As Earth temperatures cooled about 100,000 years ago, ice built up in the north to more than a mile high and spread south under its own weight, bulldozing the land. By about 18,000 years ago, the continental glacier had reached as far south as mid-New Jersey and stood at least half a mile high. Along the glacier’s leading edges, lakes formed in valleys blocked by ice and rock debris. In New Jersey, the largest of these was Glacial Lake Passaic, which covered about 30 miles by 10 miles and reached depths of 240 feet. At its maximum size about 13,000 years ago, the lake overflowed through a small gap in its rim at Liberty Corner Road just north of here, and ice-laden waters rushed through Moggy Hollow on their way to join the Raritan. After the ice melted back, a lower gap at Paterson opened, the lake drained, and it finally disappeared about 12,000 years ago. The basalt rock ledges and outcroppings within Moggy Hollow endured thousands of years of water flow and remain today, creating the remarkable rock structure within the Leonard J. Buck Garden.