An experiment has been conducted to study hydraulic fracture behavior at a geologic formation interface. Fractures were initiated above and below an interface between welded tuff with modulus of elasticity (E), Poisson's ratio (v), porosity (~) and permeability (k) of 3.B x 106 psi, 0.239, 13% and 0.01 md, respectively, overlying a bedded ash-fall tuff with E, v, ~, and k of 2.4 x 105 psi, 0.312, 45% and 0.01 md, respectively. Nine-thousand gallons and five thousand gallons of colored cement were injected into the ash-fall tuff and welded tuff, respectively, at a flow rate of 6 bbl/min. Conventional fracture design calculations indicated that this volume was sufficient to propagate 50 ft high fractures, 600 ft total length in each zone. Material property measurements from logs and laboratory tests on core samples, in situ stresses in adjacent regions, design calculations, pumping schedules and treatment operations relevant to this experiment are described. Mineback through the experiment regions has been initiated and will allow direct observation of the created fracture systems. Evaluation of the fracture behavior, particularly at the interface, will be performed during mineback and integrated with the present data to provide a better understanding of hydraulic fracturing.