Which of these pieces of basalt rock has probably been in a river the longest?

Abasalt rock is an igneous rock that is characterized by its dark color and fine-grained texture. it is mainly composed of plagioclase and pyroxene minerals. it is earth's most abundant bedrock.

in reference to the pictures shown on the questionnaire, the  flattest one among the rocks is the piece of basalt rock that has been in the river for a long period of time. this is because rivers and streams can transport pieces of rocks. as these pieces of rock are carried along by the water, they smash against each other and the river bed. as they stay long enough in such vigorous blow they gradually wear away. thus they become smaller, flatter and more rounded.