The starting point is 3.8 km from the center of El Tiemblo, on the road that goes up to Castañar de la Yedra and the El Regajo recreational area. At the crossroads there is a sign for the Senda San Gregorio, marked with the milestones of the GR-294.1 long-distance path and a mountain biking route. There is not much parking space but you can leave the car in the holes in the ditch. The trail begins along the rural road indicated to El Castañar/Ruta Bosques Mágicos. The first section runs between the walls of the farms, passes through a clearing and goes down to the bridge of the Garganta de la Hiedra, near the wall of the Hinchona reservoir. The forest environment is a beautiful gallery of riparian trees. The other large plain of the nature reserve is to the west of the sierra, it rises in the Casillas pass, collects the streams from the Casillas and Escusa peaks and the western slope of Cabeza de la Parra, flowing into the Burgillo reservoir. The first slope of the Senda San Gregorio goes up boxed in a small ravine with roots and stone steps created by the runoff, an entertaining section to warm up on the way to the mountain forests and the high peaks.