1 US gallon = 231 cubic inches
your block of ice is 36 cubic feet or 62208 cubic inches
(one cubic foot = 1728 cubic inches)
/231 gives you an approximated 269.3 gallons of ice
one gallon of ice weighs 7.65 lbs (the density of ice is less than the density of water which weighs 8.35 lbs per gallon)
Therefore Jim's block of ice weighed a whopping 2060.145lbs, not accounting for temperature or errors in his measurements.
I think anyone who has shovelled snow will agree that cold water is heavy stuff