Accessible by water only, the Montreal River Provincial Nature Reserve is notable for its ancient coastlines formed over a long time. The present Lake Superior coast consists of wave-washed outcrops of granite and diabase. Inland, there is a steep bluff and related sand deposits which formed about 6,000 years ago during the Lake Nipissing stage of the Great Lakes. Above the bluff, an impressive sequence of cobble beaches or offshore bars can be traced inland to another ancient shoreline. Such bars are usually constructed of sand in the nearshore zones of lakes and oceans. Scientists believe that these unusual cobble and boulder bars may be unique in the Great Lakes.