  Above a certain solid fraction, dense granular suspensions in water exhibit
non-Newtonian behavior, including impact-activated solidification. Although it
has been suggested that solidification depends on boundary interactions,
quantitative experiments on the boundary forces have not been reported. Using
high-speed video, tracer particles, and photoelastic boundaries, we determine
the impactor kinematics and the magnitude and timings of impactor-driven events
in the body and at the boundaries of cornstarch suspensions. We observe mass
shocks in the suspension during impact. The shockfront dynamics are strongly
correlated to those of the intruder. However, the total momentum associated
with this shock never approaches the initial impactor momentum. We also observe
a faster second front, associated with the propagation of pressure to the
boundaries of the suspension. The two fronts depend differently on the initial
impactor speed, $v_0$, and the suspension packing fraction. The speed of the
pressure wave is at least an order of magnitude smaller than (linear)
ultrasound speeds obtained for much higher frequencies, pointing to complex
amplitude and frequency response of cornstarch suspensions to compressive
strains.
