The Town of Denning is comprised of two hamlets, Sundown and Claryville, located in Ulster County, and strangely separated by Sullivan County. Sundown probably so named in the early 1800's because the narrow, deep valley, shaded by towering hemlocks seemed always to be near nightfall. Claryville was originally called Claraville after the wife of an early landowner.

Both hamlets were part of a tract given by the English Queen Anne to one Johannes Hardenberg in 1708. The Hardenberg Patent was surveyed in 1749 and divided into lots. Denning covers parts of great lots 6 and 7 which were further divided into lots which were sold to investors and called tracts. William Denning eventually purchased 19,169 acres when a Philadelphia land grant corporation failed to pay taxes. After Denning's death William H. Denning purchased some 24,000 acres which were again surveyed and divided into lots and sold.

The first recorded settler was John Bush in 1837; others soon followed to farm and operate lumber and turning mills powered by the many streams. The first school opened in 1845 and in 1849 the Town of Denning was created by an act of the State Legislature.

The vast forests of hardwood trees were converted to lumber. With the Civil War the hemlocks were cut for the bark which was used for tanning hydes imported to make leather. Land was cheap because the lumber and bark rights had already been sold and converted into money. Eventually the mills and farms were no long economically viable and Denning became a magnet for people attracted by the beautiful mountains and valleys and the excellent trout fishing afforded by the many streams.