Jean Louis Rodolphe Agassiz (28 May 1807 - 14 Dec.1873): No more ice...

"I am afraid you work too much, and (shall I tell you frankly?) that you spread your intellect over too many subjects at once. I think that you should concentrate your moral and also your pecuniary strength upon this beautiful work on fossil fishes .... In accepting considerable sums from England, you have, so to speak, contracted obligations to be met only by completing a work which will be at once a monument to your own glory and a landmark in the history of science ...[ ]...No more ice, not much of echinoderms, plenty of fish..."

2. December 1837, Alexander von Humbolt in a letter to Agassiz

Fig.1. “The period of the Diluvium, or Ice Age, with a glacier invading the land” (picture taken from UNGER 1851).

Today, celebrating the birthday of Agassiz, it’s seems surprising that till and erratic boulders were once considered proof of a flood, even if 100 years ago the field and outcrop data were the same, it needed a new way to interpret the data to recognize the true nature of these deposits. Agassiz (1807-1873) is today remembered as most famous proponent of a glacial origin of these sediments, but much earlier even Hutton (1726-1797) and his friend Playfair (1748-1819) speculated about a glaciation of the northern hemisphere. In 1826 a publication by the Danish mineralogist and mountain climber Jens Esmark (1763–1839) was translated into English, in this paper Jesmark discussed the possibilities that glaciers where much greater in the past then today. J.D. Forbes and Robert Jameson (“The sole effect they produced on me was the determination never as long as I lived to read a book on Geology or in any way to study the science.” C. Darwin in his autobiography 1876) discussed glacial theories during their lectures in Edinburgh. And even Buckland, who still in 1831 argued "northern region of the earth seems to have undergone successive changes from heat to cold", only until 1837 was converted to Lyell's uniformatism and considered that sudden changes, like an ice age, don't happen in geology. But with the introduction and forcing of the glacial theory by Agassiz between 1837 and subsequent years in the circle of British gentleman geologist, some of them finally become convinced, particularly by reinterpreting field work carried out by themselves years before. After that the most respected geologist gets convinced, the rest, as always, is history:

"advice - never try & persuade ye world of a new theory - persuade 2 or 3 of ye tip top men - & ye rest will go with ye stream, as Dr B. did with Sir H. Davy and Dr. Wollaston in case of Kirkdale Cave"

Jackson, Edward, about an advice given by his professor Buckland in 1832

References:

UNGER, F. (1851): Ideal Views of the Primitive World, in its Geological and Palaeontological Phases. Taylor and Francis, London
BOYLAN, P.J. (1998): Lyell and the dilemma of Quaternary glaciation. Geological Society, London, Special Publications 143: 145-159. doi:10.1144/GSL.SP.1998.143.01.13