Cedar Butte's geology is told in two parts: that of its bedrock, with a time scale of millions of years, and that of the glacial deposits blanketing its surface, on the much more recent time scale of roughly 100,000 to about 12,000 years ago.
And Cedar Butte does have a core of bedrock, despite its extensive glacial blanketing. There are several hard to reach exposures on the sheer north face; and it's exposed right under foot on the new trail just before the viewpoint. My (limited) understanding is that Cedar Butte's core may be an outcrop of the granitic mass known as the Snoqualmie Batholith (and exemplified in Granite Mountain, towards Snoqualmie Pass). Or it maybe a different type of volcanic rock. At any rate, it is distinctly different from the rock of Mount Si to the north.
(This is one of the reasons we know that the moraines across the ends of the valleys were created by glacier from the north, and not from the valleys: the moraines contain a distinctive greenish rock whose only known local source is on Mount Si.)
But the bedrock is largely unnoticed under a layer of glacial till deposited during the last glacial advance, approximately 16,000 to 12,000 years ago. During this time the Cordillerean Ice Sheet in Canada sent a stream of ice called the Puget Sound Lobe down the Puget Lowland. And a small part of it pushed up into the Snoqulamie Valley. The north face of Cedar Butte is so steep because it is an "ice-contact slope": it formed against a wall of ice. (The steep slope the Cedar Falls Road goes up just south of the river is also an ice-contact slope.)
All this ice had several important consequences.
First, it caused the odd behavior of the Cedar River where, nearly overlooking the Snoqualmie Valley, and with hardly any barrier to prevent it tumbling down to North Bend, it suddenly turns and go through a gap between Rattlesnake Mountain and the unnamed mountain to the south. This is explained by the Snoqualmie Valley being filled by an even bigger mountain of ice (two or even three thousand feet high over North Bend, and rising up to four thousand feet over the main lobe), and a gap in Rattlesnake Mountain due to an old ("Miocene") fault. (When the Cedar River was diverted, or even if it sometimes regains its old channel only to be diverted again by the next ice advance, is unknown.)
Second, the glacier blocked all of the valleys, creating a series of large alpine lakes. While this could be confidently inferred simply from the presence of the glacier, it is confirmed by evidence of beach terraces cut into the sides of all of these valleys. Moreover, the elevations of these terraces successively descend from north to south, showing that they drained to the south. And of course the ultimate lake, when the glacier was pressed highest against its southern and western barriers, covered all of the Puget Sound Lowland, draining out at Olympia to the Chehalis River.
(It is actually a little more complicated in that as the glacier receded different spillways would be uncovered, and the glacial lake would drop to the elevation of each spillway. So there are often multiple terraces at different elevations.)
Third, there was a LOT of water flowing around all of this ice. Perhaps not at all times, but at times, and certainly when the climate finally warmed up and the glacier started melting. Consider that all of the drainage of the west side of the Cascades from here north--the combined flows of the Snoqualmie, Skykomish, Stillaguamish, Skagit, and even the Fraser Rivers--plus all of the melt-water from the eastern side of the glacier, it all ran down the east side of glacier, right past this point.
All of that water means a lot of power to move and erode. This what created "The Slot" that sliced Little Si off from its parent: a lot of water apparently finding a weakness in an old fault. That water-- or rather, the current of flowing water--also scoured the front of Mount Washington, which is why it is so steep on the north side. But when that current hit still water, as in a lake, it lost its power, and the material it was carrying started dropping out, the bigger and heavier pieces first. This is how deltas form.
So at some time in the chilly past we have the Snoqualmie Valley filled with ice, hard up against Mount Si, and against Mount Washington, and generally closing off the Snoqualmie Valley. This creates large lakes in the valleys of the Middle and South Forks of the Snoqualmie River, and in the valley of the Cedar River.
When the large volume of water flowing through the "The Slot" hits the Middle Fork Lake it slows down and drops its load of sand and gravel. This forms a delta moraine, Grouse Ridge, across the mouth of the Middle and South Fork valleys. Again the water spills over the ice dam up against Mount Washington, cutting away at the face, but then hits the next lake and fills in the former canyon of the Cedar River. Later these deposits start accumulating to the side of the current, forming the West Ridge of Mount Washington, and a gentle slope that grades away from where the current entered the lake. The water then drains to the southwest, but no cataract forms here because (when the glacier is at its maximum) this is all under water. When the ice in this area breaks up enough so that water in the Snoqualmie Valley starts draining out to the north, the Cedar River can't escape the channel it has carved, and continues in its present course.
Cedar Butte survived being eroded away because the glacier against it and the lake above it protected it from the ravages of the current. By the time the ice melted back and the lake lowered, the flow of water was diverted elsewhere.
Between the cessation of this current and the lowering of the glacial lake, a finer clay-like sediment was deposited over the entire Cedar Embankment. The placement of this sediment, or till, is crucial to the later failure of reservoir. When Cedar Lake was first raised (by the "Crib Dam") sixteen feet, it held, because the till, acting as a sealer, was supported by the underlying deposits. This was in line with prior experience with reservoirs against moraines created by alpine glaciers. But downstream of the Crib Dam that layer of till had been scoured away by the Cedar River. When the Masonry Dam forced water against this embankment there was no seal, except at the far side. And it was not supported against pressure working from within the embankment. The result was the Boxley Blowout.