A significant overland flow and erosion event was documented and studied in an agricultural area with postglacial marine sediments (southern Norway). Field observations and UAV-photography revealed that a large volume of water abruptly exfiltrated at the end of the snowmelt season. Numerical groundwater modelling aided in interpreting our observations, as well as comparison with prior study in the same catchment. It was hypothesized that a water storage capacity threshold of the subsurface was exceeded to cause the seepage, in combination with a transition from coarse to fine marine sediments. Furthermore, the spatial distribution of snow cover and snow melt rate, as well as the presence of manmade tile drainage, were found to have significant impact on the seepage event. The recurrence frequency of the event is unknown, warranting further study.