240. If gravel be defined as material prepared by nature ready to 6. laid upon the road, then a few words are in place here concern ing iron ore. In some localities there are low-grade iron ores whi( owing to the admixture of various impurities, are unfit for use in making iron, that may be valuable for road building. These low-grade ores are widely distributed; and generally wherever limestone occurs below a considerable thickness of sandstone, the upper portion of the limy layer will be found to contain iron, and will probably make a fair road material. A lean iron ore is fre quently found in marshes; and this variety, known as bog ore, usually makes excellent roads, since it crushes readily and gives a smooth hard surface.
"In the process of retreat of the ice, the deposits which it left were accumulated under several quite diverse conditions. One of these produced the till, or commingled coarse and fine materials, which had been churned up into the ice during the time of its motion, and came down, when the melting occurred, as a broad, irregularly disposed sheet which, with rare exceptions, is to be found in all parts of the glaciated district, save where it has been swept away by streams.
"Again, from time to time during the closing stages of the ice age, the prevailingly steadfast retreat of the ice was interrupted by pauses or re-advances. In these stages there was formed along the margin of the ice-field what is called a frontal moraine, composed of debris shoved forward by the glacier or melted out of it along its front. These moraines are in most cases traceable, where they have not been washed away or buried beneath later accumulations, in the form of a ridge-like heap of waste, which, as we readily note, contains much less clay and sand and therefore a larger proportion of gravel and bowlders, than the sheet-like deposit of till above described. In some cases these moraines are very distinct features in the landscape, appearing, from the number of large bowlders which they expose, much like ruined walls of cyclopean masonry. More commonly they are found in the form of slight ridges, which may be covered with fine material, but commonly exhibit here and there projecting bowlders. In general it may be said that the
moraines afford much better sites for pits from which road mate rials are to be obtained than the till, and this because of the pre vailing absence of clay and sand in the deposits.
"Here and there in almost all glaciated districts, especially in the valleys of the greater streams, there may he found narrow ridges, often of considerable height, and almost always extending in the direction of the ice movement. These ridges are generally termed by geologists eskars, and often have a tolerable continuity for scores of miles at right angles to the ice front. A section of them shows generally a gravelly mass, nearly always free from clay and often containing little sand, though occasionally there is an abun dance of large bowlders, which have a prevailing rounded or water worn form. These eskars were doubtless formed in the caves be neath the ice through which the ancient sub-glacial streams found their way. These under-ice rivers were much- given to changing their position, and as a stream lost its impetus it was apt to fill its ancient arched-way with debris, which in its time of freest flow would have been sent forward to the ice front. At many places in New England and in New York these eskars contain large and use ful deposits of gravel, and also occasionally quantities of bowlders well fitted for crushing as regards their size and, hardness. In the Western States, because of the general coating of deep soil, these eskars are less easily found; but they exist there, and should be sought for.
"Where the eskars terminate, as they commonly do, on a morainal line, there is almost invariably found, immediately in front of their southern terminations, a delta-like deposit which, though generally composed in large measure of sand, frequently contains near the moraine extensive accumulations of useful gravel and small bowlders which are fit for crushing.
"information may be had from the banks of streams, where by chance they have cut below the deep coating of fine materials. The existence of any distinct uprise of the surface affords some reason to expect that the coarse glacial waste may be at that point not very deeply hidden. It is probable that the best method of explora tion is by any simple form of drill. Even the ordinary post-hole auger may be made to serve the purpose."