By the way, To do it in Bryce. Make a flattened cyllinder set it positive. make a flattened cone set it negative. Rotate the cyllinder and cone 90 deg X so they're on edge. Stick the cone into one face of the cyllinder. Copy the cone paste it and rotate 180 deg to stick into the other face of the cyllinder. Select all three objects and scale Y so they become tall and oval. Shift the whole cyllinder down Y a couple of clicks so the center is lower than the centers of the two cones. (this makes the top of the arch thinner than the base.) You can either stick everything half way through the ground or add another negative box to remove the bottom half. Group. Thats it.