  Observations of the interaction of an intense {\lambda}0 \approx 10 {\mu}m
laser pulse with near-critical overdense plasmas (ne = 1.8 - 3 nc) are
presented. For the first time, transverse optical probing is used to show a
recession of the front surface caused by radiation pressure driven hole-boring
by the laser pulse with an initial velocity > 10^6 ms-1, and the resulting
collisionless shocks. The collisionless shock propagates through the plasma,
dissipates into an ion-acoustic solitary wave, and eventually becomes
collisional as it slows further. These conclusions are supported by PIC
simulations which show that the initial evolution is dominated by collisionless
mechanisms.
