Portaikos Valley is an exceptional geosite full of natural beauty. It is flowed through by the Portaikos River. The Portaikos drains the southwestern part of the Thessalian basin. Its total catchment area is 294 km2.
The main branch of the river starts from the Stournareika of South Pindos, at an altitude of 1,600m. Then its course cuts almost vertically through the mountain range of Koziakas and, after passing Pyli, joins several smaller streams before finally flowing into Pinios. The main bed of the river has a total length of 31.9 km. It is located next to the watershed of its hydrological basin, which has as a consequence that its overall hydrographic network shows a completely asymmetric development around its main branch.
In the ravine crossed by the river and at a distance of about 2km from Pyli, Renz reported for the first time the existence of thin-plate limestones, which bore fossils of the genus Halobia, which places their formation during the Triassic period.