The valley descends in a northwestern direction from the watershed between the Rhine and the Thur, and is enclosed on the northeast by the chain of the Säntis () and on the southwest by that of the Churfirsten () and of the Speer (). It is a fertile valley of about in length from the source of the river to Wil on the railway line between Winterthur and St. Gallen. The upper half is traversed by an excellent road, while from Nesslau there is a railway to Wil ().