12 shares Using this 'telescope' allowed the team of US and French scientists to reveal the conditions under which these sedimentary beds first formed. Moving up through the terrain, which is several hundred feet thick, the types of bed change radically, the team explained. Lying above the lake-deposited clays that form the base of Mount Sharp, wide, tall, cross-bedded structures are a sign of the migration of wind-formed dunes. These dunes would have formed during a long, dry climate episode, thought to have been common and interspersing shorter, wet periods. Mars shifted between long dry periods and wetter eras before completely drying up to the nearly dead world we see today about three billion years ago, study shows