Gneiss: deformed pebbles

A stone with a mysterious history


These images show a few mm of the thin section of a pebble collected at the base of the hills of Lake Garda.


Desenzano and Peschiera are situated on these hills, made from strips of detritus left behind by the glacier during the Pleistocene era.


Like a conveyor belt, the glacier gathered detritus from one wide part of the Alpine chain that is north of the lake.


Our pebble was transported by the glacier; it is metamorphic rock, which has therefore been modified by high temperatures and pressure.

Name: Gneiss
Type of rock: Metamorphic rock

Quartz, plagioclase, muscovite, biotite, garnet, epidote, apatite, iron oxides. Chlorite, sericite, epidote, titanite as alteration


Fossils: Absent
Location: Lazise (N 45° 30′ 39.3″ E 010° 46′ 25.6″)

Formation (deposit): 

Formation (pebble):

pebbles of the glacial deposits of the Sarca (morainic amphitheater of the Garda)


Age (deposit): 

Age (pebble):

Pleistocene (less than a million years)


How the pebble was formed: 

How the rock was formed:

on the front of a lowland glacier



Did you know that …