These afford the clearest evidences of glaciation on a great scale in early Carboniferous times.
Local glaciation has carved the higher levels of this range into a maze of amphitheatres containing lakes, separated from each other by aretes and alpine peaks.
Of the Urals, but the question as to the glaciation of the Urals still remains open.
Glaciation was formerly much more extensive, old moraines being observed down to 12,000 ft.
No satisfactory solution of this problem has been reached; but the association of the Great Lakes and other large lakes farther north in Canada with the great North American area of strong and repeated glaciation is highly suggestive.