Harker Marsh, sometimes referred to as Sunny Slack, is a hamlet in the Allerdale district of Cumbria, located about 2 miles south-east of Maryport and ¾ mile north-east of the village of Broughton Moor. Most of the houses here are located along the road which runs between Brughton Moor and Dearham. There is a large farm, New Hall Farm, to the east of the settlement. Historically, the Harker Marsh had been part of the Little Broughton township in the ancient parish of Bridekirk. In 1887, it was transferred to Great Broughton Civil Parish. Then in 1898 it formed part of the newly established Civil Parish of Broughton Moor; except the farm, which gives its address as Sunnyslack, and is in the Civil Parish of Dearham.
By Local Government Board Order No. 20,630 (24 March, 1887), Harker Lodge Marsh and Sunny Slack were transferred from Little Broughton to Great Broughton.