all split leather is not the same. an earlier poster was correct, cowhide *is* thick. So thick, in fact, that there are several layers of epidermis (think of a sheet of plywood), most of which can be used in commercial tanning applications, but not all of which are close to being superior leather or suitable for higher quality leather goods. At the tannery, a leather hide is split into 2 layers – top grain and the lower grain or "split." The lower, bottom grain is the flesh piece and may be split one or more times. The splits are generally inferior, contain less fat, and are far less supple. Because of the lower fat content, more chromium salts have to be crammed into it during the tanning-curing process in order to make it more durable. This makes it almost as stiff as cardboard and so it needs a lot of working to be shaped to form.
This splitting process is used on a variety of hides.