Mills and Topography (adapted from Heron, 1978)

Introduction

Before supermarkets and convenience stores, mills were the economic and social centers of rural America in the 18th and 19th centuries.  Mills were the locations where raw materials were converted to usable commodities, such as grain into flour and cornmeal and logs into lumber (See Anderson (1978) for additional information on mills of the Eno.).  The choosing of a mill site was very important for the mill owners.  A mill’s location contributed to its economic success in two ways:  1) ease of construction and long-term maintenance costs of the mill house and dam; and 2) ease of access for mill customers.  The underlying geology of mill sites directly affected these aspects and contributed to the success of a mill.

Choosing a Mill Site

A water mill can be constructed just about anywhere that water is available if one can cause the water to develop a head; that is, a difference in elevation between the dam and the mill wheel.  Due to the relative abundance of mill sites in the Piedmont upland of the Eno River, it appears that local geology controlled, or at least influenced, the location of mills.  The people who selected the mill sites along the Eno River were aware of the difference between the Piedmont upland and the Triassic lowland and each area’s suitability for mill sites.  Along the Eno River in the Piedmont upland, below Hillsborough, there were more than 15 mills, but along the Eno River in the Triassic lowland there were only two mills.

Construction of a dam in the Piedmont upland was "easy" because of the following:

  • The river exposes fresh, hard igneous and metamorphic rock in the bed of the stream and in the adjacent walls of the V-shaped valley.
  • The dam could be placed on hard rock in the stream bed and could abut against the solid rock in the valley walls (dams often fail when placed on soft rock or alluvium).
  • The narrow valleys of the Piedmont upland enabled the span of a dam across the river to be short. 
  • The higher gradient of the Eno River in the Piedmont upland meant that the headrace did not have to be long in order to maintain the necessary "head" or fall of the water between the dam and the mill wheel.
  • The higher gradient also meant that for a given height of the dam, the millpond would be smaller and would not necessitate the purchase of as much land on either side of the river to contain the pond.
  • The hard rock in the river bottom would contribute to the location of a favorable ford.  With a good ford, patrons could access the mill from both sides of the river, contributing to the economic success of the mill.
  • Resistant rock bodies often form rapids and natural drops in the river.  These rapids and drops form natural mini-dams in some locations.  Mill construction was easier in these locations because the resistant rock bodies could be used as sturdy anchor points for the dam. 

One example of a Piedmont mill site is Cabe Mill.  The remains of Cabe Mill and the headrace are in the Cabe Lands portion of Eno River State Park (see geologic trail guide for Cabe Lands trail).  The former mill site is located on the edge of a small floodplain upstream from a resistant layer of rock.  The mill dam is located about 1,000 feet upstream, on another resistant rock layer, in a narrow portion of the river with rock outcrops on both sides.  The long headrace is dug in the soft sediment of the floodplain.  This headrace is longer than most on the Eno River and the length was necessary for the water to flow to the mill on the edge of the floodplain.  The headrace was relatively easy to dig because it is in the soft alluvium of the floodplain.

In the Triassic lowland (east of West Point on the Eno Park), the dams were more difficult to build and maintain.  The problems in the Durham Triassic basin are essentially the reverse of the condition in the Piedmont upland.

  • There is a lack of large, hard rock to build the dam.  Typically, the river lacks solid rock for the dam abutments.  Consequently the dam could easily wash out.  One exception is Cameron’s New Mill (pl. 1), built from local, relatively small boulders from the igneous (diabase) sill in the area.
  • Higher dams were required to provide the needed hydraulic head due to the low gradient of the river in the Triassic basin.  A higher dam required more land for the mill pond.

Reference:

Heron, D., 1978, Mill sites on the Eno River, a geological viewpoint, Papers from the seminar on water wheels and windmills, Eno Journal, volume 7 special issue.

Anderson, J., 1978, A community of men and mills, Papers from the seminar on water wheels and windmills, Eno Journal, volume 7 special issue.