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Cenozoic Pre-Belt Mesozoic Pre-Cambrian Paleozoic Cambrian and Ordovician Page 1 from http://mbmg.mtech.edu/gmr/00-old/gmr-montanageology.asp#time Geologic time, from the formation of the Earth at ~4.6 billion years ago to the present, is understood and represented by layered rocks throughout the world. By understanding the relative ages of layered and cross-cutting rocks, and the fossils they contain, geologists have developed a geologic time scale. Relative ages are cross-correlated with numerical ages derived from radioactive isotopes of elements contained in some of the geologic units. Using fossils and radiometric ages, geologists can compare the geologic strata of Montana with the "type section" of Devonian rocks exposed in the Devon area of southern England. For example, using fossils, geologists can compare certain geologic strata in Montana with the "type section" of Devonian age rocks in Devon, England and determine that their ages are the same. Radiometric dates tell us that Devonian rocks fall in a range of 369–410 million years ago. Page 2 Topsoil: The top layer of soil that consists of organic matter, minerals from decomposing rocks, and it provides nutrients and habitat for bacteria, fungi, insects, arachnids, plants, and small animals. Subsoil: In northern and eastern MT, it is predominantly Glacial Till, which is a heterogeneous mixture of sand, silt and clay. It is found between the topsoil and the bedrock. Bedrock: In places, it is weathered at the top, and becomes increasingly consolidated with greater depth. In many places of western MT, it is exposed at the ground surface, with little to no topsoil above it. Page 1 of 3 Montana Salinity Control Association www.montanasalinity.com Saline Seep FORMATION Rechar Area ge Discharge Area FORMATION: Saline seeps form when ground water brings salts from the soil up to the surface. When the topsoil is bare of plants in the recharge area, precipitation percolates through the soil profile, collecting salts along the way. It builds up the ground water table, and when the ground water gets close to the surface, a saline seep forms in the discharge area. Plants can’t grow where there is too much salt on the soil surface. RECLAMATION: To reclaim saline seeps, perennial forage (like alfalfa) is planted in the recharge area, which is uphill from the seep. This prevents the extra water from building up underground, and the salts go back down to where they don’t harm plants on the surface. Topsoil: The top layer of soil that provides nutrients for plants to live in. Saline Seep RECLAMATION Perennial Forage Subsoil: In northern MT, it is predominantly Glacial Till, which is a heterogeneous mixture of sand, silt and clay. It is found between the topsoil and the bedrock. Bedrock: The bedrock is the impermeable layer that the water table builds up on, and it is the source of salt that is found throughout the soil profile. Salt-Tolerant Forage Lowered Water Table Montana Salinity Control Association Page 2 of 3 www.montanasalinity.com Saline Seep FORMATION Bedrock Subsoil Topsoi l Recharge Area Discharge Area Page 3 of 3 Montana Salinity Control Association www.montanasalinity.com Bedrock Subsoil Topsoi l Saline Seep RECLAMATION Perennia Forage l Lowered Water Table Salt-Tolerant Forage