The remains of the old convent of San Martín del Casuar are in a spectacular location near the narrowest and most inaccessible stretch of the river gorge. The apse preserves interesting details. The trail returns to the main path and continues through the valley following the milestones of the PR-SG 6 Senda del Río trail. The PR-SG 7 goes directly towards Valdevacas de Montejo. The path follows the bank of the river near the water, it is a fantastic forest tunnel of oaks with the poplar trees lit up in yellow and orange while the beautiful days of autumn last. The path runs through the magnificent oak forest and at the mouth of the Valtejo stream appears the imposing Madrid-Burgos railway bridge, 275 meters long and 60 meters high. The line has been closed since 2011, when part of the Somosierra tunnel collapsed and the railway was closed.