Karl Prantl was able to fascinate his fellow sculptor Heinrich Deutsch and the psychologist Friedrich Czagan with the concept to the extent that they wanted to help put it into practice. He also succeeded in recruiting Gustav Hummel, the leaseholder of the St. Margarethen quarry at the time, as a stone sponsor, and consequently calcareous sandstone became the dominant material used by the expanding symposium movement. The young European sculptors who took part in the first symposium in 1959 — and in some cases launched their careers by doing so — were Dino Paolini from Italy, Sepp Wyss and Peter Meister from Switzerland, the Belgian sculptors Eugène Dodeigne, Jacques Moeschal and André Willequet, Hans Verhulst from the Netherlands, Gerson Fehrenbach and Erich Reischke from Germany. They were joined by the Slovenian sculptor Janez Lenassi and Alfred Czerny and Erwin Thorn from Austria. To raise the money needed for board, lodging and travelling expenses, several strategies were tried, including a building-brick campaign (cut sandstone from St. Margarethen) which also served as an artistic manifesto and a means of publicizing the principle behind the symposium.
The first international sculpture symposium began in August 1959 in the St. Margarethen quarry. Fourteen artists from seven countries produced large-scale sculptures in the open air. The project was concluded by a closing exhibition starting on 19 September and an exhibition catalogue.
Besides the lively exchange of artistic visions and techniques, literary and musical recitals were also part of the concept (the ensemble "die Reihe", led by Friedrich Cerha, performed in the quarry).
The success of the first exhibition in the quarry encouraged the organizers to hold another symposium the following year. To facilitate applying for the necessary subsidies, the association "Symposion Europäischer Bildhauer" (European Artists' Symposium) was founded in the autumn of 1959. Karl Prantl became the chairman, Friedrich Czagan the general secretary, Peter Meister the secretary and Jacques Moeschal deputy chairman. In 1960, the second symposium was held with eleven international sculptors.
The artists from the East and the West wrote manifestos in which they emphasized the ethical and political pretensions of their ideas and work, namely to set an example of international understanding through an alliance across national borders. The exchange of experiences with colleagues from all over the world who went on to organize similar gatherings in their home countries following Prantl's example gave rise to an international network of artists dedicated to contextual, abstract art. Karl Prantl opposed the Iron Curtain from the first, for example at the Berlin Wall, where in 1961/62 he and several colleagues placed sculptures on the Platz der Republik to send out a message of humanity in defiance of the Wall that was then being built: "We tried to bring the Wall down."
The concept of the international symposium that originated in St. Margarethen continues to this day, with symposia being held regularly in China, Japan, Taiwan, the USA, Brazil and Europe.
All in all, over 150 sculptures were created from the local calcareous sandstone in St. Margarethen by more than 110 international artists. Taken as a whole, they form a unique cultural total artwork. Today, around 50 pieces remain where they were created, scattered across the hill's southwestern slopes.