Temperatures measured at nearly equal 10-m intervals in the 30 + or - 10 m sediment at each of 12 holes at DSDP Leg 70 sites 506-509 and by temperatures of up to five thermistors on eleven 8-12 m long piston cores. High and variable total heat flow and localized hydrothermal discharge nearly equal 10 (super -8) m/s, associated with individual mounds. The combination of heat flow data and physical properties data suggests that nearly equal 50 m of sediment may be a threshold thickness for sealing of hydrothermal circulation within basement, where the topography is smooth. The formation of mounds may be associated with the forced localization of hydrothermal discharge through the sediment, as its thickness approaches this threshold value.--Modified journal abstract.