Combine crushed cookies and butter in bottom of 13 x 9 inch pan.
Freeze until firm.
Combine ice cream and all but 1/4 c crushed Heath bars.
Place on top of oreo base.
Freeze until firm.
Heat hot fudge sauce until pouring consistancy.
Pour over top of ice cream in rippled pattern. Freeze.
Just before serving, top with cool whip and reserved Heath pieces.