The Cat D6C has probably been one of the all time favorites for an intermediate sized dozer. The model shown here is a mid 1970's model equipped for logging. Indeed the photo shown here was taken in 1977. The integral arch on the back of the machine is raised above the winch to provide extra lift for the logs while at the same time guiding the cable onto the winch so it doesn't burn on the edge of the drum if one pulls a load to one side. Logs hang up on fewer things, and pull easier if they are partially lifted from the ground. Viewed here it is perhaps difficult to see why the attachment over the winch his called an arch, but the name comes from and earlier tradition of logging arches. For many years, loggers lived and suffered with a trailer mounted arch which was a wishbone shaped structure with the fairlead on top for lift and wheels on the legs. This allowed the front of the logs to hang under the fairlead. This sort of a trailer mounted arch allowed one to move larger logs with a smaller dozer. In recent years direct mounted arches have been almost universally used instead of the trailer mounted ones. It is simply a pain to drive around the woods with a trailer behind your dozer, particularly a top heavy trailer.
These dozers are just nifty. They are big enough to make a good 'spreader Cat' if you want to spread loads of rock, and if you have a dump truck or a log truck mired in the mud they are big enough to pull it loose. It is is also small enough to be readily transportable (on a modest lowboy, and is light enough to log with. Similarly it is popular for as a 'pioneer cat' in a road building operation. A D-6 won't push enough to do heavy dirt work of serious road building, and is too light to handle much in the way of stump removal, but still most road builders prefer to send a Cat of this size through the right-of-way first to clear away the organic matters which shouldn't be buried in the fills. One just doesn't like to run a 'heavy' on the side-hills and the like necessary for that pioneer run, so road builders will often have a D-6 around to complement a 'heavy' in a road building operation.
The loggers traditional road builder is a D-8 Class machine such as the HD-21A featured on this site, with the giant dozers usually being reserved for use as 'rock cats'. The D-6 is a fun dozer to have around, but it clearly doesn't belong in a rock pit trying to dig rock, and is far too light to work as a 'tail cat' to anchor the back end of a skyline logging operation. Likewise it lacks the moxy to be a proper assist in moving serious sized logging equipment. After all even a dozer cannot pull more than its own weight, and a 20 ton dozer is a bit on the light side for moving a 60 or 70 ton logging machine.
The D6C, has, of course, given way to the newer designed hi-drive model called the D6H. The elevated drive sprocket gives the manufacturer some flexibility in designing the track frames. Where the application of these machines is to be for towing loads, the track line can be extended backwards behind the rear of the dozer, to provide improved balance. This is a design feature that is impossible with a conventional dozer because the drive sprocket by definition is the back of the track line, and it has to be far enough forward for the radius of the gears to be enclosed within the innards of the dozer.