Per ora i Crinoidi li ho trovati in Valle d’Aosta!

Stefano Rossignoli 5 agosto 2012

Chi pensava che in Valle d’Aosta fosse impossibile trovare fossili?

Una moltitudine di colori!
Una moltitudine di colori!

Io ad esempio, fino a qualche anno fa, prima di studiare che i calcari si formano in mare per opera di organismi…

Sono infatti alcuni organismi invertebrati, uni o pluricellulari, animali e vegetali, a fissare nei loro gusci e scheletri il carbonato di calcio che forma in seguito le rocce calcaree che spesso troviamo in montagna!

Mi ricordo come fosse ieri!

Corricchiavo un po’ di anni fa per la Val di Rhemes e, di ritorno dal Colle della Nivoletta (che non è il Colle del Nivolet!!!), mi imbattei in rocce biancastre e giallognole su cui crescevano le Stelle Alpine.

Sapevo già che le stelle alpine hanno bisogno di un substrato di rocce calcaree per poter crescere spontaneamente.

Mi venne l’idea che le rocce su cui stavo correndo potessero essere calcescisti, ovvero dei calcari sottoposti a una certa pressione, temperatura, nonché a sforzi di taglio laterali (più o meno gli stessi sforzi a cui sottoponiamo la pasta di una torta quando la impastiamo a mano.).

Allora avevo solo una vaga idea delle dinamiche dell’orogenesi Alpina e Prealpina pur essendo quasi laureato in Scienze Naturali. Ora ne ho un’idea almeno un po’ meno vaga…

Mi ricordo ancora l’idea per giunta normale di sperimentare e provare a mettere una goccia di Acido Cloridrico (HCl)  al 5% su un campione che avevo prelevato.

Ero in Laboratorio di PalVert, sotto gli occhi dell’amico e collega Geologo Marco Rusconi, allora dottorando in Paleontologia a Milano, solo che al posto dell’HCl avevo preso l’Acido Acetico (CH3COOH) che è un acido molto debole e mi chiedevo perchè non reagisse…

Marco, giustamente, mi prese in giro e mi ha preso in giro per anni per quell’errore e magari lo farà anche ora!!!

Purtroppo però l’HCl non era disponibile, quindi la prova all’acido non la feci mai più…

Essendo venuto in possesso recentemente di una Carta Geologica della Val di Rhemes, ho potuto constatare che i Calcescisti ci sono eccome (anche se ormai lo avevo dato per scontato). In più, sarebbero state trovate piastre di Crinoidi (nei pressi di Villeneuve).

I Crinoidi sono degli Echinodermi (esistono ancora oggi e vengono chiamati Gigli di Mare), parenti di Ricci di Mare, Stelle Marine, ecc e le piastre che li compongono hanno anch’esse una simmetria pentamera, ovvero sono pentagonali o di forma riconducibile ad un pentagono regolare o ad una stella a cinque punte. A volte sono circolari…e hanno sempre un buchetto nel centro.

Essendo formati da cristalli di Calcite, sono piuttosto resistenti al METAMORFISMO, ovvero a quei processi banalmente descritti prima, quindi si possono trovare i Crinoidi in uno stato di conservazione decente anche in rocce trasformate in quel modo…

E QUINDI?

A CACCIA!!!

La Granta Parei
La Granta Parei

Vado verso i laghetti della Granta e so che passerò attraverso i Calcescisti di Rhemes. Li conosco bene ormai e vedo parecchie cose che potrebbero essere stati Crinoidi anche se non sono certo a Villeneuve, ma le piastre non sono molto riconoscibili, fino a che, come al solito, SUL SENTIERO, calpestabili e calpestati da chiunque decine di volte al giorno, ecco qui…

Una piastra di Crinoide nei Calcescisti Giurassici dell'alta Val di Rhemes
Una piastra di Crinoide nei Calcescisti Giurassici dell’alta Val di Rhemes
Una piastra di Crinoide nei Calcescisti Giurassici dell'alta Val di Rhemes-2
Una piastra di Crinoide nei Calcescisti Giurassici dell’alta Val di Rhemes-2

Pentagonale è pentagonale. Il metamorfismo lo ha un po’ massacrato ma è, o era se preferite, un Crinoide!

A tre ore a piedi o un’oretta e mezza di corsettina blanda da Thumel, Val di Rhemes Aosta!

Si trovano calcescisti a Rhemes perchè, ‘poco’ dopo l’inizio del Giurassico (circa 180 milioni di anni fa), in alcuni bacini dell’Oceano Ligure-Piemontese si depositavano calcari stratificati che poi sono metamorfosati, rimanendo imprigionati, piegati e stritolati durante l’avvicinamento tra Placca Adriatica e Sud dell’Europa e successivamente sono stati spinti verso l’alto in modo piuttosto complicato insieme al resto delle Alpi…

E allora, nonostante la fatica del divulgare qualcosa di Paleontologico in Italia, dovuta allo sciocco modo di proteggere i fossili della gente e della legge Italiana, ora, anche nella valle dei miei sogni, continuerò a calpestare questi affascinanti segni della vita passata e delle dinamiche del nostro pianeta Terra…

La testata della Val di Rhemes (sinistra idrografica)
La testata della Val di Rhemes (sinistra idrografica)
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