Geological Occurrence

This chapter introduces the geological characteristics of aggregate deposits. This type of mineral resource, regardless of whether they are rocks for crushing or sand and gravel for direct utilization, is present in numerous geological environments, all with their own unique characteristics. Sand and gravel deposits are formed by the erosion, transportation, and deposition of fragments. They are most common in glaciated areas, alluvial basins, and fluvial deposits near rivers and streams. Sand and gravel deposits are principally Pleistocene or younger and are a reflection of the region’s latest geological and climatic history. In fluvial deposits, aggregates are commonly mined directly from the river channel and/or from river terrace deposits. Glaciated regions also contain numerous deposits of this type. Marine deposits on continental shelves are large potential sources of sand and gravel, being generally sand-size and finer. Many different types of rocks are suitable for utilization as aggregates when crushed but high-quality aggregates are exclusively obtained from very hard, dense siliceous rocks. Limestone, dolostone, and sandstone are the most widely used sedimentary rocks as aggregate. Granite, basalt and andesite are typical examples of igneous rocks commonly utilized as aggregate whereas gneiss, marble, and quartzite are metamorphic rocks quarried for aggregates.