Water divide. The well-known series of peaks - the border with Guipúzcoa - from Aratz to the Arlabán pass (617 m), is the least complicated part. From this point, the geographical relief undergoes some changes. From Arlabán it heads sharply north through the Jarindo and Albertia mountains to the crags of Anboto and Urkiola. Through the rugged ravines formed by the mountainous buttresses of Bizkaia, it heads towards the summit of Gorbea. Further west, the slopes of Altube, the Guibijo and Salvada mountains definitively close the dividing wall the Cantabrian and Mediterranean slopes.