Almost 1.2 million acres of threatened land north of Toronto would be protected by legislation to be announced today by the Ontario government. The conservation effort would protect more than 90 percent of the Oak Ridges Moraine, which is the source of groundwater for much of southern Ontario. The province plans to establish a $250 million fund to purchase private land on the moraine. The decision is a reversal of previous policy, in which the provincial government had left development in the area up to the discretion of individual municipalities. Environmentalists hailed the move as a decisive blow to urban sprawl.