We have studied edge magnetoplasmons (EMP's) created by pulses of 100 ps width in a high-mobility two-dimensional electron gas, using a cryogenic transistor. We find that magnetoplasmon motion along the edge occurs only in the direction expected for negatively charged electrons; there is no hint of propagation in the opposite direction expected for positively charged quasiholes. We also measure the periods and decay rates of the edge modes continuously as a function of the magnetic field. From these characteristics we deduce the width of the EMP boundary layer as well as the magnetic-field-dependent momentum-relaxation time.