Over the centuries, trade developed. Egyptian caravans carried grain to Kush and returned to Aswan with ivory, incense, hides, and carnelian (a stone prized both as jewellery and for arrowheads) for shipment downriver. Egyptian governors particularly valued gold in Nubia and soldiers in the pharaoh's army. Egyptian military expeditions penetrated Kush periodically during the Old Kingdom. Yet there was no attempt to establish a permanent presence in the area until the Middle Kingdom (c.2100–1720 BC), when Egypt constructed a network of forts along the Nile as far south as Samnah in Lower Egypt to guard the flow of gold from mines in Wawat, the area between the First and Second Cataracts.