The Iberian peridotites recovered from Holes 897D and 899B preserve a magnetic memory of the middle Cretaceous geomagnetic field and provide a rare opportunity for dating the tectonic processes that accompanied continental breakup and opening of the North Atlantic. The fresher peridotites have Cretaceous (shallow) inclinations and a reverse polarity and are therefore most likely pre-Cretaceous Long Normal Superchron. The altered zone still has these shallow inclinations but is predominantly normal. Because other work shows that alteration was early, the normal inclination is probably of Cretaceous Long Normal Superchron age. Magnetic polarity patterns identified at these two sites appear to suggest that the emplacement of these peridotites took place sometime during the middle Cretaceous (probably at M0 time, 118 Ma), and subsequent alteration of the upper part of these peridotites probably occurred during the Cretaceous Long Normal Superchron (most likely between 117 and 84 Ma) by seawater during the opening of the North Atlantic. These peridotites were probably brought up by a combination process of mantle upwelling and lithospheric stretching and reached the last stage of their evolution about 110 Ma ago.