  We use the Chandra X-ray Observatory to analyze interactions of the blast
wave and the inhomogeneous interstellar medium on the western limb of the
Cygnus Loop supernova remnant. This field of view includes an initial
interaction between the blast wave and a large cloud, as well as the encounter
of the shock front and the shell that surrounds the cavity of the supernova
progenitor. Uniquely, the X-rays directly trace the shock front in the dense
cloud, where we measure temperature kT = 0.03 keV. We find kT~0.2 keV in
regions where reflected shocks further heat previously-shocked material.
Applying one-dimensional models to these interactions, we determine the
original blast wave velocity v_bw~330 km/s in the ambient medium. We do not
detect strong evidence for instabilities or non-equilibrium conditions on the
arcsecond scales we resolve. These sensitive, high-resolution data indicate no
exceptional abundance variations in this region of the Cygnus Loop.
