  The Galactic distribution of the high-latitude molecular clouds is
considered. It is suggested that the majority of these clouds are clustered in
two large shells around the two closest OB associations, Per OB3/Cas-Tau and
Sco OB2. The most prominent shell of high-latitude clouds is centred around the
Per OB3/ Cas-Tau association which is also at the centre of the Gould's Belt.
However, the Per OB3/Cas-Tau group of high-latitude clouds appears as an
elliptical shell at nearly right angles to the plane of the Gould's Belt. Its
kinematic age (~10 Myr) is much smaller than the expansion age (~35 Myr) of the
ring of intertellar matter associated with the Gould's Belt. It is suggested
that while, as is generally understood, the larger expanding ring of gas
associated with the Gould's Belt was created by stellar winds and supernova
explosions of the more massive OB stars of the Per OB3/Cas-Tau association ~35
Myr ago, the Per OB3 shell of high-latitude clouds was formed from the
back-falling gas as it was swept up by a more recent supernova explosion of an
early B type star in Per OB3 ~10 Myr ago. More distant OB associations also
produce similar shells of clouds at large heights from the Galactic plane
having relatively smaller angular sizes due to their greater distances. These
can be seen as higher latitude extensions of cloud complexes at lower Galactic
latitudes as, for example, in the case of Vela OB2.
