DISTRIBUTION OF GRAVEL. The gravel beds of the glacial drift furnish excellent road-making materials. The glacial ice sheet, often a mile or more thick, covered New England and Canada and all of the United States north of an irregular line start ing on the Atlantic Coast a little south of New York City and run ning thence successively to the southwest corner of the State of New York, to Cincinnati, to a point a little north of the mouth of the Ohio river, to the mouth of the Missouri river, to Topeka, Kansas, thence north and west a little west and south of the Missouri river to the head waters of that stream, and thence west to the Pacific ocean. All of the area north of the above described line was cov ered with the ice sheet except small portions of southeastern Minne sota, northeastern Iowa, northwestern Illinois, and a considerable portion of southwestern Wisconsin. As this ice sheet •crept to the southward, it rent great quantities of stone from the bed rocks; and these materials were borne southward, either in the slow-moving ice or hurried along by the violent currents of water which swept forward to the margin of the ice field. Thus impelled the under ice streams were able to bear toward the margin of the glacier great quantities of stone. The original range of the glacial gravels has been greatly extended here and there by the streams, which, flowing southward beyond the drift belt, have often carried quantities of the hard detritus for many miles beyond the limits of the ice field.
Unfortunately the glacial gravel deposits have not been studied from the point of view of the road-maker. However, it is known that east of the Hudson river the glacial supply of road gravels is only here and there of economic importance, for in most of that field the glacial waste lies on native rocks which are suitable for road making; and that from the Hudson to the Mississippi, the glacial deposits of bowlders and gravel afford better road-building mate rials than any of the native rocks. Glacial gravels exist in consid erable quantities in western Pennsylvania, in the greater part of Ohio, in northern Indiana, and in northern Illinois, and to some extent in several of the states of the Northwest.
South of the glacial district, the rocks exposed to the weather have decayed by a process of leaching, which in many cases has removed strata hundreds of feet thick. The rocky portion is removed in proportion to its solubility; and, as a result, there are often left concretions of cherty matter which were originally con tained in beds of limestone. This cherty residuum of flinty mate rial generally lies in a comparatively thin sheet of fragments min gled with sand and clay; but occasionally it is found in deposits from which the clay and sand have been removed by recent or ancient streams, leaving the material well suited for spreading upon a road. Sometimes this cherty residuum is found in layers of fragments many feet thick, and is valuable for road-building in a locality where suitable material is scarce. The presence of chert is often revealed by the gullies in the plowed fields and along the streams. In some localities very good roadways are constructed simply by shoveling these fragments from the stream beds &AL depositing them on the road.* This cherty deposit is a valuable road material in the southern portion of the Appalachian mountains, and along the Ozark foot hills in southern Illinois (particularly in Alexander and Union counties), in southern Missouri, and in northern Arkansas. Chert is found in some of the states of the Northwest where the glacial erosion was small, so that the rocks that had decayed before the glacial time were not entirely removed. In southwestern Arkan sas the gravels consist of fragments of novaculite or razor stone—a material of nearly the same geological origin and physical charac teristics as chert. In many places in that state the novaculite gravels form extensive beds, 20 or more feet thick. At the southern extremity of the Appalachian mountain system is a wide-spread deposit of gravel, termed the La Fayette formation, whose geologi cal origin is not determined. This deposit often attains a thickness of 40 to 50 feet, and is a valuable source of mad-building material.