The St-Omer area is a rapidly subsiding basin in which long sequences of Weichselian and Holocene sediments are preserved. A core 21 m long, extracted from near St-Momelin about ten km north of St-Omer, has been analysed for insect fossils. Four Faunal Units (SMi-1 to SMi-4) are described based on changes in both coleopteran and trichopteran assemblages. The basal Faunal Unit (SMi-1) includes many cold-adapted species and is attributed to the Younger Dryas chronozone. The transition to Holocene sedimentation was abrupt. Faunal units SMi-2 to SMi-4 are attributable to the Holocene. They lack all the cold-adapted species found in the basal sediments and in their place are insect assemblages very similar to the present day fauna in this region. This sequence spans the period from the Pre-Boreal to the Sub-Boreal. Faunal Unit SMi-2 includes insects from the Pre-Boreal, the Boreal and most of the Atlantic periods. This fauna was fairly sparse and made up largely of species living in freshwater habitats. The Faunal Unit SMi-3 includes the insect assemblages from the Late-Atlantic to Sub-Boreal periods. This fauna was much more diverse and indicated a river meandering across its floodplain and bordered by a mature forest at a time when the climate was warmer than that of the present day. This chronostratigraphical sequence of events is supported by 14C dates and by lithological data. Thermal climatic conditions have been quantified using the Mutual Climatic Range method. During the Younger Dryas period (SMi-1), Tmax (the mean July temperature) was in the region of 10 °C and Tmin (the mean temperature of January/February) was close to − 11/− 12 °C. After the sudden climatic amelioration at the start of the Holocene (SMi-2 to SMi-4) Tmax probably fluctuated throughout the early Holocene between 16 °C to 19 °C and Tmin between 0 °C to 5 °C; figures that are close to those of the present day. This climatic history is compared with others in northern Europe.