Approximately 201 to 66 million years ago, during the Jurassic and Cretaceous periods, Greece was continuously submerged under the sea. The Tethys Ocean was a vast ocean that existed between two lithospheric plates (crustal fragments). On the seabed of this ocean, sediments accumulated, which today form limestone rocks. The Pindus Ocean was a part of the Tethys Ocean, extending to its east, in the area where the Pindus mountain range is located today. Later, during the Paleogene to Oligocene periods (66 to 23 million years ago), even though Greece remained beneath the sea, the Tethys Ocean began to retreat due to the collision of lithospheric plates. This led to the uplift of the Greek landmass and the formation of the first mountains. Material from these early mountains was transported by rivers into marine basins (Tethys and Pindus), forming new sediments over the older ones, which today make up the flysch. As the plate convergence continued, rocks from both marine basins shifted and were displaced into adjacent positions within the UNESCO Global Geopark of Vikos–Aoos.