The Petitcodiac River watershed covers an area north and east of the Caledonian Highlands; a low-elevation (on average 67 m; ) region with rolling hills, valleys, and ridges. In fact, most of the region was below sea level, beneath the former DeGeer and Goldthwait seas, during the quaternary period. Non-calcareous Pennsylvanian (and younger) sandstones, conglomerates, silt, and red- to grey-tinted mudstone now compose the majority of the bedrock. The northern shore of the Petitcodiac River, including the Anagance and North rivers, is primarily made up of shale with volcanic rocks, mixed igneous rocks, and felsic pebble conglomerates. The southern shore, including the Little and Pollett rivers, is composed of Precambrian or Lower-Palaeozoic sedimentary, igneous, and volcanic rocks, and limestone. Both shores include red to grey Mississippian sandstone. Red beds, or red-tinted sedimentary rocks, have a higher occurrence around Port Elgin. Other significant Mississippian-era rock patches appear in two areas around the watershed. The first is located near the north-western border of the watershed, near Lutes Mountain and Cornhill. The second ranges from the east of the Memramcook River to the west of Hillsborough, in Beech Hill. The Petitcodiac River watershed also features karst topography, where gypsum and limestone around Mississippian rocks dilute into the circulating groundwater. This creates sinkholes, depressions, and caves. Examples of rare karst occurrences appear west of the village of Petitcodiac, where large sinkholes alternate between small ridges to form honeycomb-shaped patterns. Hillsborough is the home of one of the longest gypsum cave networks in eastern Canada, and is a key habitat for bats in hibernation.