Pebble formation on Mars indicates ancient river

BBC : Pebbles imaged by NASA’s Curiosity rover in Mars’s Gale crater were probably formed by flowing water some 3.5 billion years ago, researchers have concluded. Ranging in size from 2 mm to 40 mm in diameter, the stones are too big to have been blown there by wind. Their different colors indicate they came from different locations, the smoothness of their surfaces suggests abrasion by moving water, and the way they’re stacked reveals examples of imbrication , or the toppled-domino formation indicative of past river activity. The data support earlier satellite observations of the planet’s surface that show a network of valleys and channels, which could have been carved by water. Over the next weeks, as Curiosity retraces its path, scientists hope to get even better photos using its MAHLI camera, designed to capture close-up, high-resolution images of rocks and soil.