A research paper on the Fex slabs written by Andrea Kuhn in the 1970s provides evidence of mining activities in the "gianda da las plattas" area near the Sils Alp in the Fex Valley shortly after the middle of the 19th century. People from Sils interested in quarrying stone slabs concluded a concession agreement with the municipality of Sils on 17 April 1864. However, stone slabs had probably been quarried there much earlier. In the contract it was pointed out that the right of the inhabitants of Sils to quarry slabs for their buildings in the Fex Valley was not to be diminished by the concession contract. An old Engadine house, the former post office in Sils-Maria, which bore the date 1659 and unfortunately had to make way for today's post office square, was covered with beautiful Fexer slabs on the roof. These can be admired today in the hotel/restaurant Chesa Grischa in Sils-Baselgia. The importance of the Fexer tiles was based on the fact that they could be split very thin, up to about 15 mm thick. Therefore, because of the weight saving, they were the ideal means of covering roofs. The stone - a gneiss (mica slate containing minerals) - can only be split in winter when frozen. So some men from nearby Val Malenco and Sils had a modest but welcome income in winter. Today, the Fexer slabs are highly sought after, especially because of their beautiful colours. Museum only open on Tuesdays with guided tour of the quarry, which takes place from 13.00 - 14.30.