This is a report of our geological surveys in the Nachola-Samburu Hills areas，northern Kenya，in 1998-1999．Unfortunately Dr．Nakayama，the first author of this paper，died in a car accident in Kenya in 2001．We worked as members of the Japan-Kenya project of expedition Middle Miocene hominoids fossils for human evolution，which was financially supported by the Ministry of Education，Culture，Sports，Science and Technology．
One of the purposes of our project was to determine the geological background and environments in which these hominoids lived．In 1998-1999，geological survey and sample collection was carried out as follows ： regional mapping by Y.Sawada，sedimentological survey by K.Nakayama and T.Saneyoshi，and sample collection for magnetostratigraphy by Y.Sawada and C.Kashine．Sedimentological survey and detailed descriptions with columnar sections has been completed in the lowest part of the Middle Miocene Aka Aitheputh Formation，which yielded Nachilapithecus，and the Namurungule Formation of 10-9 Ma，which included Samburupithecus．
Sediments of the Aka Aitheputh and Namunmgule Formations are divided into nine facies，as summarized in Table 1．The following columnar sections are shown in this report；（1）Site SH-22：Samburupithecus was discovered at this site，（2）Site SH-23：Many mammal and birds foot prints occur in the sedimentary rocks at this site，（3）middle reaches of the Nakaporeterado River．