Combine the graham
cracker crumbs, melted butter, and 1/2 cup sugar in a mixing bowl. Mix
until evenly moistened, reserve 1/4 cup of the mixture for the topping.
Press the remaining mixture into the bottom of a 9×13-inch baking dish.
Heat marshmallows and
milk in a large saucepan over low heat and stir until the marshmallows
are completely melted. Remove from heat and cool.
Whip cream in a large
bowl until soft peaks form. Beat in 1/3 cup sugar until the cream forms
stiff peaks. Fold the whipped cream into the cooled marshmallow mixture.
Spread 1/2 the cream
mixture over the crust, arrange the peaches on top of the cream, then
spread the remaining cream mixture over the peaches. Sprinkle the
reserved crumb mixture over the cream. Refrigerate until serving.