Nonrenewable resources, in contrast to renewable resources, do not naturally replenish themselves within time limits that permit sustained yield (i.e., minerals and hydrocarbons, such as phosphate rock, limestone, petroleum). Nonrenewable resources may be called stock resources because of their fixed supply. Some resources, such as soil and water, can be termed either nonrenewable or renewable depending on circumstances. For example, some underground reservoirs replenish so slowly they are effectively nonrenewable, such as the Ogalala Aquifer. Soil that is eroding faster than its T value faces eventual depletion.