South of this boundary, many of the present-day lakes are artificial — rivers impounded by dams. The boundary is formed by the southern terminus of the Laurentide Ice Sheet, a massive continental glacier that covered most of Canada and many northern U.S. states, including New York. The eventual melting of this ice sheet transformed the hydrology of the entire continent, but in areas where the glacier covered the land, it gave birth to many lakes and ponds.

There are several different ways in which a melting glacier can leave a landscape dotted with pools of standing water. Here, we will focus on the examples of Lake Placid and Mirror Lake.

At the height of the last ice age, the ice sheet covered nearly all of New York state and began melting away around 21,000 years ago. Later, as ice was melting out of the valleys of the West Branch and East Branch of the AuSable River, it blocked their drainage to the north/northeast. Large glacial lakes filled the valleys with excess water escaping through mountain passes into adjacent watersheds.

The post-glacial history of the West Branch AuSable River valley was somewhat different than the East Branch, mostly due to the higher elevation of the West Branch valley above Wilmington Notch. As the ice retreated north of the Sentinel Range, water from the upper West Branch valley emptied into the lower West Branch and East Branch valleys, which, in turn, found an outlet through Chapel Pond Pass, between St. Huberts and Underwood, and drained to a glacial lake in the Boquet watershed.

After drainage through the Notch was reestablished, the result was a lack of large glacial lakes in the upper West Branch valley and the deposition of an end moraine in the Lake Placid Basin. These types of moraine deposits are not found in the valley of the East Branch.

An end moraine is formed when the terminus of a glacier persists at the same location for an extended period of time. The glacier does not advance or retreat due to climate and leaves a mound of deposits as material is moved through the glacier and dropped at the terminus by melting ice. These mounds can later act as dams if there is a natural depression behind them. Any end moraines that may have been left behind as the glacier retreated from the East Branch valley were likely destroyed by the deep lakes which persisted there long after the West Branch valley had drained.

Lake Placid sits in two narrow fault valleys that follow the general northeasterly trend of faults and fractures found in this portion of the Adirondacks. The two northeast-trending fault valleys are connected by smaller fault valleys, giving it the instantly recognizable “ladder” shape.

A moraine was deposited at the southern end of the depression as a stranded lobe of ice on the southwestern flank of Whiteface Mountain wasted away. The deposit blocked the drainage of the fault valleys and created a moraine-dammed lake.

Mirror Lake sits in a shallow depression on the moraine dam. The high elevation kept the moraine deposits from being breached and re-worked by waters moving within and between the large glacial lakes that filled and spanned portions of the East Branch, West Branch, Saranac and Boquet watersheds.

It would be an oversimplification to say that Lake Placid and Mirror Lake are born entirely of the Laurentide Ice Sheet. The deep scars within the billion-year-old bedrock and the uplift of the Adirondack Dome exert a considerable influence on local hydrology.

These lakes formed at the confluence of geology, climate, topographic position and good fortune to create two jewels among the many natural lakes that dot our landscape in the Adirondacks.

(Gary Henry is a stream restoration associate at the Ausable River Association.)