Fortismere was a large estate stretching from what is now Birchwood Avenue, to the edge of the playing field in Woodside Avenue in the south and to Collngwood Avenue in the west. Some of the trees along the back garden fences of Grand Avenue and Collingwood Avenue may be the ones shown on the 1865 map, not repoduced here. A narrow, sinuous lake lay about half way down the slope between Fortis Green and Grand Avenue. Nearly five hundred yards long, it stretched from the west side of Firs Avenue to the east side of Leaside Avenue, with an island in Birchwood Avenue and a boathouse on the back fence between Leaside Avenue and Fortismere Avenue.
This was a considerable area of water, yet it was almost at the top of a hill. Where did the water come from?
Presumably the lake was fed by springs produced as a result of the ice action ten thousand years ago. The ice brought a mixed deposit of stones and sand from far afield and laid it on top of the London Clay. Long before the ice had deposited its debris, rivers and streams had run on top of the London Clay and laid down gravel beds. Thus the glacial drifts now rest on clay and irregular patches of gravel.