This phenomenon is very common in the north of Burgos and Palencia provinces. They constitute a group of several churches. Sometimes it is just a gap in the rock that the hermits used as a place to rest or to pray. This is the example of Reverillas.Sometimes it is a real church with its nave,apses, vaults and altar separated by columns, all of them carved in the rock. They also have one only bench arround, also carved in the stone. This is the example of Santa María de Valverde which has a nave and two aisles and where they still celebrate the Sunday Mass. In order to visit it we have to ask for the key in the nearby tavern (Mesón). It is evident the fact that the aim of these constructions was a hiding place to defend the christians from the attacks of the muslims when these lands were in the border between the christian lands and the muslims themselves. Others say that these churches were used by the new christians who had come from the neighbouring moorish lands and who had not been accepted by the old christians. The authors of these opinions, date them in the 10th century. Others think that these hermitages were made in the 7th century right before the arrival of the Arabians. Another remarkable church is the one of Arroyuelos. A two storey church which has a column , right in the middle of the building to support a 4 metre high vault. Sometimes , as they were used by a community they have some burial places excavated in the rock such is the example of Puente del Valle where there is also an hermitage. It is worth mentioning a fountain which is also excavated in the rocki in La Serna , in Castrillo de Valdelomar.