Which rock formation is often indicative of past river environments?

Sandstone is often indicative of past river environments due to its formation process and the conditions under which it typically accumulates. Sandstone is primarily composed of sand-sized particles, usually derived from the erosion of pre-existing rocks. These sediments are carried by water and can accumulate in riverbeds, floodplains, or deltas, where they are often deposited in layers.

The characteristics of sandstone, such as its sorting and grain size, can reflect the energy and dynamics of river systems. For example, in fast-flowing rivers, larger grains may be transported and deposited, while finer grains may settle in more tranquil areas or slower-moving waters. The presence of cross-bedding within sandstone layers can also be an indicator of river channel movement, illustrating how sediment was transported and deposited in a flowing environment.

In contrast, basalt is an igneous rock formed from volcanic activity, limestone typically originates from marine environments and consists of calcium carbonate, while slate is a metamorphic rock that results from the alteration of shale under heat and pressure. These other rock types do not correlate with historical river environments in the same way sandstone does.