The landforms that make up Minnesota today were left behind during the time period that is now known as the Wisconsin Glaciation. Tongues of ice formed and retreated many times. Early history of these episodes are speculative because evidence has been obscured by later glacial activity. Wood from beneath the oldest deposit goes beyond the capability of radio carbon dating and so must be 40,000 years or more. The next advance of ice was perhaps 34,000 years ago and it left behind significant amounts of limestone fragments. Till deposited in this advance is visible on the Iron Range (in the open pit mines), in the Twin Cities, and along the Minnesota River. Landforms in west-central Minnesota show that the Wadena Lobe shrank, then readvanced to form the Alexandria Moraine, which is impressive in height and breadth. Eventually the flow of ice stopped and collapsed into a moraine dotted with lakes. Almost 20,000 years ago, the glacier making processes slowed. The huge glacier known as the Rainy-Superior melted and developed tunnels of meltwater. The hydrostatic power of the melt water was so great it created cut deep valleys into the floor. Eventually the tunnels collapsed and deposited ribbons of sand and gravel within those valleys. Glaciation had not entirely ceased. 14,000 years ago there was a reservoir of ice in the north of what is now the Canadian border. There were several short advances before full retreat. After that, the now familiar landscape of present-day Minnesota emerged from the former ice sheet.