Hack Fall Wood, otherwise known as Hackfall, is a Site of Special Scientific Interest, or SSSI, of 44.8687 hectares (0.4487 km2; 0.1732 sq mi), lying north-east of the village of Grewelthorpe, North Yorkshire, England.

During the 18th century it was landscaped in the picturesque style by landowner William Aislabie, who created views by engineering streams and pools, planting trees and building follies.

He bought it for its timber, and perhaps also for its lime kiln, the coal pits near Limehouse Hill, sandstone quarries for repairing Ripon Cathedral, and the sawmill.

His son William Aislabie (1700–1781), with an eye to the sublime aesthetic, landscaped the site in a natural, picturesque style with follies, an artificial waterfall, temples and grottoes among the trees, and the kinds of views and glades which were fashionable at the time.

[3] Aislabie used the Banqueting House, now known as the Mowbray Point Ruin, to entertain friends, and in the 19th century this became a tea room for tourists, when Hackfall was the property of Lord Ripon and available to those could pay for entry.

[10][13] "Nineteenth century writers hailed [Hackfall] as one of the most beautiful woodlands in the country;" J. M. W. Turner and William Sawrey Gilpin painted here.

[3][13][14] In March 1933 after the National Trust failed to purchase it due to lack of funds, timber merchant John Green bought the wood,[10][14] and most of it was clear-felled, then partially replanted with conifers.

By 1937 Hackfall was a commercial farm and woodland, then during World War II it was allowed to degenerate, and fell prey to vandalism.

[3] The wood was, however, allowed to regenerate naturally until the 1980s, and a small part still remains of the Sandbed Hut near Limehouse Hill, and the entrance Gate Pillars.

[1][14][15] When the property was offered for sale in 1987 and a threat of commercial development was noted, the Hackfall Trust was formed to raise funds for restoration of the landscaping.

[14] Hack Fall Wood is a 44.8687 hectares (0.4487 km2; 0.1732 sq mi) biological Site of Special Scientific Interest (SSSI),[16] designated because "it is important as a largely undisturbed example of ancient semi-natural broadleaved woodland."

This "ancient woodland" with its large variety of flora over a mixed geology holds a valued resource of plant life.

[3] It sits on the north and east slopes where the River Ure flows through the Hackfall Gorge,[2] the site being adjacent to the north-east side of Grewelthorpe, and south of Masham.

For example, around the Brimham Grits cliffs and steep slopes the soil is acidic, and woodland trees are holly, rowan, with a predominant canopy of silver birch and sessile oak.

Other woodland floor plants here are primrose, wood anemone, enchanter's nightshade, woodruff or sweet-scented bedstraw, ramsons or wild garlic and dog's mercury.

[2] Lesser celandine, early dog violet, opposite-leaved golden-saxifrage (Chrysosplenium oppositifolium) and wood sorrel are also present in spring.

On the wettest ground is tufted hair-grass, pendulous sedge, great horsetail and meadowsweet, below spindle, bird cherry and alder.

[22][23] Animals seen here are stoat, badger, red fox,[20] brown hare, rabbit, grey squirrel, mole, roe deer,[3] and otter.

[2][24] The main principles of maintenance in this case are to make sure that the woodland is appropriate for the site's history, geology and geography, that it can continue to regenerate following the clearcutting of the 1930s, and that the protected habitat and biota can be supported.

Between August and February (to avoid the breeding season), non-Indigenous trees and shrubs may be cleared, and in some areas thinning may take place to maintain variety of woodland structure.

Some areas of the wood should be left completely unmanaged, with fallen trees permitted to pile up, providing habitat and insect food for a variety of wildlife.

Unit One, 17.5165 hectares (0.1752 km2; 0.06763 sq mi) in the north, had mature woodland with old and young trees, fallen deadwood and some sycamore but not too much.

Unit Two, 17.3069 hectares (0.1731 km2; 0.06682 sq mi) in the centre of the site, had been in less favourable condition but was now acceptable, with varied woodland, diverse flora and sycamore whose expansion had been cut back sufficiently but still required monitoring.

[27] Harrogate Borough Council's conservation document of 2011 provides full information on the quality of the site and public consultation obligations.

[3] There are about eight SSSIs in the Harrogate region,[28] others being Bishop Monkton Ings,[29] Brimham Rocks,[30] Cow Myers,[31] Farnham Mires,[32] Hay-a-Park Gravel Pit,[33] Kirk Deighton,[34] Mar Field Fen,[35] Quarry Moor,[36] and Ripon Parks.