An Aptian rudist limestone unit, up to 200m thick and extending more than 3km, lies 2km to the
south of Bilbao. The stratigraphy and mapping of the limestones in the Bolintxu valley reveal diachronous
lower and upper boundaries grading laterally to sandy and marly facies respectively. A carbonate ramp
model with southeastwards backstepping in three pulses indicates a deepening upward trend. Depositional
dip of the ramp down to the west suggests westward tilting and development of enhanced subsident areas
to the west. The orbitolinids indicate an Early Aptian (Late Bedoulian) age for the nuclear part of the ramp