Much of the work was through a sandy soil, but in various places its course could not avoid ledges of solid rock. These taxed the effort and patience of the laborers, who were mostly native born, as emigration from Ireland, Italy and Hungary was then but slight. The shore of Medford Pond, or, as it is now termed, Mystic Lake, was originally intended for the southern terminus, but the canal was built six miles further, and ended at the Charlestown mill-pond, special legislation authorizing the extension. But for this, the remaining distance would have been covered by the pond and Mystic, or, as then called, Medford River. Whether the shallowness of the upper river, or its serpentine course caused the continuation of the artificial channel is unknown.
Briefly described, the canal was a ditch thirty feet wide and four feet deep below its banks. Sometimes these were below the natural surface of the ground, but in many places, artificial embankments were required to preserve the various levels, of which there were eight. Between these latter, which varied from one to six miles in length, sixteen locks, like huge steps, were built, overcoming a rise of one hundred and four feet from tide water at Charlestown, to the Concord at Billerica, and a descent of twenty-six feet to the Merrimack at Chelmsford. Five others provided entrance into these rivers, and also into the Mystic at Medford, while suitable waste weirs were placed contiguous to natural water courses. Filling this ditch to within a foot of the embankment
