Temperatures were measured in six boreholes (~380 m deep) in northcentral Oklahoma (36.36 ¿N, 96.70 ¿W) in May 1993. Temperatures in the upper 150 m of the boreholes appeared to be anomalously warm when the average thermal gradient was extrapolated from below 150 m depth. The average thermal gradient below 150 m depth was 37.5 ¿C/km; the average thermal gradient from 20 to 150 m depth was 27.4 ¿C/km. Several hundred laboratory measurements of thermal conductivity were made on cores collected at the study site; 65 specific heat capacity measurements were also made. In situ porosity was estimated from density logs and measurements of rock matrix density. Average thermal diffusivity of the upper 110 m of the stratigraphic section was estimated to be 20.3¿2.0 m2/yr (6.43 ¿10-7m2/s). Heat flow was estimated to be 52¿6 mW/m2 from 20 to 110 m depth and 69¿7 mW/m2 from 277 to 305 m depth. The observed energy imbalance and anomalously warm temperatures in the upper 150 m of the boreholes could not be explained solely by hypotheses related to topographic gradients, vegetation, heat refraction, groundwater flow, or land use changes. The only hypothesis which satisfactorily explained all of the observations was an apparent increase (1.25-1.50¿0.5 ¿C) in ground surface temperature (GST) related to a climatic warming starting in the middle to early 19th century or before (1835,+50,-150 years). When constraints from surface air temperatures (SATs) were used to interpret borehole temperatures, a better match to observations was obtained, suggesting that changes in SATs at the study site were tracked by changes in GSTs. ¿ American Geophysical Union 1995