Three types of geological phenomena independently suggest that the Témiscouata–Madawaska Valley was affected by one or more seismic events following its deglaciation:(1) Subbottom acoustic profiling of Lac Témiscouata and Grand lac Squatec revealed disturbance of bottom sediments by mass transport processes in both lakes. Erosional truncation of preexisting, acoustically laminated sediments and accumulation of hummocky, chaotic deposits over older hummocky surfaces or laminated sequences both result from mass transport processes. Unidirectional mass flows from several points in these symmetrical basins, in situ disruption of laminated sediment beneath flat bottoms, and the large area of the lake floors affected suggest strong similarities in sedimentation style with lakes that have been disturbed during strong earthquakes.(2) Southeast of Lac Témiscouata, in Saint-Jacques, New Brunswick, two separate mass flow deposits, made up largely of coarse (> 0.5 m), angular boulders of local bedrock, occur on opposite sides of the Madawaska River valley. These deposits have different source areas and transport directions, but occupy more or less the same stratigraphic position within sediments deposited in glacial Lake Madawaska.(3) At one site in Saint-Jacques, a near-vertical fault displaces a glacially striated bedrock surface at least 7 cm, suggesting a response to postglacial compressive stress similar to that observed on outcrops in the nearby epicentral region of the 1982 Miramichi earthquake.Although the Témiscouata–Madawaska Valley lacks historical evidence of seismic activity, and many of the phenomena observed could, individually, have been generated by aseismic processes, we conclude that the close proximity of diverse features related to mass transport and faulting suggest that the valley has been the locus of seismic activity from the time of its deglaciation to the recent, but prehistorical, past.