There are certain features in the construction of this fortification which have attracted attention, but their purpose has not been, and probably cannot be, explained. There are not less than seventy gaps or openings leading out of the embankments. It has been supposed that these were intended to allow the escape of water from the interior. There is another theory which has been suggested, according to which it is thought possible that they were openings made with the object of enabling the Indians to rush out at several points to repel their enemies, and that they were fenced by stockades. It, however, happens that these gaps are sometimes in positions where the slopes of the hill are so steep as to be practically inaccessible, and at other places they are on the level ground from which no surplus waters could drain away. They seem to have formed part of the system of fortification, for they occur in the same inexplicable manner at another hill work of defence, built under similar conditions, on the summit of a promontory with precipitous slopes, about forty miles to the south-east of this position, which was evidently built by the same race. This large earthwork is called Fort Hill, and it is singular in the respect of having afforded to its surveyor the means of forming a judgment upon the question of its antiquity. Consequently it has become possible to establish well-founded conclusions with respect to the dates of the construction of earthworks of a similar character. Professor Locke, in his report on the geology of that part of Ohio, stated that on the top of the wall of Fort Hill stood a chestnut tree six feet in diameter. “Counting and measuring,” he observes, “the annual layers of wood where an axeman had cut into the trunk, I found them at nearly 200 to the foot, which would give to this tree the age of 600 years. A poplar tree, seven feet in diameter, standing in the ditch, allowing the thickness to the layers which I have found in like poplars, 170 to the foot, would give nearly the same result, 607 years.”[25] Accepting the deductions of Professor Locke as being correct, it follows that the period when this hill fort was constructed was not later than the thirteenth century. Admitting that the thirteenth century, is therefore the latest age that can be ascribed to works of this type, they may be much older, for the forest trees within the inclosures may have succeeded earlier growths. It is not possible to form an estimate of the age of earthworks from their appearance,[26] and it is only by counting the annual rings of trees that happen to have been growing upon them, that any safe theories respecting their antiquity can be adopted.