern, reaching as far as to lap, in the most intense glaciation phases, the present-day settlements of Caluso and Agliè.
The glacial phases of the Pleistocene were traditionally divided by geological literature into Mindel, Riss and Würm. In the past, the designation of the three main moraine circles constituting the AMI was borrowed directly from these sub-periods, the temporal subdivision of which was determined mainly on the basis of studies of the effects of glaciation north of the Alps. Later this classification was no longer considered sufficiently accurate to describe the geological evolution of the basins located south of the Alpine chain, so that the current subdivision of the moraine circles became the one shown in the following table (reworked from the Geological Map of the Serra Morainic Amphitheater): PeriodEarly period (millions of years ago)Moraine circleOld attributionLate Pleistocene0,13Bollengo-Strambino GroupWürmMiddle Pleistocene0,73Serra d'Ivrea GroupRissEarly Pleistocene1,65San Michele - Borgo GroupMindel The deposits left by the oldest of the three main glacial pulsations (San Michele - Borgo group) are the outermost and are best identified on the left side of the AMI, in the Biella area of the Serra. The frontal moraine and much of the right moraine have in fact been covered by the debris left by the later phase, which began about 700,000 years ago. During this period the best preserved of the three moraine circles, the Serra d'Ivrea group, was deposited. In addition to the main body of the Serra this includes a large part of the frontal moraine observable today (the area roughly between Moncrivello and Torre Canavese) and part of the reliefs located at the outlet of the Valchiusella. The hilly reliefs due to this glacial phase are the ones that reach the greatest heights partly because they were to some extent curbed by the presence of the moraines left by the previous pulsation, which resulted in a higher elevation than would have occurred in the case of undisturbed deposition on a flat area. The third group of deposits, referred to as the Bollengo-Strambino (or also Bollengo-Albiano), is the most recent and is located within the previous two; it includes some moraine cordons of lesser elevation development as well as the Serretta, a low hill that breaks away from the main body of the Serra near Bollengo.
The other glacial episodes that have marked Pleistocene history left no traces in the area because their deposits were later covered and/or displaced by the sedimentary masses related to the three main morainic circles.