I thought that creating another mask would work however it did not, instead it replaced the landscape with just black. Another thought was using the paint tool to fill in the black but I would have to use the paint tool all across the time line of the gate moving. I then thought about using a shape or solid layer to try and cover the landscape.
I copied the key points over to the shape layer and tried to make the shape of the covered mask, however the movement of the shape layer didn't match the main action and I couldn't get the correct shape.
I then realised that it would be easier to copy the mask on the layer where the mask was covered by the landscape and copy this on a solid layer, using the subtract mode for the mask. This worked really well as it did not cover the landscape or the spikes with in the gate, and I only had to move the anchor point to ensure that the key points would be matched with the main movement.