I've taken several photos like this over the holidays, with the low sun creating graphic layers in the hills. I took a very similar photograph to this almost a year ago to the day (see 'X-Mas Rays').
... only photography enthusiasts need read on ...
My mini-review of the 50mm f1.8 yesterday reminded me that I haven't said how I'm getting on with my Canon EF 400mm f5.6 'L'. Frankly, I'm loving it. It's in a league of its own for sharpness and it has great reach – with a 1.6x crop it's the equivalent of 640mm in 35mm terms. It's so much better than my old Canon 75-300mm f4-5.6 it's not even funny.
400mm prime v 100-400mm zoom
The burning question everyone asks is whether the 400mm prime is better than the 100-400mm f4.5-5.6 L IS zoom lens, which is a similar price and only slightly heavier. I briefly had a loan of the zoom and got on really well with it. I chose to buy the prime because it's widely acknowledged to be sharper (at 400mm) and faster focusing.
Do I miss the versatility of the zoom? Well, there's only been a few occasions where I've missed a shot because I needed to change the 400mm for a shorter focal length. A bigger problem has been placing wildlife in the camera viewfinder (otters in particular) because I couldn't zoom out, find the animal, and then zoom in again.
The other big issue of course is image stabilization (IS) - which the zoom has and the prime doesn't. I have to admit it's difficult to get sharp shots hand-holding the prime in poor light and that IS sure would be handy. But, I cart a beanbag and a tripod with me in the car and, providing I bother to use one of them, it's not that hard to prevent camera-shake.
The other thing to bear in mind is that IS doesn't do anything to prevent motion blur. When it comes to photographing fast-moving wildlife, the prime's slightly faster autofocus gives it an advantage.
Choosing between the two lenses comes down to the kind of photography you do and whether you prioritise the versatility of the zoom or the outright optical quality of the prime.